Source file src/runtime/mgcpacer.go
1 // Copyright 2021 The Go Authors. All rights reserved. 2 // Use of this source code is governed by a BSD-style 3 // license that can be found in the LICENSE file. 4 5 package runtime 6 7 import ( 8 "internal/cpu" 9 "internal/goexperiment" 10 "internal/runtime/atomic" 11 _ "unsafe" // for go:linkname 12 ) 13 14 const ( 15 // gcGoalUtilization is the goal CPU utilization for 16 // marking as a fraction of GOMAXPROCS. 17 // 18 // Increasing the goal utilization will shorten GC cycles as the GC 19 // has more resources behind it, lessening costs from the write barrier, 20 // but comes at the cost of increasing mutator latency. 21 gcGoalUtilization = gcBackgroundUtilization 22 23 // gcBackgroundUtilization is the fixed CPU utilization for background 24 // marking. It must be <= gcGoalUtilization. The difference between 25 // gcGoalUtilization and gcBackgroundUtilization will be made up by 26 // mark assists. The scheduler will aim to use within 50% of this 27 // goal. 28 // 29 // As a general rule, there's little reason to set gcBackgroundUtilization 30 // < gcGoalUtilization. One reason might be in mostly idle applications, 31 // where goroutines are unlikely to assist at all, so the actual 32 // utilization will be lower than the goal. But this is moot point 33 // because the idle mark workers already soak up idle CPU resources. 34 // These two values are still kept separate however because they are 35 // distinct conceptually, and in previous iterations of the pacer the 36 // distinction was more important. 37 gcBackgroundUtilization = 0.25 38 39 // gcCreditSlack is the amount of scan work credit that can 40 // accumulate locally before updating gcController.heapScanWork and, 41 // optionally, gcController.bgScanCredit. Lower values give a more 42 // accurate assist ratio and make it more likely that assists will 43 // successfully steal background credit. Higher values reduce memory 44 // contention. 45 gcCreditSlack = 2000 46 47 // gcAssistTimeSlack is the nanoseconds of mutator assist time that 48 // can accumulate on a P before updating gcController.assistTime. 49 gcAssistTimeSlack = 5000 50 51 // gcOverAssistWork determines how many extra units of scan work a GC 52 // assist does when an assist happens. This amortizes the cost of an 53 // assist by pre-paying for this many bytes of future allocations. 54 gcOverAssistWork = 64 << 10 55 56 // defaultHeapMinimum is the value of heapMinimum for GOGC==100. 57 defaultHeapMinimum = (goexperiment.HeapMinimum512KiBInt)*(512<<10) + 58 (1-goexperiment.HeapMinimum512KiBInt)*(4<<20) 59 60 // maxStackScanSlack is the bytes of stack space allocated or freed 61 // that can accumulate on a P before updating gcController.stackSize. 62 maxStackScanSlack = 8 << 10 63 64 // memoryLimitMinHeapGoalHeadroom is the minimum amount of headroom the 65 // pacer gives to the heap goal when operating in the memory-limited regime. 66 // That is, it'll reduce the heap goal by this many extra bytes off of the 67 // base calculation, at minimum. 68 memoryLimitMinHeapGoalHeadroom = 1 << 20 69 70 // memoryLimitHeapGoalHeadroomPercent is how headroom the memory-limit-based 71 // heap goal should have as a percent of the maximum possible heap goal allowed 72 // to maintain the memory limit. 73 memoryLimitHeapGoalHeadroomPercent = 3 74 ) 75 76 // gcController implements the GC pacing controller that determines 77 // when to trigger concurrent garbage collection and how much marking 78 // work to do in mutator assists and background marking. 79 // 80 // It calculates the ratio between the allocation rate (in terms of CPU 81 // time) and the GC scan throughput to determine the heap size at which to 82 // trigger a GC cycle such that no GC assists are required to finish on time. 83 // This algorithm thus optimizes GC CPU utilization to the dedicated background 84 // mark utilization of 25% of GOMAXPROCS by minimizing GC assists. 85 // GOMAXPROCS. The high-level design of this algorithm is documented 86 // at https://github.com/golang/proposal/blob/master/design/44167-gc-pacer-redesign.md. 87 // See https://golang.org/s/go15gcpacing for additional historical context. 88 var gcController gcControllerState 89 90 type gcControllerState struct { 91 // Initialized from GOGC. GOGC=off means no GC. 92 gcPercent atomic.Int32 93 94 // memoryLimit is the soft memory limit in bytes. 95 // 96 // Initialized from GOMEMLIMIT. GOMEMLIMIT=off is equivalent to MaxInt64 97 // which means no soft memory limit in practice. 98 // 99 // This is an int64 instead of a uint64 to more easily maintain parity with 100 // the SetMemoryLimit API, which sets a maximum at MaxInt64. This value 101 // should never be negative. 102 memoryLimit atomic.Int64 103 104 // heapMinimum is the minimum heap size at which to trigger GC. 105 // For small heaps, this overrides the usual GOGC*live set rule. 106 // 107 // When there is a very small live set but a lot of allocation, simply 108 // collecting when the heap reaches GOGC*live results in many GC 109 // cycles and high total per-GC overhead. This minimum amortizes this 110 // per-GC overhead while keeping the heap reasonably small. 111 // 112 // During initialization this is set to 4MB*GOGC/100. In the case of 113 // GOGC==0, this will set heapMinimum to 0, resulting in constant 114 // collection even when the heap size is small, which is useful for 115 // debugging. 116 heapMinimum uint64 117 118 // runway is the amount of runway in heap bytes allocated by the 119 // application that we want to give the GC once it starts. 120 // 121 // This is computed from consMark during mark termination. 122 runway atomic.Uint64 123 124 // consMark is the estimated per-CPU consMark ratio for the application. 125 // 126 // It represents the ratio between the application's allocation 127 // rate, as bytes allocated per CPU-time, and the GC's scan rate, 128 // as bytes scanned per CPU-time. 129 // The units of this ratio are (B / cpu-ns) / (B / cpu-ns). 130 // 131 // At a high level, this value is computed as the bytes of memory 132 // allocated (cons) per unit of scan work completed (mark) in a GC 133 // cycle, divided by the CPU time spent on each activity. 134 // 135 // Updated at the end of each GC cycle, in endCycle. 136 consMark float64 137 138 // lastConsMark is the computed cons/mark value for the previous 4 GC 139 // cycles. Note that this is *not* the last value of consMark, but the 140 // measured cons/mark value in endCycle. 141 lastConsMark [4]float64 142 143 // gcPercentHeapGoal is the goal heapLive for when next GC ends derived 144 // from gcPercent. 145 // 146 // Set to ^uint64(0) if gcPercent is disabled. 147 gcPercentHeapGoal atomic.Uint64 148 149 // sweepDistMinTrigger is the minimum trigger to ensure a minimum 150 // sweep distance. 151 // 152 // This bound is also special because it applies to both the trigger 153 // *and* the goal (all other trigger bounds must be based *on* the goal). 154 // 155 // It is computed ahead of time, at commit time. The theory is that, 156 // absent a sudden change to a parameter like gcPercent, the trigger 157 // will be chosen to always give the sweeper enough headroom. However, 158 // such a change might dramatically and suddenly move up the trigger, 159 // in which case we need to ensure the sweeper still has enough headroom. 160 sweepDistMinTrigger atomic.Uint64 161 162 // triggered is the point at which the current GC cycle actually triggered. 163 // Only valid during the mark phase of a GC cycle, otherwise set to ^uint64(0). 164 // 165 // Updated while the world is stopped. 166 triggered uint64 167 168 // lastHeapGoal is the value of heapGoal at the moment the last GC 169 // ended. Note that this is distinct from the last value heapGoal had, 170 // because it could change if e.g. gcPercent changes. 171 // 172 // Read and written with the world stopped or with mheap_.lock held. 173 lastHeapGoal uint64 174 175 // heapLive is the number of bytes considered live by the GC. 176 // That is: retained by the most recent GC plus allocated 177 // since then. heapLive ≤ memstats.totalAlloc-memstats.totalFree, since 178 // heapAlloc includes unmarked objects that have not yet been swept (and 179 // hence goes up as we allocate and down as we sweep) while heapLive 180 // excludes these objects (and hence only goes up between GCs). 181 // 182 // To reduce contention, this is updated only when obtaining a span 183 // from an mcentral and at this point it counts all of the unallocated 184 // slots in that span (which will be allocated before that mcache 185 // obtains another span from that mcentral). Hence, it slightly 186 // overestimates the "true" live heap size. It's better to overestimate 187 // than to underestimate because 1) this triggers the GC earlier than 188 // necessary rather than potentially too late and 2) this leads to a 189 // conservative GC rate rather than a GC rate that is potentially too 190 // low. 191 // 192 // Whenever this is updated, call traceHeapAlloc() and 193 // this gcControllerState's revise() method. 194 heapLive atomic.Uint64 195 196 // heapScan is the number of bytes of "scannable" heap. This is the 197 // live heap (as counted by heapLive), but omitting no-scan objects and 198 // no-scan tails of objects. 199 // 200 // This value is fixed at the start of a GC cycle. It represents the 201 // maximum scannable heap. 202 heapScan atomic.Uint64 203 204 // lastHeapScan is the number of bytes of heap that were scanned 205 // last GC cycle. It is the same as heapMarked, but only 206 // includes the "scannable" parts of objects. 207 // 208 // Updated when the world is stopped. 209 lastHeapScan uint64 210 211 // lastStackScan is the number of bytes of stack that were scanned 212 // last GC cycle. 213 lastStackScan atomic.Uint64 214 215 // maxStackScan is the amount of allocated goroutine stack space in 216 // use by goroutines. 217 // 218 // This number tracks allocated goroutine stack space rather than used 219 // goroutine stack space (i.e. what is actually scanned) because used 220 // goroutine stack space is much harder to measure cheaply. By using 221 // allocated space, we make an overestimate; this is OK, it's better 222 // to conservatively overcount than undercount. 223 maxStackScan atomic.Uint64 224 225 // globalsScan is the total amount of global variable space 226 // that is scannable. 227 globalsScan atomic.Uint64 228 229 // heapMarked is the number of bytes marked by the previous 230 // GC. After mark termination, heapLive == heapMarked, but 231 // unlike heapLive, heapMarked does not change until the 232 // next mark termination. 233 heapMarked uint64 234 235 // heapScanWork is the total heap scan work performed this cycle. 236 // stackScanWork is the total stack scan work performed this cycle. 237 // globalsScanWork is the total globals scan work performed this cycle. 238 // 239 // These are updated atomically during the cycle. Updates occur in 240 // bounded batches, since they are both written and read 241 // throughout the cycle. At the end of the cycle, heapScanWork is how 242 // much of the retained heap is scannable. 243 // 244 // Currently these are measured in bytes. For most uses, this is an 245 // opaque unit of work, but for estimation the definition is important. 246 // 247 // Note that stackScanWork includes only stack space scanned, not all 248 // of the allocated stack. 249 heapScanWork atomic.Int64 250 stackScanWork atomic.Int64 251 globalsScanWork atomic.Int64 252 253 // bgScanCredit is the scan work credit accumulated by the concurrent 254 // background scan. This credit is accumulated by the background scan 255 // and stolen by mutator assists. Updates occur in bounded batches, 256 // since it is both written and read throughout the cycle. 257 bgScanCredit atomic.Int64 258 259 // assistTime is the nanoseconds spent in mutator assists 260 // during this cycle. This is updated atomically, and must also 261 // be updated atomically even during a STW, because it is read 262 // by sysmon. Updates occur in bounded batches, since it is both 263 // written and read throughout the cycle. 264 assistTime atomic.Int64 265 266 // dedicatedMarkTime is the nanoseconds spent in dedicated mark workers 267 // during this cycle. This is updated at the end of the concurrent mark 268 // phase. 269 dedicatedMarkTime atomic.Int64 270 271 // fractionalMarkTime is the nanoseconds spent in the fractional mark 272 // worker during this cycle. This is updated throughout the cycle and 273 // will be up-to-date if the fractional mark worker is not currently 274 // running. 275 fractionalMarkTime atomic.Int64 276 277 // idleMarkTime is the nanoseconds spent in idle marking during this 278 // cycle. This is updated throughout the cycle. 279 idleMarkTime atomic.Int64 280 281 // markStartTime is the absolute start time in nanoseconds 282 // that assists and background mark workers started. 283 markStartTime int64 284 285 // dedicatedMarkWorkersNeeded is the number of dedicated mark workers 286 // that need to be started. This is computed at the beginning of each 287 // cycle and decremented as dedicated mark workers get started. 288 dedicatedMarkWorkersNeeded atomic.Int64 289 290 // idleMarkWorkers is two packed int32 values in a single uint64. 291 // These two values are always updated simultaneously. 292 // 293 // The bottom int32 is the current number of idle mark workers executing. 294 // 295 // The top int32 is the maximum number of idle mark workers allowed to 296 // execute concurrently. Normally, this number is just gomaxprocs. However, 297 // during periodic GC cycles it is set to 0 because the system is idle 298 // anyway; there's no need to go full blast on all of GOMAXPROCS. 299 // 300 // The maximum number of idle mark workers is used to prevent new workers 301 // from starting, but it is not a hard maximum. It is possible (but 302 // exceedingly rare) for the current number of idle mark workers to 303 // transiently exceed the maximum. This could happen if the maximum changes 304 // just after a GC ends, and an M with no P. 305 // 306 // Note that if we have no dedicated mark workers, we set this value to 307 // 1 in this case we only have fractional GC workers which aren't scheduled 308 // strictly enough to ensure GC progress. As a result, idle-priority mark 309 // workers are vital to GC progress in these situations. 310 // 311 // For example, consider a situation in which goroutines block on the GC 312 // (such as via runtime.GOMAXPROCS) and only fractional mark workers are 313 // scheduled (e.g. GOMAXPROCS=1). Without idle-priority mark workers, the 314 // last running M might skip scheduling a fractional mark worker if its 315 // utilization goal is met, such that once it goes to sleep (because there's 316 // nothing to do), there will be nothing else to spin up a new M for the 317 // fractional worker in the future, stalling GC progress and causing a 318 // deadlock. However, idle-priority workers will *always* run when there is 319 // nothing left to do, ensuring the GC makes progress. 320 // 321 // See github.com/golang/go/issues/44163 for more details. 322 idleMarkWorkers atomic.Uint64 323 324 // assistWorkPerByte is the ratio of scan work to allocated 325 // bytes that should be performed by mutator assists. This is 326 // computed at the beginning of each cycle and updated every 327 // time heapScan is updated. 328 assistWorkPerByte atomic.Float64 329 330 // assistBytesPerWork is 1/assistWorkPerByte. 331 // 332 // Note that because this is read and written independently 333 // from assistWorkPerByte users may notice a skew between 334 // the two values, and such a state should be safe. 335 assistBytesPerWork atomic.Float64 336 337 // fractionalUtilizationGoal is the fraction of wall clock 338 // time that should be spent in the fractional mark worker on 339 // each P that isn't running a dedicated worker. 340 // 341 // For example, if the utilization goal is 25% and there are 342 // no dedicated workers, this will be 0.25. If the goal is 343 // 25%, there is one dedicated worker, and GOMAXPROCS is 5, 344 // this will be 0.05 to make up the missing 5%. 345 // 346 // If this is zero, no fractional workers are needed. 347 fractionalUtilizationGoal float64 348 349 // These memory stats are effectively duplicates of fields from 350 // memstats.heapStats but are updated atomically or with the world 351 // stopped and don't provide the same consistency guarantees. 352 // 353 // Because the runtime is responsible for managing a memory limit, it's 354 // useful to couple these stats more tightly to the gcController, which 355 // is intimately connected to how that memory limit is maintained. 356 heapInUse sysMemStat // bytes in mSpanInUse spans 357 heapReleased sysMemStat // bytes released to the OS 358 heapFree sysMemStat // bytes not in any span, but not released to the OS 359 totalAlloc atomic.Uint64 // total bytes allocated 360 totalFree atomic.Uint64 // total bytes freed 361 mappedReady atomic.Uint64 // total virtual memory in the Ready state (see mem.go). 362 363 // test indicates that this is a test-only copy of gcControllerState. 364 test bool 365 366 _ cpu.CacheLinePad 367 } 368 369 func (c *gcControllerState) init(gcPercent int32, memoryLimit int64) { 370 c.heapMinimum = defaultHeapMinimum 371 c.triggered = ^uint64(0) 372 c.setGCPercent(gcPercent) 373 c.setMemoryLimit(memoryLimit) 374 c.commit(true) // No sweep phase in the first GC cycle. 375 // N.B. Don't bother calling traceHeapGoal. Tracing is never enabled at 376 // initialization time. 377 // N.B. No need to call revise; there's no GC enabled during 378 // initialization. 379 } 380 381 // startCycle resets the GC controller's state and computes estimates 382 // for a new GC cycle. The caller must hold worldsema and the world 383 // must be stopped. 384 func (c *gcControllerState) startCycle(markStartTime int64, procs int, trigger gcTrigger) { 385 c.heapScanWork.Store(0) 386 c.stackScanWork.Store(0) 387 c.globalsScanWork.Store(0) 388 c.bgScanCredit.Store(0) 389 c.assistTime.Store(0) 390 c.dedicatedMarkTime.Store(0) 391 c.fractionalMarkTime.Store(0) 392 c.idleMarkTime.Store(0) 393 c.markStartTime = markStartTime 394 c.triggered = c.heapLive.Load() 395 396 // Compute the background mark utilization goal. In general, 397 // this may not come out exactly. We round the number of 398 // dedicated workers so that the utilization is closest to 399 // 25%. For small GOMAXPROCS, this would introduce too much 400 // error, so we add fractional workers in that case. 401 totalUtilizationGoal := float64(procs) * gcBackgroundUtilization 402 dedicatedMarkWorkersNeeded := int64(totalUtilizationGoal + 0.5) 403 utilError := float64(dedicatedMarkWorkersNeeded)/totalUtilizationGoal - 1 404 const maxUtilError = 0.3 405 if utilError < -maxUtilError || utilError > maxUtilError { 406 // Rounding put us more than 30% off our goal. With 407 // gcBackgroundUtilization of 25%, this happens for 408 // GOMAXPROCS<=3 or GOMAXPROCS=6. Enable fractional 409 // workers to compensate. 410 if float64(dedicatedMarkWorkersNeeded) > totalUtilizationGoal { 411 // Too many dedicated workers. 412 dedicatedMarkWorkersNeeded-- 413 } 414 c.fractionalUtilizationGoal = (totalUtilizationGoal - float64(dedicatedMarkWorkersNeeded)) / float64(procs) 415 } else { 416 c.fractionalUtilizationGoal = 0 417 } 418 419 // In STW mode, we just want dedicated workers. 420 if debug.gcstoptheworld > 0 { 421 dedicatedMarkWorkersNeeded = int64(procs) 422 c.fractionalUtilizationGoal = 0 423 } 424 425 // Clear per-P state 426 for _, p := range allp { 427 p.gcAssistTime = 0 428 p.gcFractionalMarkTime = 0 429 } 430 431 if trigger.kind == gcTriggerTime { 432 // During a periodic GC cycle, reduce the number of idle mark workers 433 // required. However, we need at least one dedicated mark worker or 434 // idle GC worker to ensure GC progress in some scenarios (see comment 435 // on maxIdleMarkWorkers). 436 if dedicatedMarkWorkersNeeded > 0 { 437 c.setMaxIdleMarkWorkers(0) 438 } else { 439 // TODO(mknyszek): The fundamental reason why we need this is because 440 // we can't count on the fractional mark worker to get scheduled. 441 // Fix that by ensuring it gets scheduled according to its quota even 442 // if the rest of the application is idle. 443 c.setMaxIdleMarkWorkers(1) 444 } 445 } else { 446 // N.B. gomaxprocs and dedicatedMarkWorkersNeeded are guaranteed not to 447 // change during a GC cycle. 448 c.setMaxIdleMarkWorkers(int32(procs) - int32(dedicatedMarkWorkersNeeded)) 449 } 450 451 // Compute initial values for controls that are updated 452 // throughout the cycle. 453 c.dedicatedMarkWorkersNeeded.Store(dedicatedMarkWorkersNeeded) 454 c.revise() 455 456 if debug.gcpacertrace > 0 { 457 heapGoal := c.heapGoal() 458 assistRatio := c.assistWorkPerByte.Load() 459 print("pacer: assist ratio=", assistRatio, 460 " (scan ", gcController.heapScan.Load()>>20, " MB in ", 461 work.initialHeapLive>>20, "->", 462 heapGoal>>20, " MB)", 463 " workers=", dedicatedMarkWorkersNeeded, 464 "+", c.fractionalUtilizationGoal, "\n") 465 } 466 } 467 468 // revise updates the assist ratio during the GC cycle to account for 469 // improved estimates. This should be called whenever gcController.heapScan, 470 // gcController.heapLive, or if any inputs to gcController.heapGoal are 471 // updated. It is safe to call concurrently, but it may race with other 472 // calls to revise. 473 // 474 // The result of this race is that the two assist ratio values may not line 475 // up or may be stale. In practice this is OK because the assist ratio 476 // moves slowly throughout a GC cycle, and the assist ratio is a best-effort 477 // heuristic anyway. Furthermore, no part of the heuristic depends on 478 // the two assist ratio values being exact reciprocals of one another, since 479 // the two values are used to convert values from different sources. 480 // 481 // The worst case result of this raciness is that we may miss a larger shift 482 // in the ratio (say, if we decide to pace more aggressively against the 483 // hard heap goal) but even this "hard goal" is best-effort (see #40460). 484 // The dedicated GC should ensure we don't exceed the hard goal by too much 485 // in the rare case we do exceed it. 486 // 487 // It should only be called when gcBlackenEnabled != 0 (because this 488 // is when assists are enabled and the necessary statistics are 489 // available). 490 func (c *gcControllerState) revise() { 491 gcPercent := c.gcPercent.Load() 492 if gcPercent < 0 { 493 // If GC is disabled but we're running a forced GC, 494 // act like GOGC is huge for the below calculations. 495 gcPercent = 100000 496 } 497 live := c.heapLive.Load() 498 scan := c.heapScan.Load() 499 work := c.heapScanWork.Load() + c.stackScanWork.Load() + c.globalsScanWork.Load() 500 501 // Assume we're under the soft goal. Pace GC to complete at 502 // heapGoal assuming the heap is in steady-state. 503 heapGoal := int64(c.heapGoal()) 504 505 // The expected scan work is computed as the amount of bytes scanned last 506 // GC cycle (both heap and stack), plus our estimate of globals work for this cycle. 507 scanWorkExpected := int64(c.lastHeapScan + c.lastStackScan.Load() + c.globalsScan.Load()) 508 509 // maxScanWork is a worst-case estimate of the amount of scan work that 510 // needs to be performed in this GC cycle. Specifically, it represents 511 // the case where *all* scannable memory turns out to be live, and 512 // *all* allocated stack space is scannable. 513 maxStackScan := c.maxStackScan.Load() 514 maxScanWork := int64(scan + maxStackScan + c.globalsScan.Load()) 515 if work > scanWorkExpected { 516 // We've already done more scan work than expected. Because our expectation 517 // is based on a steady-state scannable heap size, we assume this means our 518 // heap is growing. Compute a new heap goal that takes our existing runway 519 // computed for scanWorkExpected and extrapolates it to maxScanWork, the worst-case 520 // scan work. This keeps our assist ratio stable if the heap continues to grow. 521 // 522 // The effect of this mechanism is that assists stay flat in the face of heap 523 // growths. It's OK to use more memory this cycle to scan all the live heap, 524 // because the next GC cycle is inevitably going to use *at least* that much 525 // memory anyway. 526 extHeapGoal := int64(float64(heapGoal-int64(c.triggered))/float64(scanWorkExpected)*float64(maxScanWork)) + int64(c.triggered) 527 scanWorkExpected = maxScanWork 528 529 // hardGoal is a hard limit on the amount that we're willing to push back the 530 // heap goal, and that's twice the heap goal (i.e. if GOGC=100 and the heap and/or 531 // stacks and/or globals grow to twice their size, this limits the current GC cycle's 532 // growth to 4x the original live heap's size). 533 // 534 // This maintains the invariant that we use no more memory than the next GC cycle 535 // will anyway. 536 hardGoal := int64((1.0 + float64(gcPercent)/100.0) * float64(heapGoal)) 537 if extHeapGoal > hardGoal { 538 extHeapGoal = hardGoal 539 } 540 heapGoal = extHeapGoal 541 } 542 if int64(live) > heapGoal { 543 // We're already past our heap goal, even the extrapolated one. 544 // Leave ourselves some extra runway, so in the worst case we 545 // finish by that point. 546 const maxOvershoot = 1.1 547 heapGoal = int64(float64(heapGoal) * maxOvershoot) 548 549 // Compute the upper bound on the scan work remaining. 550 scanWorkExpected = maxScanWork 551 } 552 553 // Compute the remaining scan work estimate. 554 // 555 // Note that we currently count allocations during GC as both 556 // scannable heap (heapScan) and scan work completed 557 // (scanWork), so allocation will change this difference 558 // slowly in the soft regime and not at all in the hard 559 // regime. 560 scanWorkRemaining := scanWorkExpected - work 561 if scanWorkRemaining < 1000 { 562 // We set a somewhat arbitrary lower bound on 563 // remaining scan work since if we aim a little high, 564 // we can miss by a little. 565 // 566 // We *do* need to enforce that this is at least 1, 567 // since marking is racy and double-scanning objects 568 // may legitimately make the remaining scan work 569 // negative, even in the hard goal regime. 570 scanWorkRemaining = 1000 571 } 572 573 // Compute the heap distance remaining. 574 heapRemaining := heapGoal - int64(live) 575 if heapRemaining <= 0 { 576 // This shouldn't happen, but if it does, avoid 577 // dividing by zero or setting the assist negative. 578 heapRemaining = 1 579 } 580 581 // Compute the mutator assist ratio so by the time the mutator 582 // allocates the remaining heap bytes up to heapGoal, it will 583 // have done (or stolen) the remaining amount of scan work. 584 // Note that the assist ratio values are updated atomically 585 // but not together. This means there may be some degree of 586 // skew between the two values. This is generally OK as the 587 // values shift relatively slowly over the course of a GC 588 // cycle. 589 assistWorkPerByte := float64(scanWorkRemaining) / float64(heapRemaining) 590 assistBytesPerWork := float64(heapRemaining) / float64(scanWorkRemaining) 591 c.assistWorkPerByte.Store(assistWorkPerByte) 592 c.assistBytesPerWork.Store(assistBytesPerWork) 593 } 594 595 // endCycle computes the consMark estimate for the next cycle. 596 // userForced indicates whether the current GC cycle was forced 597 // by the application. 598 func (c *gcControllerState) endCycle(now int64, procs int, userForced bool) { 599 // Record last heap goal for the scavenger. 600 // We'll be updating the heap goal soon. 601 gcController.lastHeapGoal = c.heapGoal() 602 603 // Compute the duration of time for which assists were turned on. 604 assistDuration := now - c.markStartTime 605 606 // Assume background mark hit its utilization goal. 607 utilization := gcBackgroundUtilization 608 // Add assist utilization; avoid divide by zero. 609 if assistDuration > 0 { 610 utilization += float64(c.assistTime.Load()) / float64(assistDuration*int64(procs)) 611 } 612 613 if c.heapLive.Load() <= c.triggered { 614 // Shouldn't happen, but let's be very safe about this in case the 615 // GC is somehow extremely short. 616 // 617 // In this case though, the only reasonable value for c.heapLive-c.triggered 618 // would be 0, which isn't really all that useful, i.e. the GC was so short 619 // that it didn't matter. 620 // 621 // Ignore this case and don't update anything. 622 return 623 } 624 idleUtilization := 0.0 625 if assistDuration > 0 { 626 idleUtilization = float64(c.idleMarkTime.Load()) / float64(assistDuration*int64(procs)) 627 } 628 // Determine the cons/mark ratio. 629 // 630 // The units we want for the numerator and denominator are both B / cpu-ns. 631 // We get this by taking the bytes allocated or scanned, and divide by the amount of 632 // CPU time it took for those operations. For allocations, that CPU time is 633 // 634 // assistDuration * procs * (1 - utilization) 635 // 636 // Where utilization includes just background GC workers and assists. It does *not* 637 // include idle GC work time, because in theory the mutator is free to take that at 638 // any point. 639 // 640 // For scanning, that CPU time is 641 // 642 // assistDuration * procs * (utilization + idleUtilization) 643 // 644 // In this case, we *include* idle utilization, because that is additional CPU time that 645 // the GC had available to it. 646 // 647 // In effect, idle GC time is sort of double-counted here, but it's very weird compared 648 // to other kinds of GC work, because of how fluid it is. Namely, because the mutator is 649 // *always* free to take it. 650 // 651 // So this calculation is really: 652 // (heapLive-trigger) / (assistDuration * procs * (1-utilization)) / 653 // (scanWork) / (assistDuration * procs * (utilization+idleUtilization)) 654 // 655 // Note that because we only care about the ratio, assistDuration and procs cancel out. 656 scanWork := c.heapScanWork.Load() + c.stackScanWork.Load() + c.globalsScanWork.Load() 657 currentConsMark := (float64(c.heapLive.Load()-c.triggered) * (utilization + idleUtilization)) / 658 (float64(scanWork) * (1 - utilization)) 659 660 // Update our cons/mark estimate. This is the maximum of the value we just computed and the last 661 // 4 cons/mark values we measured. The reason we take the maximum here is to bias a noisy 662 // cons/mark measurement toward fewer assists at the expense of additional GC cycles (starting 663 // earlier). 664 oldConsMark := c.consMark 665 c.consMark = currentConsMark 666 for i := range c.lastConsMark { 667 if c.lastConsMark[i] > c.consMark { 668 c.consMark = c.lastConsMark[i] 669 } 670 } 671 copy(c.lastConsMark[:], c.lastConsMark[1:]) 672 c.lastConsMark[len(c.lastConsMark)-1] = currentConsMark 673 674 if debug.gcpacertrace > 0 { 675 printlock() 676 goal := gcGoalUtilization * 100 677 print("pacer: ", int(utilization*100), "% CPU (", int(goal), " exp.) for ") 678 print(c.heapScanWork.Load(), "+", c.stackScanWork.Load(), "+", c.globalsScanWork.Load(), " B work (", c.lastHeapScan+c.lastStackScan.Load()+c.globalsScan.Load(), " B exp.) ") 679 live := c.heapLive.Load() 680 print("in ", c.triggered, " B -> ", live, " B (∆goal ", int64(live)-int64(c.lastHeapGoal), ", cons/mark ", oldConsMark, ")") 681 println() 682 printunlock() 683 } 684 } 685 686 // enlistWorker encourages another dedicated mark worker to start on 687 // another P if there are spare worker slots. It is used by putfull 688 // when more work is made available. 689 // 690 //go:nowritebarrier 691 func (c *gcControllerState) enlistWorker() { 692 // If there are idle Ps, wake one so it will run an idle worker. 693 // NOTE: This is suspected of causing deadlocks. See golang.org/issue/19112. 694 // 695 // if sched.npidle.Load() != 0 && sched.nmspinning.Load() == 0 { 696 // wakep() 697 // return 698 // } 699 700 // There are no idle Ps. If we need more dedicated workers, 701 // try to preempt a running P so it will switch to a worker. 702 if c.dedicatedMarkWorkersNeeded.Load() <= 0 { 703 return 704 } 705 // Pick a random other P to preempt. 706 if gomaxprocs <= 1 { 707 return 708 } 709 gp := getg() 710 if gp == nil || gp.m == nil || gp.m.p == 0 { 711 return 712 } 713 myID := gp.m.p.ptr().id 714 for tries := 0; tries < 5; tries++ { 715 id := int32(cheaprandn(uint32(gomaxprocs - 1))) 716 if id >= myID { 717 id++ 718 } 719 p := allp[id] 720 if p.status != _Prunning { 721 continue 722 } 723 if preemptone(p) { 724 return 725 } 726 } 727 } 728 729 // findRunnableGCWorker returns a background mark worker for pp if it 730 // should be run. This must only be called when gcBlackenEnabled != 0. 731 func (c *gcControllerState) findRunnableGCWorker(pp *p, now int64) (*g, int64) { 732 if gcBlackenEnabled == 0 { 733 throw("gcControllerState.findRunnable: blackening not enabled") 734 } 735 736 // Since we have the current time, check if the GC CPU limiter 737 // hasn't had an update in a while. This check is necessary in 738 // case the limiter is on but hasn't been checked in a while and 739 // so may have left sufficient headroom to turn off again. 740 if now == 0 { 741 now = nanotime() 742 } 743 if gcCPULimiter.needUpdate(now) { 744 gcCPULimiter.update(now) 745 } 746 747 if !gcMarkWorkAvailable(pp) { 748 // No work to be done right now. This can happen at 749 // the end of the mark phase when there are still 750 // assists tapering off. Don't bother running a worker 751 // now because it'll just return immediately. 752 return nil, now 753 } 754 755 if c.dedicatedMarkWorkersNeeded.Load() <= 0 && c.fractionalUtilizationGoal == 0 { 756 // No current need for dedicated workers, and no need at all for 757 // fractional workers. Check before trying to acquire a worker; when 758 // GOMAXPROCS is large, that can be expensive and is often unnecessary. 759 // 760 // When a dedicated worker stops running, the gcBgMarkWorker loop notes 761 // the need for the worker before returning it to the pool. If we don't 762 // see the need now, we wouldn't have found it in the pool anyway. 763 return nil, now 764 } 765 766 // Grab a worker before we commit to running below. 767 node := (*gcBgMarkWorkerNode)(gcBgMarkWorkerPool.pop()) 768 if node == nil { 769 // There is at least one worker per P, so normally there are 770 // enough workers to run on all Ps, if necessary. However, once 771 // a worker enters gcMarkDone it may park without rejoining the 772 // pool, thus freeing a P with no corresponding worker. 773 // gcMarkDone never depends on another worker doing work, so it 774 // is safe to simply do nothing here. 775 // 776 // If gcMarkDone bails out without completing the mark phase, 777 // it will always do so with queued global work. Thus, that P 778 // will be immediately eligible to re-run the worker G it was 779 // just using, ensuring work can complete. 780 return nil, now 781 } 782 783 decIfPositive := func(val *atomic.Int64) bool { 784 for { 785 v := val.Load() 786 if v <= 0 { 787 return false 788 } 789 790 if val.CompareAndSwap(v, v-1) { 791 return true 792 } 793 } 794 } 795 796 if decIfPositive(&c.dedicatedMarkWorkersNeeded) { 797 // This P is now dedicated to marking until the end of 798 // the concurrent mark phase. 799 pp.gcMarkWorkerMode = gcMarkWorkerDedicatedMode 800 } else if c.fractionalUtilizationGoal == 0 { 801 // No need for fractional workers. 802 gcBgMarkWorkerPool.push(&node.node) 803 return nil, now 804 } else { 805 // Is this P behind on the fractional utilization 806 // goal? 807 // 808 // This should be kept in sync with pollFractionalWorkerExit. 809 delta := now - c.markStartTime 810 if delta > 0 && float64(pp.gcFractionalMarkTime)/float64(delta) > c.fractionalUtilizationGoal { 811 // Nope. No need to run a fractional worker. 812 gcBgMarkWorkerPool.push(&node.node) 813 return nil, now 814 } 815 // Run a fractional worker. 816 pp.gcMarkWorkerMode = gcMarkWorkerFractionalMode 817 } 818 819 // Run the background mark worker. 820 gp := node.gp.ptr() 821 trace := traceAcquire() 822 casgstatus(gp, _Gwaiting, _Grunnable) 823 if trace.ok() { 824 trace.GoUnpark(gp, 0) 825 traceRelease(trace) 826 } 827 return gp, now 828 } 829 830 // resetLive sets up the controller state for the next mark phase after the end 831 // of the previous one. Must be called after endCycle and before commit, before 832 // the world is started. 833 // 834 // The world must be stopped. 835 func (c *gcControllerState) resetLive(bytesMarked uint64) { 836 c.heapMarked = bytesMarked 837 c.heapLive.Store(bytesMarked) 838 c.heapScan.Store(uint64(c.heapScanWork.Load())) 839 c.lastHeapScan = uint64(c.heapScanWork.Load()) 840 c.lastStackScan.Store(uint64(c.stackScanWork.Load())) 841 c.triggered = ^uint64(0) // Reset triggered. 842 843 // heapLive was updated, so emit a trace event. 844 trace := traceAcquire() 845 if trace.ok() { 846 trace.HeapAlloc(bytesMarked) 847 traceRelease(trace) 848 } 849 } 850 851 // markWorkerStop must be called whenever a mark worker stops executing. 852 // 853 // It updates mark work accounting in the controller by a duration of 854 // work in nanoseconds and other bookkeeping. 855 // 856 // Safe to execute at any time. 857 func (c *gcControllerState) markWorkerStop(mode gcMarkWorkerMode, duration int64) { 858 switch mode { 859 case gcMarkWorkerDedicatedMode: 860 c.dedicatedMarkTime.Add(duration) 861 c.dedicatedMarkWorkersNeeded.Add(1) 862 case gcMarkWorkerFractionalMode: 863 c.fractionalMarkTime.Add(duration) 864 case gcMarkWorkerIdleMode: 865 c.idleMarkTime.Add(duration) 866 c.removeIdleMarkWorker() 867 default: 868 throw("markWorkerStop: unknown mark worker mode") 869 } 870 } 871 872 func (c *gcControllerState) update(dHeapLive, dHeapScan int64) { 873 if dHeapLive != 0 { 874 trace := traceAcquire() 875 live := gcController.heapLive.Add(dHeapLive) 876 if trace.ok() { 877 // gcController.heapLive changed. 878 trace.HeapAlloc(live) 879 traceRelease(trace) 880 } 881 } 882 if gcBlackenEnabled == 0 { 883 // Update heapScan when we're not in a current GC. It is fixed 884 // at the beginning of a cycle. 885 if dHeapScan != 0 { 886 gcController.heapScan.Add(dHeapScan) 887 } 888 } else { 889 // gcController.heapLive changed. 890 c.revise() 891 } 892 } 893 894 func (c *gcControllerState) addScannableStack(pp *p, amount int64) { 895 if pp == nil { 896 c.maxStackScan.Add(amount) 897 return 898 } 899 pp.maxStackScanDelta += amount 900 if pp.maxStackScanDelta >= maxStackScanSlack || pp.maxStackScanDelta <= -maxStackScanSlack { 901 c.maxStackScan.Add(pp.maxStackScanDelta) 902 pp.maxStackScanDelta = 0 903 } 904 } 905 906 func (c *gcControllerState) addGlobals(amount int64) { 907 c.globalsScan.Add(amount) 908 } 909 910 // heapGoal returns the current heap goal. 911 func (c *gcControllerState) heapGoal() uint64 { 912 goal, _ := c.heapGoalInternal() 913 return goal 914 } 915 916 // heapGoalInternal is the implementation of heapGoal which returns additional 917 // information that is necessary for computing the trigger. 918 // 919 // The returned minTrigger is always <= goal. 920 func (c *gcControllerState) heapGoalInternal() (goal, minTrigger uint64) { 921 // Start with the goal calculated for gcPercent. 922 goal = c.gcPercentHeapGoal.Load() 923 924 // Check if the memory-limit-based goal is smaller, and if so, pick that. 925 if newGoal := c.memoryLimitHeapGoal(); newGoal < goal { 926 goal = newGoal 927 } else { 928 // We're not limited by the memory limit goal, so perform a series of 929 // adjustments that might move the goal forward in a variety of circumstances. 930 931 sweepDistTrigger := c.sweepDistMinTrigger.Load() 932 if sweepDistTrigger > goal { 933 // Set the goal to maintain a minimum sweep distance since 934 // the last call to commit. Note that we never want to do this 935 // if we're in the memory limit regime, because it could push 936 // the goal up. 937 goal = sweepDistTrigger 938 } 939 // Since we ignore the sweep distance trigger in the memory 940 // limit regime, we need to ensure we don't propagate it to 941 // the trigger, because it could cause a violation of the 942 // invariant that the trigger < goal. 943 minTrigger = sweepDistTrigger 944 945 // Ensure that the heap goal is at least a little larger than 946 // the point at which we triggered. This may not be the case if GC 947 // start is delayed or if the allocation that pushed gcController.heapLive 948 // over trigger is large or if the trigger is really close to 949 // GOGC. Assist is proportional to this distance, so enforce a 950 // minimum distance, even if it means going over the GOGC goal 951 // by a tiny bit. 952 // 953 // Ignore this if we're in the memory limit regime: we'd prefer to 954 // have the GC respond hard about how close we are to the goal than to 955 // push the goal back in such a manner that it could cause us to exceed 956 // the memory limit. 957 const minRunway = 64 << 10 958 if c.triggered != ^uint64(0) && goal < c.triggered+minRunway { 959 goal = c.triggered + minRunway 960 } 961 } 962 return 963 } 964 965 // memoryLimitHeapGoal returns a heap goal derived from memoryLimit. 966 func (c *gcControllerState) memoryLimitHeapGoal() uint64 { 967 // Start by pulling out some values we'll need. Be careful about overflow. 968 var heapFree, heapAlloc, mappedReady uint64 969 for { 970 heapFree = c.heapFree.load() // Free and unscavenged memory. 971 heapAlloc = c.totalAlloc.Load() - c.totalFree.Load() // Heap object bytes in use. 972 mappedReady = c.mappedReady.Load() // Total unreleased mapped memory. 973 if heapFree+heapAlloc <= mappedReady { 974 break 975 } 976 // It is impossible for total unreleased mapped memory to exceed heap memory, but 977 // because these stats are updated independently, we may observe a partial update 978 // including only some values. Thus, we appear to break the invariant. However, 979 // this condition is necessarily transient, so just try again. In the case of a 980 // persistent accounting error, we'll deadlock here. 981 } 982 983 // Below we compute a goal from memoryLimit. There are a few things to be aware of. 984 // Firstly, the memoryLimit does not easily compare to the heap goal: the former 985 // is total mapped memory by the runtime that hasn't been released, while the latter is 986 // only heap object memory. Intuitively, the way we convert from one to the other is to 987 // subtract everything from memoryLimit that both contributes to the memory limit (so, 988 // ignore scavenged memory) and doesn't contain heap objects. This isn't quite what 989 // lines up with reality, but it's a good starting point. 990 // 991 // In practice this computation looks like the following: 992 // 993 // goal := memoryLimit - ((mappedReady - heapFree - heapAlloc) + max(mappedReady - memoryLimit, 0)) 994 // ^1 ^2 995 // goal -= goal / 100 * memoryLimitHeapGoalHeadroomPercent 996 // ^3 997 // 998 // Let's break this down. 999 // 1000 // The first term (marker 1) is everything that contributes to the memory limit and isn't 1001 // or couldn't become heap objects. It represents, broadly speaking, non-heap overheads. 1002 // One oddity you may have noticed is that we also subtract out heapFree, i.e. unscavenged 1003 // memory that may contain heap objects in the future. 1004 // 1005 // Let's take a step back. In an ideal world, this term would look something like just 1006 // the heap goal. That is, we "reserve" enough space for the heap to grow to the heap 1007 // goal, and subtract out everything else. This is of course impossible; the definition 1008 // is circular! However, this impossible definition contains a key insight: the amount 1009 // we're *going* to use matters just as much as whatever we're currently using. 1010 // 1011 // Consider if the heap shrinks to 1/10th its size, leaving behind lots of free and 1012 // unscavenged memory. mappedReady - heapAlloc will be quite large, because of that free 1013 // and unscavenged memory, pushing the goal down significantly. 1014 // 1015 // heapFree is also safe to exclude from the memory limit because in the steady-state, it's 1016 // just a pool of memory for future heap allocations, and making new allocations from heapFree 1017 // memory doesn't increase overall memory use. In transient states, the scavenger and the 1018 // allocator actively manage the pool of heapFree memory to maintain the memory limit. 1019 // 1020 // The second term (marker 2) is the amount of memory we've exceeded the limit by, and is 1021 // intended to help recover from such a situation. By pushing the heap goal down, we also 1022 // push the trigger down, triggering and finishing a GC sooner in order to make room for 1023 // other memory sources. Note that since we're effectively reducing the heap goal by X bytes, 1024 // we're actually giving more than X bytes of headroom back, because the heap goal is in 1025 // terms of heap objects, but it takes more than X bytes (e.g. due to fragmentation) to store 1026 // X bytes worth of objects. 1027 // 1028 // The final adjustment (marker 3) reduces the maximum possible memory limit heap goal by 1029 // memoryLimitHeapGoalPercent. As the name implies, this is to provide additional headroom in 1030 // the face of pacing inaccuracies, and also to leave a buffer of unscavenged memory so the 1031 // allocator isn't constantly scavenging. The reduction amount also has a fixed minimum 1032 // (memoryLimitMinHeapGoalHeadroom, not pictured) because the aforementioned pacing inaccuracies 1033 // disproportionately affect small heaps: as heaps get smaller, the pacer's inputs get fuzzier. 1034 // Shorter GC cycles and less GC work means noisy external factors like the OS scheduler have a 1035 // greater impact. 1036 1037 memoryLimit := uint64(c.memoryLimit.Load()) 1038 1039 // Compute term 1. 1040 nonHeapMemory := mappedReady - heapFree - heapAlloc 1041 1042 // Compute term 2. 1043 var overage uint64 1044 if mappedReady > memoryLimit { 1045 overage = mappedReady - memoryLimit 1046 } 1047 1048 if nonHeapMemory+overage >= memoryLimit { 1049 // We're at a point where non-heap memory exceeds the memory limit on its own. 1050 // There's honestly not much we can do here but just trigger GCs continuously 1051 // and let the CPU limiter reign that in. Something has to give at this point. 1052 // Set it to heapMarked, the lowest possible goal. 1053 return c.heapMarked 1054 } 1055 1056 // Compute the goal. 1057 goal := memoryLimit - (nonHeapMemory + overage) 1058 1059 // Apply some headroom to the goal to account for pacing inaccuracies and to reduce 1060 // the impact of scavenging at allocation time in response to a high allocation rate 1061 // when GOGC=off. See issue #57069. Also, be careful about small limits. 1062 headroom := goal / 100 * memoryLimitHeapGoalHeadroomPercent 1063 if headroom < memoryLimitMinHeapGoalHeadroom { 1064 // Set a fixed minimum to deal with the particularly large effect pacing inaccuracies 1065 // have for smaller heaps. 1066 headroom = memoryLimitMinHeapGoalHeadroom 1067 } 1068 if goal < headroom || goal-headroom < headroom { 1069 goal = headroom 1070 } else { 1071 goal = goal - headroom 1072 } 1073 // Don't let us go below the live heap. A heap goal below the live heap doesn't make sense. 1074 if goal < c.heapMarked { 1075 goal = c.heapMarked 1076 } 1077 return goal 1078 } 1079 1080 const ( 1081 // These constants determine the bounds on the GC trigger as a fraction 1082 // of heap bytes allocated between the start of a GC (heapLive == heapMarked) 1083 // and the end of a GC (heapLive == heapGoal). 1084 // 1085 // The constants are obscured in this way for efficiency. The denominator 1086 // of the fraction is always a power-of-two for a quick division, so that 1087 // the numerator is a single constant integer multiplication. 1088 triggerRatioDen = 64 1089 1090 // The minimum trigger constant was chosen empirically: given a sufficiently 1091 // fast/scalable allocator with 48 Ps that could drive the trigger ratio 1092 // to <0.05, this constant causes applications to retain the same peak 1093 // RSS compared to not having this allocator. 1094 minTriggerRatioNum = 45 // ~0.7 1095 1096 // The maximum trigger constant is chosen somewhat arbitrarily, but the 1097 // current constant has served us well over the years. 1098 maxTriggerRatioNum = 61 // ~0.95 1099 ) 1100 1101 // trigger returns the current point at which a GC should trigger along with 1102 // the heap goal. 1103 // 1104 // The returned value may be compared against heapLive to determine whether 1105 // the GC should trigger. Thus, the GC trigger condition should be (but may 1106 // not be, in the case of small movements for efficiency) checked whenever 1107 // the heap goal may change. 1108 func (c *gcControllerState) trigger() (uint64, uint64) { 1109 goal, minTrigger := c.heapGoalInternal() 1110 1111 // Invariant: the trigger must always be less than the heap goal. 1112 // 1113 // Note that the memory limit sets a hard maximum on our heap goal, 1114 // but the live heap may grow beyond it. 1115 1116 if c.heapMarked >= goal { 1117 // The goal should never be smaller than heapMarked, but let's be 1118 // defensive about it. The only reasonable trigger here is one that 1119 // causes a continuous GC cycle at heapMarked, but respect the goal 1120 // if it came out as smaller than that. 1121 return goal, goal 1122 } 1123 1124 // Below this point, c.heapMarked < goal. 1125 1126 // heapMarked is our absolute minimum, and it's possible the trigger 1127 // bound we get from heapGoalinternal is less than that. 1128 if minTrigger < c.heapMarked { 1129 minTrigger = c.heapMarked 1130 } 1131 1132 // If we let the trigger go too low, then if the application 1133 // is allocating very rapidly we might end up in a situation 1134 // where we're allocating black during a nearly always-on GC. 1135 // The result of this is a growing heap and ultimately an 1136 // increase in RSS. By capping us at a point >0, we're essentially 1137 // saying that we're OK using more CPU during the GC to prevent 1138 // this growth in RSS. 1139 triggerLowerBound := ((goal-c.heapMarked)/triggerRatioDen)*minTriggerRatioNum + c.heapMarked 1140 if minTrigger < triggerLowerBound { 1141 minTrigger = triggerLowerBound 1142 } 1143 1144 // For small heaps, set the max trigger point at maxTriggerRatio of the way 1145 // from the live heap to the heap goal. This ensures we always have *some* 1146 // headroom when the GC actually starts. For larger heaps, set the max trigger 1147 // point at the goal, minus the minimum heap size. 1148 // 1149 // This choice follows from the fact that the minimum heap size is chosen 1150 // to reflect the costs of a GC with no work to do. With a large heap but 1151 // very little scan work to perform, this gives us exactly as much runway 1152 // as we would need, in the worst case. 1153 maxTrigger := ((goal-c.heapMarked)/triggerRatioDen)*maxTriggerRatioNum + c.heapMarked 1154 if goal > defaultHeapMinimum && goal-defaultHeapMinimum > maxTrigger { 1155 maxTrigger = goal - defaultHeapMinimum 1156 } 1157 maxTrigger = max(maxTrigger, minTrigger) 1158 1159 // Compute the trigger from our bounds and the runway stored by commit. 1160 var trigger uint64 1161 runway := c.runway.Load() 1162 if runway > goal { 1163 trigger = minTrigger 1164 } else { 1165 trigger = goal - runway 1166 } 1167 trigger = max(trigger, minTrigger) 1168 trigger = min(trigger, maxTrigger) 1169 if trigger > goal { 1170 print("trigger=", trigger, " heapGoal=", goal, "\n") 1171 print("minTrigger=", minTrigger, " maxTrigger=", maxTrigger, "\n") 1172 throw("produced a trigger greater than the heap goal") 1173 } 1174 return trigger, goal 1175 } 1176 1177 // commit recomputes all pacing parameters needed to derive the 1178 // trigger and the heap goal. Namely, the gcPercent-based heap goal, 1179 // and the amount of runway we want to give the GC this cycle. 1180 // 1181 // This can be called any time. If GC is the in the middle of a 1182 // concurrent phase, it will adjust the pacing of that phase. 1183 // 1184 // isSweepDone should be the result of calling isSweepDone(), 1185 // unless we're testing or we know we're executing during a GC cycle. 1186 // 1187 // This depends on gcPercent, gcController.heapMarked, and 1188 // gcController.heapLive. These must be up to date. 1189 // 1190 // Callers must call gcControllerState.revise after calling this 1191 // function if the GC is enabled. 1192 // 1193 // mheap_.lock must be held or the world must be stopped. 1194 func (c *gcControllerState) commit(isSweepDone bool) { 1195 if !c.test { 1196 assertWorldStoppedOrLockHeld(&mheap_.lock) 1197 } 1198 1199 if isSweepDone { 1200 // The sweep is done, so there aren't any restrictions on the trigger 1201 // we need to think about. 1202 c.sweepDistMinTrigger.Store(0) 1203 } else { 1204 // Concurrent sweep happens in the heap growth 1205 // from gcController.heapLive to trigger. Make sure we 1206 // give the sweeper some runway if it doesn't have enough. 1207 c.sweepDistMinTrigger.Store(c.heapLive.Load() + sweepMinHeapDistance) 1208 } 1209 1210 // Compute the next GC goal, which is when the allocated heap 1211 // has grown by GOGC/100 over where it started the last cycle, 1212 // plus additional runway for non-heap sources of GC work. 1213 gcPercentHeapGoal := ^uint64(0) 1214 if gcPercent := c.gcPercent.Load(); gcPercent >= 0 { 1215 gcPercentHeapGoal = c.heapMarked + (c.heapMarked+c.lastStackScan.Load()+c.globalsScan.Load())*uint64(gcPercent)/100 1216 } 1217 // Apply the minimum heap size here. It's defined in terms of gcPercent 1218 // and is only updated by functions that call commit. 1219 if gcPercentHeapGoal < c.heapMinimum { 1220 gcPercentHeapGoal = c.heapMinimum 1221 } 1222 c.gcPercentHeapGoal.Store(gcPercentHeapGoal) 1223 1224 // Compute the amount of runway we want the GC to have by using our 1225 // estimate of the cons/mark ratio. 1226 // 1227 // The idea is to take our expected scan work, and multiply it by 1228 // the cons/mark ratio to determine how long it'll take to complete 1229 // that scan work in terms of bytes allocated. This gives us our GC's 1230 // runway. 1231 // 1232 // However, the cons/mark ratio is a ratio of rates per CPU-second, but 1233 // here we care about the relative rates for some division of CPU 1234 // resources among the mutator and the GC. 1235 // 1236 // To summarize, we have B / cpu-ns, and we want B / ns. We get that 1237 // by multiplying by our desired division of CPU resources. We choose 1238 // to express CPU resources as GOMAPROCS*fraction. Note that because 1239 // we're working with a ratio here, we can omit the number of CPU cores, 1240 // because they'll appear in the numerator and denominator and cancel out. 1241 // As a result, this is basically just "weighing" the cons/mark ratio by 1242 // our desired division of resources. 1243 // 1244 // Furthermore, by setting the runway so that CPU resources are divided 1245 // this way, assuming that the cons/mark ratio is correct, we make that 1246 // division a reality. 1247 c.runway.Store(uint64((c.consMark * (1 - gcGoalUtilization) / (gcGoalUtilization)) * float64(c.lastHeapScan+c.lastStackScan.Load()+c.globalsScan.Load()))) 1248 } 1249 1250 // setGCPercent updates gcPercent. commit must be called after. 1251 // Returns the old value of gcPercent. 1252 // 1253 // The world must be stopped, or mheap_.lock must be held. 1254 func (c *gcControllerState) setGCPercent(in int32) int32 { 1255 if !c.test { 1256 assertWorldStoppedOrLockHeld(&mheap_.lock) 1257 } 1258 1259 out := c.gcPercent.Load() 1260 if in < 0 { 1261 in = -1 1262 } 1263 c.heapMinimum = defaultHeapMinimum * uint64(in) / 100 1264 c.gcPercent.Store(in) 1265 1266 return out 1267 } 1268 1269 //go:linkname setGCPercent runtime/debug.setGCPercent 1270 func setGCPercent(in int32) (out int32) { 1271 // Run on the system stack since we grab the heap lock. 1272 systemstack(func() { 1273 lock(&mheap_.lock) 1274 out = gcController.setGCPercent(in) 1275 gcControllerCommit() 1276 unlock(&mheap_.lock) 1277 }) 1278 1279 // If we just disabled GC, wait for any concurrent GC mark to 1280 // finish so we always return with no GC running. 1281 if in < 0 { 1282 gcWaitOnMark(work.cycles.Load()) 1283 } 1284 1285 return out 1286 } 1287 1288 func readGOGC() int32 { 1289 p := gogetenv("GOGC") 1290 if p == "off" { 1291 return -1 1292 } 1293 if n, ok := atoi32(p); ok { 1294 return n 1295 } 1296 return 100 1297 } 1298 1299 // setMemoryLimit updates memoryLimit. commit must be called after 1300 // Returns the old value of memoryLimit. 1301 // 1302 // The world must be stopped, or mheap_.lock must be held. 1303 func (c *gcControllerState) setMemoryLimit(in int64) int64 { 1304 if !c.test { 1305 assertWorldStoppedOrLockHeld(&mheap_.lock) 1306 } 1307 1308 out := c.memoryLimit.Load() 1309 if in >= 0 { 1310 c.memoryLimit.Store(in) 1311 } 1312 1313 return out 1314 } 1315 1316 //go:linkname setMemoryLimit runtime/debug.setMemoryLimit 1317 func setMemoryLimit(in int64) (out int64) { 1318 // Run on the system stack since we grab the heap lock. 1319 systemstack(func() { 1320 lock(&mheap_.lock) 1321 out = gcController.setMemoryLimit(in) 1322 if in < 0 || out == in { 1323 // If we're just checking the value or not changing 1324 // it, there's no point in doing the rest. 1325 unlock(&mheap_.lock) 1326 return 1327 } 1328 gcControllerCommit() 1329 unlock(&mheap_.lock) 1330 }) 1331 return out 1332 } 1333 1334 func readGOMEMLIMIT() int64 { 1335 p := gogetenv("GOMEMLIMIT") 1336 if p == "" || p == "off" { 1337 return maxInt64 1338 } 1339 n, ok := parseByteCount(p) 1340 if !ok { 1341 print("GOMEMLIMIT=", p, "\n") 1342 throw("malformed GOMEMLIMIT; see `go doc runtime/debug.SetMemoryLimit`") 1343 } 1344 return n 1345 } 1346 1347 // addIdleMarkWorker attempts to add a new idle mark worker. 1348 // 1349 // If this returns true, the caller must become an idle mark worker unless 1350 // there's no background mark worker goroutines in the pool. This case is 1351 // harmless because there are already background mark workers running. 1352 // If this returns false, the caller must NOT become an idle mark worker. 1353 // 1354 // nosplit because it may be called without a P. 1355 // 1356 //go:nosplit 1357 func (c *gcControllerState) addIdleMarkWorker() bool { 1358 for { 1359 old := c.idleMarkWorkers.Load() 1360 n, max := int32(old&uint64(^uint32(0))), int32(old>>32) 1361 if n >= max { 1362 // See the comment on idleMarkWorkers for why 1363 // n > max is tolerated. 1364 return false 1365 } 1366 if n < 0 { 1367 print("n=", n, " max=", max, "\n") 1368 throw("negative idle mark workers") 1369 } 1370 new := uint64(uint32(n+1)) | (uint64(max) << 32) 1371 if c.idleMarkWorkers.CompareAndSwap(old, new) { 1372 return true 1373 } 1374 } 1375 } 1376 1377 // needIdleMarkWorker is a hint as to whether another idle mark worker is needed. 1378 // 1379 // The caller must still call addIdleMarkWorker to become one. This is mainly 1380 // useful for a quick check before an expensive operation. 1381 // 1382 // nosplit because it may be called without a P. 1383 // 1384 //go:nosplit 1385 func (c *gcControllerState) needIdleMarkWorker() bool { 1386 p := c.idleMarkWorkers.Load() 1387 n, max := int32(p&uint64(^uint32(0))), int32(p>>32) 1388 return n < max 1389 } 1390 1391 // removeIdleMarkWorker must be called when a new idle mark worker stops executing. 1392 func (c *gcControllerState) removeIdleMarkWorker() { 1393 for { 1394 old := c.idleMarkWorkers.Load() 1395 n, max := int32(old&uint64(^uint32(0))), int32(old>>32) 1396 if n-1 < 0 { 1397 print("n=", n, " max=", max, "\n") 1398 throw("negative idle mark workers") 1399 } 1400 new := uint64(uint32(n-1)) | (uint64(max) << 32) 1401 if c.idleMarkWorkers.CompareAndSwap(old, new) { 1402 return 1403 } 1404 } 1405 } 1406 1407 // setMaxIdleMarkWorkers sets the maximum number of idle mark workers allowed. 1408 // 1409 // This method is optimistic in that it does not wait for the number of 1410 // idle mark workers to reduce to max before returning; it assumes the workers 1411 // will deschedule themselves. 1412 func (c *gcControllerState) setMaxIdleMarkWorkers(max int32) { 1413 for { 1414 old := c.idleMarkWorkers.Load() 1415 n := int32(old & uint64(^uint32(0))) 1416 if n < 0 { 1417 print("n=", n, " max=", max, "\n") 1418 throw("negative idle mark workers") 1419 } 1420 new := uint64(uint32(n)) | (uint64(max) << 32) 1421 if c.idleMarkWorkers.CompareAndSwap(old, new) { 1422 return 1423 } 1424 } 1425 } 1426 1427 // gcControllerCommit is gcController.commit, but passes arguments from live 1428 // (non-test) data. It also updates any consumers of the GC pacing, such as 1429 // sweep pacing and the background scavenger. 1430 // 1431 // Calls gcController.commit. 1432 // 1433 // The heap lock must be held, so this must be executed on the system stack. 1434 // 1435 //go:systemstack 1436 func gcControllerCommit() { 1437 assertWorldStoppedOrLockHeld(&mheap_.lock) 1438 1439 gcController.commit(isSweepDone()) 1440 1441 // Update mark pacing. 1442 if gcphase != _GCoff { 1443 gcController.revise() 1444 } 1445 1446 // TODO(mknyszek): This isn't really accurate any longer because the heap 1447 // goal is computed dynamically. Still useful to snapshot, but not as useful. 1448 trace := traceAcquire() 1449 if trace.ok() { 1450 trace.HeapGoal() 1451 traceRelease(trace) 1452 } 1453 1454 trigger, heapGoal := gcController.trigger() 1455 gcPaceSweeper(trigger) 1456 gcPaceScavenger(gcController.memoryLimit.Load(), heapGoal, gcController.lastHeapGoal) 1457 } 1458