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 // Grab a worker before we commit to running below. 756 node := (*gcBgMarkWorkerNode)(gcBgMarkWorkerPool.pop()) 757 if node == nil { 758 // There is at least one worker per P, so normally there are 759 // enough workers to run on all Ps, if necessary. However, once 760 // a worker enters gcMarkDone it may park without rejoining the 761 // pool, thus freeing a P with no corresponding worker. 762 // gcMarkDone never depends on another worker doing work, so it 763 // is safe to simply do nothing here. 764 // 765 // If gcMarkDone bails out without completing the mark phase, 766 // it will always do so with queued global work. Thus, that P 767 // will be immediately eligible to re-run the worker G it was 768 // just using, ensuring work can complete. 769 return nil, now 770 } 771 772 decIfPositive := func(val *atomic.Int64) bool { 773 for { 774 v := val.Load() 775 if v <= 0 { 776 return false 777 } 778 779 if val.CompareAndSwap(v, v-1) { 780 return true 781 } 782 } 783 } 784 785 if decIfPositive(&c.dedicatedMarkWorkersNeeded) { 786 // This P is now dedicated to marking until the end of 787 // the concurrent mark phase. 788 pp.gcMarkWorkerMode = gcMarkWorkerDedicatedMode 789 } else if c.fractionalUtilizationGoal == 0 { 790 // No need for fractional workers. 791 gcBgMarkWorkerPool.push(&node.node) 792 return nil, now 793 } else { 794 // Is this P behind on the fractional utilization 795 // goal? 796 // 797 // This should be kept in sync with pollFractionalWorkerExit. 798 delta := now - c.markStartTime 799 if delta > 0 && float64(pp.gcFractionalMarkTime)/float64(delta) > c.fractionalUtilizationGoal { 800 // Nope. No need to run a fractional worker. 801 gcBgMarkWorkerPool.push(&node.node) 802 return nil, now 803 } 804 // Run a fractional worker. 805 pp.gcMarkWorkerMode = gcMarkWorkerFractionalMode 806 } 807 808 // Run the background mark worker. 809 gp := node.gp.ptr() 810 trace := traceAcquire() 811 casgstatus(gp, _Gwaiting, _Grunnable) 812 if trace.ok() { 813 trace.GoUnpark(gp, 0) 814 traceRelease(trace) 815 } 816 return gp, now 817 } 818 819 // resetLive sets up the controller state for the next mark phase after the end 820 // of the previous one. Must be called after endCycle and before commit, before 821 // the world is started. 822 // 823 // The world must be stopped. 824 func (c *gcControllerState) resetLive(bytesMarked uint64) { 825 c.heapMarked = bytesMarked 826 c.heapLive.Store(bytesMarked) 827 c.heapScan.Store(uint64(c.heapScanWork.Load())) 828 c.lastHeapScan = uint64(c.heapScanWork.Load()) 829 c.lastStackScan.Store(uint64(c.stackScanWork.Load())) 830 c.triggered = ^uint64(0) // Reset triggered. 831 832 // heapLive was updated, so emit a trace event. 833 trace := traceAcquire() 834 if trace.ok() { 835 trace.HeapAlloc(bytesMarked) 836 traceRelease(trace) 837 } 838 } 839 840 // markWorkerStop must be called whenever a mark worker stops executing. 841 // 842 // It updates mark work accounting in the controller by a duration of 843 // work in nanoseconds and other bookkeeping. 844 // 845 // Safe to execute at any time. 846 func (c *gcControllerState) markWorkerStop(mode gcMarkWorkerMode, duration int64) { 847 switch mode { 848 case gcMarkWorkerDedicatedMode: 849 c.dedicatedMarkTime.Add(duration) 850 c.dedicatedMarkWorkersNeeded.Add(1) 851 case gcMarkWorkerFractionalMode: 852 c.fractionalMarkTime.Add(duration) 853 case gcMarkWorkerIdleMode: 854 c.idleMarkTime.Add(duration) 855 c.removeIdleMarkWorker() 856 default: 857 throw("markWorkerStop: unknown mark worker mode") 858 } 859 } 860 861 func (c *gcControllerState) update(dHeapLive, dHeapScan int64) { 862 if dHeapLive != 0 { 863 trace := traceAcquire() 864 live := gcController.heapLive.Add(dHeapLive) 865 if trace.ok() { 866 // gcController.heapLive changed. 867 trace.HeapAlloc(live) 868 traceRelease(trace) 869 } 870 } 871 if gcBlackenEnabled == 0 { 872 // Update heapScan when we're not in a current GC. It is fixed 873 // at the beginning of a cycle. 874 if dHeapScan != 0 { 875 gcController.heapScan.Add(dHeapScan) 876 } 877 } else { 878 // gcController.heapLive changed. 879 c.revise() 880 } 881 } 882 883 func (c *gcControllerState) addScannableStack(pp *p, amount int64) { 884 if pp == nil { 885 c.maxStackScan.Add(amount) 886 return 887 } 888 pp.maxStackScanDelta += amount 889 if pp.maxStackScanDelta >= maxStackScanSlack || pp.maxStackScanDelta <= -maxStackScanSlack { 890 c.maxStackScan.Add(pp.maxStackScanDelta) 891 pp.maxStackScanDelta = 0 892 } 893 } 894 895 func (c *gcControllerState) addGlobals(amount int64) { 896 c.globalsScan.Add(amount) 897 } 898 899 // heapGoal returns the current heap goal. 900 func (c *gcControllerState) heapGoal() uint64 { 901 goal, _ := c.heapGoalInternal() 902 return goal 903 } 904 905 // heapGoalInternal is the implementation of heapGoal which returns additional 906 // information that is necessary for computing the trigger. 907 // 908 // The returned minTrigger is always <= goal. 909 func (c *gcControllerState) heapGoalInternal() (goal, minTrigger uint64) { 910 // Start with the goal calculated for gcPercent. 911 goal = c.gcPercentHeapGoal.Load() 912 913 // Check if the memory-limit-based goal is smaller, and if so, pick that. 914 if newGoal := c.memoryLimitHeapGoal(); newGoal < goal { 915 goal = newGoal 916 } else { 917 // We're not limited by the memory limit goal, so perform a series of 918 // adjustments that might move the goal forward in a variety of circumstances. 919 920 sweepDistTrigger := c.sweepDistMinTrigger.Load() 921 if sweepDistTrigger > goal { 922 // Set the goal to maintain a minimum sweep distance since 923 // the last call to commit. Note that we never want to do this 924 // if we're in the memory limit regime, because it could push 925 // the goal up. 926 goal = sweepDistTrigger 927 } 928 // Since we ignore the sweep distance trigger in the memory 929 // limit regime, we need to ensure we don't propagate it to 930 // the trigger, because it could cause a violation of the 931 // invariant that the trigger < goal. 932 minTrigger = sweepDistTrigger 933 934 // Ensure that the heap goal is at least a little larger than 935 // the point at which we triggered. This may not be the case if GC 936 // start is delayed or if the allocation that pushed gcController.heapLive 937 // over trigger is large or if the trigger is really close to 938 // GOGC. Assist is proportional to this distance, so enforce a 939 // minimum distance, even if it means going over the GOGC goal 940 // by a tiny bit. 941 // 942 // Ignore this if we're in the memory limit regime: we'd prefer to 943 // have the GC respond hard about how close we are to the goal than to 944 // push the goal back in such a manner that it could cause us to exceed 945 // the memory limit. 946 const minRunway = 64 << 10 947 if c.triggered != ^uint64(0) && goal < c.triggered+minRunway { 948 goal = c.triggered + minRunway 949 } 950 } 951 return 952 } 953 954 // memoryLimitHeapGoal returns a heap goal derived from memoryLimit. 955 func (c *gcControllerState) memoryLimitHeapGoal() uint64 { 956 // Start by pulling out some values we'll need. Be careful about overflow. 957 var heapFree, heapAlloc, mappedReady uint64 958 for { 959 heapFree = c.heapFree.load() // Free and unscavenged memory. 960 heapAlloc = c.totalAlloc.Load() - c.totalFree.Load() // Heap object bytes in use. 961 mappedReady = c.mappedReady.Load() // Total unreleased mapped memory. 962 if heapFree+heapAlloc <= mappedReady { 963 break 964 } 965 // It is impossible for total unreleased mapped memory to exceed heap memory, but 966 // because these stats are updated independently, we may observe a partial update 967 // including only some values. Thus, we appear to break the invariant. However, 968 // this condition is necessarily transient, so just try again. In the case of a 969 // persistent accounting error, we'll deadlock here. 970 } 971 972 // Below we compute a goal from memoryLimit. There are a few things to be aware of. 973 // Firstly, the memoryLimit does not easily compare to the heap goal: the former 974 // is total mapped memory by the runtime that hasn't been released, while the latter is 975 // only heap object memory. Intuitively, the way we convert from one to the other is to 976 // subtract everything from memoryLimit that both contributes to the memory limit (so, 977 // ignore scavenged memory) and doesn't contain heap objects. This isn't quite what 978 // lines up with reality, but it's a good starting point. 979 // 980 // In practice this computation looks like the following: 981 // 982 // goal := memoryLimit - ((mappedReady - heapFree - heapAlloc) + max(mappedReady - memoryLimit, 0)) 983 // ^1 ^2 984 // goal -= goal / 100 * memoryLimitHeapGoalHeadroomPercent 985 // ^3 986 // 987 // Let's break this down. 988 // 989 // The first term (marker 1) is everything that contributes to the memory limit and isn't 990 // or couldn't become heap objects. It represents, broadly speaking, non-heap overheads. 991 // One oddity you may have noticed is that we also subtract out heapFree, i.e. unscavenged 992 // memory that may contain heap objects in the future. 993 // 994 // Let's take a step back. In an ideal world, this term would look something like just 995 // the heap goal. That is, we "reserve" enough space for the heap to grow to the heap 996 // goal, and subtract out everything else. This is of course impossible; the definition 997 // is circular! However, this impossible definition contains a key insight: the amount 998 // we're *going* to use matters just as much as whatever we're currently using. 999 // 1000 // Consider if the heap shrinks to 1/10th its size, leaving behind lots of free and 1001 // unscavenged memory. mappedReady - heapAlloc will be quite large, because of that free 1002 // and unscavenged memory, pushing the goal down significantly. 1003 // 1004 // heapFree is also safe to exclude from the memory limit because in the steady-state, it's 1005 // just a pool of memory for future heap allocations, and making new allocations from heapFree 1006 // memory doesn't increase overall memory use. In transient states, the scavenger and the 1007 // allocator actively manage the pool of heapFree memory to maintain the memory limit. 1008 // 1009 // The second term (marker 2) is the amount of memory we've exceeded the limit by, and is 1010 // intended to help recover from such a situation. By pushing the heap goal down, we also 1011 // push the trigger down, triggering and finishing a GC sooner in order to make room for 1012 // other memory sources. Note that since we're effectively reducing the heap goal by X bytes, 1013 // we're actually giving more than X bytes of headroom back, because the heap goal is in 1014 // terms of heap objects, but it takes more than X bytes (e.g. due to fragmentation) to store 1015 // X bytes worth of objects. 1016 // 1017 // The final adjustment (marker 3) reduces the maximum possible memory limit heap goal by 1018 // memoryLimitHeapGoalPercent. As the name implies, this is to provide additional headroom in 1019 // the face of pacing inaccuracies, and also to leave a buffer of unscavenged memory so the 1020 // allocator isn't constantly scavenging. The reduction amount also has a fixed minimum 1021 // (memoryLimitMinHeapGoalHeadroom, not pictured) because the aforementioned pacing inaccuracies 1022 // disproportionately affect small heaps: as heaps get smaller, the pacer's inputs get fuzzier. 1023 // Shorter GC cycles and less GC work means noisy external factors like the OS scheduler have a 1024 // greater impact. 1025 1026 memoryLimit := uint64(c.memoryLimit.Load()) 1027 1028 // Compute term 1. 1029 nonHeapMemory := mappedReady - heapFree - heapAlloc 1030 1031 // Compute term 2. 1032 var overage uint64 1033 if mappedReady > memoryLimit { 1034 overage = mappedReady - memoryLimit 1035 } 1036 1037 if nonHeapMemory+overage >= memoryLimit { 1038 // We're at a point where non-heap memory exceeds the memory limit on its own. 1039 // There's honestly not much we can do here but just trigger GCs continuously 1040 // and let the CPU limiter reign that in. Something has to give at this point. 1041 // Set it to heapMarked, the lowest possible goal. 1042 return c.heapMarked 1043 } 1044 1045 // Compute the goal. 1046 goal := memoryLimit - (nonHeapMemory + overage) 1047 1048 // Apply some headroom to the goal to account for pacing inaccuracies and to reduce 1049 // the impact of scavenging at allocation time in response to a high allocation rate 1050 // when GOGC=off. See issue #57069. Also, be careful about small limits. 1051 headroom := goal / 100 * memoryLimitHeapGoalHeadroomPercent 1052 if headroom < memoryLimitMinHeapGoalHeadroom { 1053 // Set a fixed minimum to deal with the particularly large effect pacing inaccuracies 1054 // have for smaller heaps. 1055 headroom = memoryLimitMinHeapGoalHeadroom 1056 } 1057 if goal < headroom || goal-headroom < headroom { 1058 goal = headroom 1059 } else { 1060 goal = goal - headroom 1061 } 1062 // Don't let us go below the live heap. A heap goal below the live heap doesn't make sense. 1063 if goal < c.heapMarked { 1064 goal = c.heapMarked 1065 } 1066 return goal 1067 } 1068 1069 const ( 1070 // These constants determine the bounds on the GC trigger as a fraction 1071 // of heap bytes allocated between the start of a GC (heapLive == heapMarked) 1072 // and the end of a GC (heapLive == heapGoal). 1073 // 1074 // The constants are obscured in this way for efficiency. The denominator 1075 // of the fraction is always a power-of-two for a quick division, so that 1076 // the numerator is a single constant integer multiplication. 1077 triggerRatioDen = 64 1078 1079 // The minimum trigger constant was chosen empirically: given a sufficiently 1080 // fast/scalable allocator with 48 Ps that could drive the trigger ratio 1081 // to <0.05, this constant causes applications to retain the same peak 1082 // RSS compared to not having this allocator. 1083 minTriggerRatioNum = 45 // ~0.7 1084 1085 // The maximum trigger constant is chosen somewhat arbitrarily, but the 1086 // current constant has served us well over the years. 1087 maxTriggerRatioNum = 61 // ~0.95 1088 ) 1089 1090 // trigger returns the current point at which a GC should trigger along with 1091 // the heap goal. 1092 // 1093 // The returned value may be compared against heapLive to determine whether 1094 // the GC should trigger. Thus, the GC trigger condition should be (but may 1095 // not be, in the case of small movements for efficiency) checked whenever 1096 // the heap goal may change. 1097 func (c *gcControllerState) trigger() (uint64, uint64) { 1098 goal, minTrigger := c.heapGoalInternal() 1099 1100 // Invariant: the trigger must always be less than the heap goal. 1101 // 1102 // Note that the memory limit sets a hard maximum on our heap goal, 1103 // but the live heap may grow beyond it. 1104 1105 if c.heapMarked >= goal { 1106 // The goal should never be smaller than heapMarked, but let's be 1107 // defensive about it. The only reasonable trigger here is one that 1108 // causes a continuous GC cycle at heapMarked, but respect the goal 1109 // if it came out as smaller than that. 1110 return goal, goal 1111 } 1112 1113 // Below this point, c.heapMarked < goal. 1114 1115 // heapMarked is our absolute minimum, and it's possible the trigger 1116 // bound we get from heapGoalinternal is less than that. 1117 if minTrigger < c.heapMarked { 1118 minTrigger = c.heapMarked 1119 } 1120 1121 // If we let the trigger go too low, then if the application 1122 // is allocating very rapidly we might end up in a situation 1123 // where we're allocating black during a nearly always-on GC. 1124 // The result of this is a growing heap and ultimately an 1125 // increase in RSS. By capping us at a point >0, we're essentially 1126 // saying that we're OK using more CPU during the GC to prevent 1127 // this growth in RSS. 1128 triggerLowerBound := ((goal-c.heapMarked)/triggerRatioDen)*minTriggerRatioNum + c.heapMarked 1129 if minTrigger < triggerLowerBound { 1130 minTrigger = triggerLowerBound 1131 } 1132 1133 // For small heaps, set the max trigger point at maxTriggerRatio of the way 1134 // from the live heap to the heap goal. This ensures we always have *some* 1135 // headroom when the GC actually starts. For larger heaps, set the max trigger 1136 // point at the goal, minus the minimum heap size. 1137 // 1138 // This choice follows from the fact that the minimum heap size is chosen 1139 // to reflect the costs of a GC with no work to do. With a large heap but 1140 // very little scan work to perform, this gives us exactly as much runway 1141 // as we would need, in the worst case. 1142 maxTrigger := ((goal-c.heapMarked)/triggerRatioDen)*maxTriggerRatioNum + c.heapMarked 1143 if goal > defaultHeapMinimum && goal-defaultHeapMinimum > maxTrigger { 1144 maxTrigger = goal - defaultHeapMinimum 1145 } 1146 maxTrigger = max(maxTrigger, minTrigger) 1147 1148 // Compute the trigger from our bounds and the runway stored by commit. 1149 var trigger uint64 1150 runway := c.runway.Load() 1151 if runway > goal { 1152 trigger = minTrigger 1153 } else { 1154 trigger = goal - runway 1155 } 1156 trigger = max(trigger, minTrigger) 1157 trigger = min(trigger, maxTrigger) 1158 if trigger > goal { 1159 print("trigger=", trigger, " heapGoal=", goal, "\n") 1160 print("minTrigger=", minTrigger, " maxTrigger=", maxTrigger, "\n") 1161 throw("produced a trigger greater than the heap goal") 1162 } 1163 return trigger, goal 1164 } 1165 1166 // commit recomputes all pacing parameters needed to derive the 1167 // trigger and the heap goal. Namely, the gcPercent-based heap goal, 1168 // and the amount of runway we want to give the GC this cycle. 1169 // 1170 // This can be called any time. If GC is the in the middle of a 1171 // concurrent phase, it will adjust the pacing of that phase. 1172 // 1173 // isSweepDone should be the result of calling isSweepDone(), 1174 // unless we're testing or we know we're executing during a GC cycle. 1175 // 1176 // This depends on gcPercent, gcController.heapMarked, and 1177 // gcController.heapLive. These must be up to date. 1178 // 1179 // Callers must call gcControllerState.revise after calling this 1180 // function if the GC is enabled. 1181 // 1182 // mheap_.lock must be held or the world must be stopped. 1183 func (c *gcControllerState) commit(isSweepDone bool) { 1184 if !c.test { 1185 assertWorldStoppedOrLockHeld(&mheap_.lock) 1186 } 1187 1188 if isSweepDone { 1189 // The sweep is done, so there aren't any restrictions on the trigger 1190 // we need to think about. 1191 c.sweepDistMinTrigger.Store(0) 1192 } else { 1193 // Concurrent sweep happens in the heap growth 1194 // from gcController.heapLive to trigger. Make sure we 1195 // give the sweeper some runway if it doesn't have enough. 1196 c.sweepDistMinTrigger.Store(c.heapLive.Load() + sweepMinHeapDistance) 1197 } 1198 1199 // Compute the next GC goal, which is when the allocated heap 1200 // has grown by GOGC/100 over where it started the last cycle, 1201 // plus additional runway for non-heap sources of GC work. 1202 gcPercentHeapGoal := ^uint64(0) 1203 if gcPercent := c.gcPercent.Load(); gcPercent >= 0 { 1204 gcPercentHeapGoal = c.heapMarked + (c.heapMarked+c.lastStackScan.Load()+c.globalsScan.Load())*uint64(gcPercent)/100 1205 } 1206 // Apply the minimum heap size here. It's defined in terms of gcPercent 1207 // and is only updated by functions that call commit. 1208 if gcPercentHeapGoal < c.heapMinimum { 1209 gcPercentHeapGoal = c.heapMinimum 1210 } 1211 c.gcPercentHeapGoal.Store(gcPercentHeapGoal) 1212 1213 // Compute the amount of runway we want the GC to have by using our 1214 // estimate of the cons/mark ratio. 1215 // 1216 // The idea is to take our expected scan work, and multiply it by 1217 // the cons/mark ratio to determine how long it'll take to complete 1218 // that scan work in terms of bytes allocated. This gives us our GC's 1219 // runway. 1220 // 1221 // However, the cons/mark ratio is a ratio of rates per CPU-second, but 1222 // here we care about the relative rates for some division of CPU 1223 // resources among the mutator and the GC. 1224 // 1225 // To summarize, we have B / cpu-ns, and we want B / ns. We get that 1226 // by multiplying by our desired division of CPU resources. We choose 1227 // to express CPU resources as GOMAPROCS*fraction. Note that because 1228 // we're working with a ratio here, we can omit the number of CPU cores, 1229 // because they'll appear in the numerator and denominator and cancel out. 1230 // As a result, this is basically just "weighing" the cons/mark ratio by 1231 // our desired division of resources. 1232 // 1233 // Furthermore, by setting the runway so that CPU resources are divided 1234 // this way, assuming that the cons/mark ratio is correct, we make that 1235 // division a reality. 1236 c.runway.Store(uint64((c.consMark * (1 - gcGoalUtilization) / (gcGoalUtilization)) * float64(c.lastHeapScan+c.lastStackScan.Load()+c.globalsScan.Load()))) 1237 } 1238 1239 // setGCPercent updates gcPercent. commit must be called after. 1240 // Returns the old value of gcPercent. 1241 // 1242 // The world must be stopped, or mheap_.lock must be held. 1243 func (c *gcControllerState) setGCPercent(in int32) int32 { 1244 if !c.test { 1245 assertWorldStoppedOrLockHeld(&mheap_.lock) 1246 } 1247 1248 out := c.gcPercent.Load() 1249 if in < 0 { 1250 in = -1 1251 } 1252 c.heapMinimum = defaultHeapMinimum * uint64(in) / 100 1253 c.gcPercent.Store(in) 1254 1255 return out 1256 } 1257 1258 //go:linkname setGCPercent runtime/debug.setGCPercent 1259 func setGCPercent(in int32) (out int32) { 1260 // Run on the system stack since we grab the heap lock. 1261 systemstack(func() { 1262 lock(&mheap_.lock) 1263 out = gcController.setGCPercent(in) 1264 gcControllerCommit() 1265 unlock(&mheap_.lock) 1266 }) 1267 1268 // If we just disabled GC, wait for any concurrent GC mark to 1269 // finish so we always return with no GC running. 1270 if in < 0 { 1271 gcWaitOnMark(work.cycles.Load()) 1272 } 1273 1274 return out 1275 } 1276 1277 func readGOGC() int32 { 1278 p := gogetenv("GOGC") 1279 if p == "off" { 1280 return -1 1281 } 1282 if n, ok := atoi32(p); ok { 1283 return n 1284 } 1285 return 100 1286 } 1287 1288 // setMemoryLimit updates memoryLimit. commit must be called after 1289 // Returns the old value of memoryLimit. 1290 // 1291 // The world must be stopped, or mheap_.lock must be held. 1292 func (c *gcControllerState) setMemoryLimit(in int64) int64 { 1293 if !c.test { 1294 assertWorldStoppedOrLockHeld(&mheap_.lock) 1295 } 1296 1297 out := c.memoryLimit.Load() 1298 if in >= 0 { 1299 c.memoryLimit.Store(in) 1300 } 1301 1302 return out 1303 } 1304 1305 //go:linkname setMemoryLimit runtime/debug.setMemoryLimit 1306 func setMemoryLimit(in int64) (out int64) { 1307 // Run on the system stack since we grab the heap lock. 1308 systemstack(func() { 1309 lock(&mheap_.lock) 1310 out = gcController.setMemoryLimit(in) 1311 if in < 0 || out == in { 1312 // If we're just checking the value or not changing 1313 // it, there's no point in doing the rest. 1314 unlock(&mheap_.lock) 1315 return 1316 } 1317 gcControllerCommit() 1318 unlock(&mheap_.lock) 1319 }) 1320 return out 1321 } 1322 1323 func readGOMEMLIMIT() int64 { 1324 p := gogetenv("GOMEMLIMIT") 1325 if p == "" || p == "off" { 1326 return maxInt64 1327 } 1328 n, ok := parseByteCount(p) 1329 if !ok { 1330 print("GOMEMLIMIT=", p, "\n") 1331 throw("malformed GOMEMLIMIT; see `go doc runtime/debug.SetMemoryLimit`") 1332 } 1333 return n 1334 } 1335 1336 // addIdleMarkWorker attempts to add a new idle mark worker. 1337 // 1338 // If this returns true, the caller must become an idle mark worker unless 1339 // there's no background mark worker goroutines in the pool. This case is 1340 // harmless because there are already background mark workers running. 1341 // If this returns false, the caller must NOT become an idle mark worker. 1342 // 1343 // nosplit because it may be called without a P. 1344 // 1345 //go:nosplit 1346 func (c *gcControllerState) addIdleMarkWorker() bool { 1347 for { 1348 old := c.idleMarkWorkers.Load() 1349 n, max := int32(old&uint64(^uint32(0))), int32(old>>32) 1350 if n >= max { 1351 // See the comment on idleMarkWorkers for why 1352 // n > max is tolerated. 1353 return false 1354 } 1355 if n < 0 { 1356 print("n=", n, " max=", max, "\n") 1357 throw("negative idle mark workers") 1358 } 1359 new := uint64(uint32(n+1)) | (uint64(max) << 32) 1360 if c.idleMarkWorkers.CompareAndSwap(old, new) { 1361 return true 1362 } 1363 } 1364 } 1365 1366 // needIdleMarkWorker is a hint as to whether another idle mark worker is needed. 1367 // 1368 // The caller must still call addIdleMarkWorker to become one. This is mainly 1369 // useful for a quick check before an expensive operation. 1370 // 1371 // nosplit because it may be called without a P. 1372 // 1373 //go:nosplit 1374 func (c *gcControllerState) needIdleMarkWorker() bool { 1375 p := c.idleMarkWorkers.Load() 1376 n, max := int32(p&uint64(^uint32(0))), int32(p>>32) 1377 return n < max 1378 } 1379 1380 // removeIdleMarkWorker must be called when a new idle mark worker stops executing. 1381 func (c *gcControllerState) removeIdleMarkWorker() { 1382 for { 1383 old := c.idleMarkWorkers.Load() 1384 n, max := int32(old&uint64(^uint32(0))), int32(old>>32) 1385 if n-1 < 0 { 1386 print("n=", n, " max=", max, "\n") 1387 throw("negative idle mark workers") 1388 } 1389 new := uint64(uint32(n-1)) | (uint64(max) << 32) 1390 if c.idleMarkWorkers.CompareAndSwap(old, new) { 1391 return 1392 } 1393 } 1394 } 1395 1396 // setMaxIdleMarkWorkers sets the maximum number of idle mark workers allowed. 1397 // 1398 // This method is optimistic in that it does not wait for the number of 1399 // idle mark workers to reduce to max before returning; it assumes the workers 1400 // will deschedule themselves. 1401 func (c *gcControllerState) setMaxIdleMarkWorkers(max int32) { 1402 for { 1403 old := c.idleMarkWorkers.Load() 1404 n := int32(old & uint64(^uint32(0))) 1405 if n < 0 { 1406 print("n=", n, " max=", max, "\n") 1407 throw("negative idle mark workers") 1408 } 1409 new := uint64(uint32(n)) | (uint64(max) << 32) 1410 if c.idleMarkWorkers.CompareAndSwap(old, new) { 1411 return 1412 } 1413 } 1414 } 1415 1416 // gcControllerCommit is gcController.commit, but passes arguments from live 1417 // (non-test) data. It also updates any consumers of the GC pacing, such as 1418 // sweep pacing and the background scavenger. 1419 // 1420 // Calls gcController.commit. 1421 // 1422 // The heap lock must be held, so this must be executed on the system stack. 1423 // 1424 //go:systemstack 1425 func gcControllerCommit() { 1426 assertWorldStoppedOrLockHeld(&mheap_.lock) 1427 1428 gcController.commit(isSweepDone()) 1429 1430 // Update mark pacing. 1431 if gcphase != _GCoff { 1432 gcController.revise() 1433 } 1434 1435 // TODO(mknyszek): This isn't really accurate any longer because the heap 1436 // goal is computed dynamically. Still useful to snapshot, but not as useful. 1437 trace := traceAcquire() 1438 if trace.ok() { 1439 trace.HeapGoal() 1440 traceRelease(trace) 1441 } 1442 1443 trigger, heapGoal := gcController.trigger() 1444 gcPaceSweeper(trigger) 1445 gcPaceScavenger(gcController.memoryLimit.Load(), heapGoal, gcController.lastHeapGoal) 1446 } 1447