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 "runtime/internal/atomic" 10 "unsafe" 11 ) 12 13 const ( 14 // gcGoalUtilization is the goal CPU utilization for 15 // marking as a fraction of GOMAXPROCS. 16 gcGoalUtilization = 0.30 17 18 // gcBackgroundUtilization is the fixed CPU utilization for background 19 // marking. It must be <= gcGoalUtilization. The difference between 20 // gcGoalUtilization and gcBackgroundUtilization will be made up by 21 // mark assists. The scheduler will aim to use within 50% of this 22 // goal. 23 // 24 // Setting this to < gcGoalUtilization avoids saturating the trigger 25 // feedback controller when there are no assists, which allows it to 26 // better control CPU and heap growth. However, the larger the gap, 27 // the more mutator assists are expected to happen, which impact 28 // mutator latency. 29 gcBackgroundUtilization = 0.25 30 31 // gcCreditSlack is the amount of scan work credit that can 32 // accumulate locally before updating gcController.scanWork and, 33 // optionally, gcController.bgScanCredit. Lower values give a more 34 // accurate assist ratio and make it more likely that assists will 35 // successfully steal background credit. Higher values reduce memory 36 // contention. 37 gcCreditSlack = 2000 38 39 // gcAssistTimeSlack is the nanoseconds of mutator assist time that 40 // can accumulate on a P before updating gcController.assistTime. 41 gcAssistTimeSlack = 5000 42 43 // gcOverAssistWork determines how many extra units of scan work a GC 44 // assist does when an assist happens. This amortizes the cost of an 45 // assist by pre-paying for this many bytes of future allocations. 46 gcOverAssistWork = 64 << 10 47 48 // defaultHeapMinimum is the value of heapMinimum for GOGC==100. 49 defaultHeapMinimum = 4 << 20 50 ) 51 52 func init() { 53 if offset := unsafe.Offsetof(gcController.heapLive); offset%8 != 0 { 54 println(offset) 55 throw("gcController.heapLive not aligned to 8 bytes") 56 } 57 } 58 59 // gcController implements the GC pacing controller that determines 60 // when to trigger concurrent garbage collection and how much marking 61 // work to do in mutator assists and background marking. 62 // 63 // It uses a feedback control algorithm to adjust the gcController.trigger 64 // trigger based on the heap growth and GC CPU utilization each cycle. 65 // This algorithm optimizes for heap growth to match GOGC and for CPU 66 // utilization between assist and background marking to be 25% of 67 // GOMAXPROCS. The high-level design of this algorithm is documented 68 // at https://golang.org/s/go15gcpacing. 69 // 70 // All fields of gcController are used only during a single mark 71 // cycle. 72 var gcController gcControllerState 73 74 type gcControllerState struct { 75 // Initialized from $GOGC. GOGC=off means no GC. 76 gcPercent int32 77 78 _ uint32 // padding so following 64-bit values are 8-byte aligned 79 80 // heapMinimum is the minimum heap size at which to trigger GC. 81 // For small heaps, this overrides the usual GOGC*live set rule. 82 // 83 // When there is a very small live set but a lot of allocation, simply 84 // collecting when the heap reaches GOGC*live results in many GC 85 // cycles and high total per-GC overhead. This minimum amortizes this 86 // per-GC overhead while keeping the heap reasonably small. 87 // 88 // During initialization this is set to 4MB*GOGC/100. In the case of 89 // GOGC==0, this will set heapMinimum to 0, resulting in constant 90 // collection even when the heap size is small, which is useful for 91 // debugging. 92 heapMinimum uint64 93 94 // triggerRatio is the heap growth ratio that triggers marking. 95 // 96 // E.g., if this is 0.6, then GC should start when the live 97 // heap has reached 1.6 times the heap size marked by the 98 // previous cycle. This should be ≤ GOGC/100 so the trigger 99 // heap size is less than the goal heap size. This is set 100 // during mark termination for the next cycle's trigger. 101 // 102 // Protected by mheap_.lock or a STW. 103 triggerRatio float64 104 105 // trigger is the heap size that triggers marking. 106 // 107 // When heapLive ≥ trigger, the mark phase will start. 108 // This is also the heap size by which proportional sweeping 109 // must be complete. 110 // 111 // This is computed from triggerRatio during mark termination 112 // for the next cycle's trigger. 113 // 114 // Protected by mheap_.lock or a STW. 115 trigger uint64 116 117 // heapGoal is the goal heapLive for when next GC ends. 118 // Set to ^uint64(0) if disabled. 119 // 120 // Read and written atomically, unless the world is stopped. 121 heapGoal uint64 122 123 // lastHeapGoal is the value of heapGoal for the previous GC. 124 // Note that this is distinct from the last value heapGoal had, 125 // because it could change if e.g. gcPercent changes. 126 // 127 // Read and written with the world stopped or with mheap_.lock held. 128 lastHeapGoal uint64 129 130 // heapLive is the number of bytes considered live by the GC. 131 // That is: retained by the most recent GC plus allocated 132 // since then. heapLive ≤ memstats.heapAlloc, since heapAlloc includes 133 // unmarked objects that have not yet been swept (and hence goes up as we 134 // allocate and down as we sweep) while heapLive excludes these 135 // objects (and hence only goes up between GCs). 136 // 137 // This is updated atomically without locking. To reduce 138 // contention, this is updated only when obtaining a span from 139 // an mcentral and at this point it counts all of the 140 // unallocated slots in that span (which will be allocated 141 // before that mcache obtains another span from that 142 // mcentral). Hence, it slightly overestimates the "true" live 143 // heap size. It's better to overestimate than to 144 // underestimate because 1) this triggers the GC earlier than 145 // necessary rather than potentially too late and 2) this 146 // leads to a conservative GC rate rather than a GC rate that 147 // is potentially too low. 148 // 149 // Reads should likewise be atomic (or during STW). 150 // 151 // Whenever this is updated, call traceHeapAlloc() and 152 // this gcControllerState's revise() method. 153 heapLive uint64 154 155 // heapScan is the number of bytes of "scannable" heap. This 156 // is the live heap (as counted by heapLive), but omitting 157 // no-scan objects and no-scan tails of objects. 158 // 159 // Whenever this is updated, call this gcControllerState's 160 // revise() method. 161 // 162 // Read and written atomically or with the world stopped. 163 heapScan uint64 164 165 // heapMarked is the number of bytes marked by the previous 166 // GC. After mark termination, heapLive == heapMarked, but 167 // unlike heapLive, heapMarked does not change until the 168 // next mark termination. 169 heapMarked uint64 170 171 // scanWork is the total scan work performed this cycle. This 172 // is updated atomically during the cycle. Updates occur in 173 // bounded batches, since it is both written and read 174 // throughout the cycle. At the end of the cycle, this is how 175 // much of the retained heap is scannable. 176 // 177 // Currently this is the bytes of heap scanned. For most uses, 178 // this is an opaque unit of work, but for estimation the 179 // definition is important. 180 scanWork int64 181 182 // bgScanCredit is the scan work credit accumulated by the 183 // concurrent background scan. This credit is accumulated by 184 // the background scan and stolen by mutator assists. This is 185 // updated atomically. Updates occur in bounded batches, since 186 // it is both written and read throughout the cycle. 187 bgScanCredit int64 188 189 // assistTime is the nanoseconds spent in mutator assists 190 // during this cycle. This is updated atomically. Updates 191 // occur in bounded batches, since it is both written and read 192 // throughout the cycle. 193 assistTime int64 194 195 // dedicatedMarkTime is the nanoseconds spent in dedicated 196 // mark workers during this cycle. This is updated atomically 197 // at the end of the concurrent mark phase. 198 dedicatedMarkTime int64 199 200 // fractionalMarkTime is the nanoseconds spent in the 201 // fractional mark worker during this cycle. This is updated 202 // atomically throughout the cycle and will be up-to-date if 203 // the fractional mark worker is not currently running. 204 fractionalMarkTime int64 205 206 // idleMarkTime is the nanoseconds spent in idle marking 207 // during this cycle. This is updated atomically throughout 208 // the cycle. 209 idleMarkTime int64 210 211 // markStartTime is the absolute start time in nanoseconds 212 // that assists and background mark workers started. 213 markStartTime int64 214 215 // dedicatedMarkWorkersNeeded is the number of dedicated mark 216 // workers that need to be started. This is computed at the 217 // beginning of each cycle and decremented atomically as 218 // dedicated mark workers get started. 219 dedicatedMarkWorkersNeeded int64 220 221 // assistWorkPerByte is the ratio of scan work to allocated 222 // bytes that should be performed by mutator assists. This is 223 // computed at the beginning of each cycle and updated every 224 // time heapScan is updated. 225 // 226 // Stored as a uint64, but it's actually a float64. Use 227 // float64frombits to get the value. 228 // 229 // Read and written atomically. 230 assistWorkPerByte uint64 231 232 // assistBytesPerWork is 1/assistWorkPerByte. 233 // 234 // Stored as a uint64, but it's actually a float64. Use 235 // float64frombits to get the value. 236 // 237 // Read and written atomically. 238 // 239 // Note that because this is read and written independently 240 // from assistWorkPerByte users may notice a skew between 241 // the two values, and such a state should be safe. 242 assistBytesPerWork uint64 243 244 // fractionalUtilizationGoal is the fraction of wall clock 245 // time that should be spent in the fractional mark worker on 246 // each P that isn't running a dedicated worker. 247 // 248 // For example, if the utilization goal is 25% and there are 249 // no dedicated workers, this will be 0.25. If the goal is 250 // 25%, there is one dedicated worker, and GOMAXPROCS is 5, 251 // this will be 0.05 to make up the missing 5%. 252 // 253 // If this is zero, no fractional workers are needed. 254 fractionalUtilizationGoal float64 255 256 _ cpu.CacheLinePad 257 } 258 259 func (c *gcControllerState) init(gcPercent int32) { 260 c.heapMinimum = defaultHeapMinimum 261 262 // Set a reasonable initial GC trigger. 263 c.triggerRatio = 7 / 8.0 264 265 // Fake a heapMarked value so it looks like a trigger at 266 // heapMinimum is the appropriate growth from heapMarked. 267 // This will go into computing the initial GC goal. 268 c.heapMarked = uint64(float64(c.heapMinimum) / (1 + c.triggerRatio)) 269 270 // This will also compute and set the GC trigger and goal. 271 c.setGCPercent(gcPercent) 272 } 273 274 // startCycle resets the GC controller's state and computes estimates 275 // for a new GC cycle. The caller must hold worldsema and the world 276 // must be stopped. 277 func (c *gcControllerState) startCycle() { 278 c.scanWork = 0 279 c.bgScanCredit = 0 280 c.assistTime = 0 281 c.dedicatedMarkTime = 0 282 c.fractionalMarkTime = 0 283 c.idleMarkTime = 0 284 285 // Ensure that the heap goal is at least a little larger than 286 // the current live heap size. This may not be the case if GC 287 // start is delayed or if the allocation that pushed gcController.heapLive 288 // over trigger is large or if the trigger is really close to 289 // GOGC. Assist is proportional to this distance, so enforce a 290 // minimum distance, even if it means going over the GOGC goal 291 // by a tiny bit. 292 if c.heapGoal < c.heapLive+1024*1024 { 293 c.heapGoal = c.heapLive + 1024*1024 294 } 295 296 // Compute the background mark utilization goal. In general, 297 // this may not come out exactly. We round the number of 298 // dedicated workers so that the utilization is closest to 299 // 25%. For small GOMAXPROCS, this would introduce too much 300 // error, so we add fractional workers in that case. 301 totalUtilizationGoal := float64(gomaxprocs) * gcBackgroundUtilization 302 c.dedicatedMarkWorkersNeeded = int64(totalUtilizationGoal + 0.5) 303 utilError := float64(c.dedicatedMarkWorkersNeeded)/totalUtilizationGoal - 1 304 const maxUtilError = 0.3 305 if utilError < -maxUtilError || utilError > maxUtilError { 306 // Rounding put us more than 30% off our goal. With 307 // gcBackgroundUtilization of 25%, this happens for 308 // GOMAXPROCS<=3 or GOMAXPROCS=6. Enable fractional 309 // workers to compensate. 310 if float64(c.dedicatedMarkWorkersNeeded) > totalUtilizationGoal { 311 // Too many dedicated workers. 312 c.dedicatedMarkWorkersNeeded-- 313 } 314 c.fractionalUtilizationGoal = (totalUtilizationGoal - float64(c.dedicatedMarkWorkersNeeded)) / float64(gomaxprocs) 315 } else { 316 c.fractionalUtilizationGoal = 0 317 } 318 319 // In STW mode, we just want dedicated workers. 320 if debug.gcstoptheworld > 0 { 321 c.dedicatedMarkWorkersNeeded = int64(gomaxprocs) 322 c.fractionalUtilizationGoal = 0 323 } 324 325 // Clear per-P state 326 for _, p := range allp { 327 p.gcAssistTime = 0 328 p.gcFractionalMarkTime = 0 329 } 330 331 // Compute initial values for controls that are updated 332 // throughout the cycle. 333 c.revise() 334 335 if debug.gcpacertrace > 0 { 336 assistRatio := float64frombits(atomic.Load64(&c.assistWorkPerByte)) 337 print("pacer: assist ratio=", assistRatio, 338 " (scan ", gcController.heapScan>>20, " MB in ", 339 work.initialHeapLive>>20, "->", 340 c.heapGoal>>20, " MB)", 341 " workers=", c.dedicatedMarkWorkersNeeded, 342 "+", c.fractionalUtilizationGoal, "\n") 343 } 344 } 345 346 // revise updates the assist ratio during the GC cycle to account for 347 // improved estimates. This should be called whenever gcController.heapScan, 348 // gcController.heapLive, or gcController.heapGoal is updated. It is safe to 349 // call concurrently, but it may race with other calls to revise. 350 // 351 // The result of this race is that the two assist ratio values may not line 352 // up or may be stale. In practice this is OK because the assist ratio 353 // moves slowly throughout a GC cycle, and the assist ratio is a best-effort 354 // heuristic anyway. Furthermore, no part of the heuristic depends on 355 // the two assist ratio values being exact reciprocals of one another, since 356 // the two values are used to convert values from different sources. 357 // 358 // The worst case result of this raciness is that we may miss a larger shift 359 // in the ratio (say, if we decide to pace more aggressively against the 360 // hard heap goal) but even this "hard goal" is best-effort (see #40460). 361 // The dedicated GC should ensure we don't exceed the hard goal by too much 362 // in the rare case we do exceed it. 363 // 364 // It should only be called when gcBlackenEnabled != 0 (because this 365 // is when assists are enabled and the necessary statistics are 366 // available). 367 func (c *gcControllerState) revise() { 368 gcPercent := c.gcPercent 369 if gcPercent < 0 { 370 // If GC is disabled but we're running a forced GC, 371 // act like GOGC is huge for the below calculations. 372 gcPercent = 100000 373 } 374 live := atomic.Load64(&c.heapLive) 375 scan := atomic.Load64(&c.heapScan) 376 work := atomic.Loadint64(&c.scanWork) 377 378 // Assume we're under the soft goal. Pace GC to complete at 379 // heapGoal assuming the heap is in steady-state. 380 heapGoal := int64(atomic.Load64(&c.heapGoal)) 381 382 // Compute the expected scan work remaining. 383 // 384 // This is estimated based on the expected 385 // steady-state scannable heap. For example, with 386 // GOGC=100, only half of the scannable heap is 387 // expected to be live, so that's what we target. 388 // 389 // (This is a float calculation to avoid overflowing on 390 // 100*heapScan.) 391 scanWorkExpected := int64(float64(scan) * 100 / float64(100+gcPercent)) 392 393 if int64(live) > heapGoal || work > scanWorkExpected { 394 // We're past the soft goal, or we've already done more scan 395 // work than we expected. Pace GC so that in the worst case it 396 // will complete by the hard goal. 397 const maxOvershoot = 1.1 398 heapGoal = int64(float64(heapGoal) * maxOvershoot) 399 400 // Compute the upper bound on the scan work remaining. 401 scanWorkExpected = int64(scan) 402 } 403 404 // Compute the remaining scan work estimate. 405 // 406 // Note that we currently count allocations during GC as both 407 // scannable heap (heapScan) and scan work completed 408 // (scanWork), so allocation will change this difference 409 // slowly in the soft regime and not at all in the hard 410 // regime. 411 scanWorkRemaining := scanWorkExpected - work 412 if scanWorkRemaining < 1000 { 413 // We set a somewhat arbitrary lower bound on 414 // remaining scan work since if we aim a little high, 415 // we can miss by a little. 416 // 417 // We *do* need to enforce that this is at least 1, 418 // since marking is racy and double-scanning objects 419 // may legitimately make the remaining scan work 420 // negative, even in the hard goal regime. 421 scanWorkRemaining = 1000 422 } 423 424 // Compute the heap distance remaining. 425 heapRemaining := heapGoal - int64(live) 426 if heapRemaining <= 0 { 427 // This shouldn't happen, but if it does, avoid 428 // dividing by zero or setting the assist negative. 429 heapRemaining = 1 430 } 431 432 // Compute the mutator assist ratio so by the time the mutator 433 // allocates the remaining heap bytes up to heapGoal, it will 434 // have done (or stolen) the remaining amount of scan work. 435 // Note that the assist ratio values are updated atomically 436 // but not together. This means there may be some degree of 437 // skew between the two values. This is generally OK as the 438 // values shift relatively slowly over the course of a GC 439 // cycle. 440 assistWorkPerByte := float64(scanWorkRemaining) / float64(heapRemaining) 441 assistBytesPerWork := float64(heapRemaining) / float64(scanWorkRemaining) 442 atomic.Store64(&c.assistWorkPerByte, float64bits(assistWorkPerByte)) 443 atomic.Store64(&c.assistBytesPerWork, float64bits(assistBytesPerWork)) 444 } 445 446 // endCycle computes the trigger ratio for the next cycle. 447 // userForced indicates whether the current GC cycle was forced 448 // by the application. 449 func (c *gcControllerState) endCycle(userForced bool) float64 { 450 if userForced { 451 // Forced GC means this cycle didn't start at the 452 // trigger, so where it finished isn't good 453 // information about how to adjust the trigger. 454 // Just leave it where it is. 455 return c.triggerRatio 456 } 457 458 // Proportional response gain for the trigger controller. Must 459 // be in [0, 1]. Lower values smooth out transient effects but 460 // take longer to respond to phase changes. Higher values 461 // react to phase changes quickly, but are more affected by 462 // transient changes. Values near 1 may be unstable. 463 const triggerGain = 0.5 464 465 // Compute next cycle trigger ratio. First, this computes the 466 // "error" for this cycle; that is, how far off the trigger 467 // was from what it should have been, accounting for both heap 468 // growth and GC CPU utilization. We compute the actual heap 469 // growth during this cycle and scale that by how far off from 470 // the goal CPU utilization we were (to estimate the heap 471 // growth if we had the desired CPU utilization). The 472 // difference between this estimate and the GOGC-based goal 473 // heap growth is the error. 474 goalGrowthRatio := c.effectiveGrowthRatio() 475 actualGrowthRatio := float64(c.heapLive)/float64(c.heapMarked) - 1 476 assistDuration := nanotime() - c.markStartTime 477 478 // Assume background mark hit its utilization goal. 479 utilization := gcBackgroundUtilization 480 // Add assist utilization; avoid divide by zero. 481 if assistDuration > 0 { 482 utilization += float64(c.assistTime) / float64(assistDuration*int64(gomaxprocs)) 483 } 484 485 triggerError := goalGrowthRatio - c.triggerRatio - utilization/gcGoalUtilization*(actualGrowthRatio-c.triggerRatio) 486 487 // Finally, we adjust the trigger for next time by this error, 488 // damped by the proportional gain. 489 triggerRatio := c.triggerRatio + triggerGain*triggerError 490 491 if debug.gcpacertrace > 0 { 492 // Print controller state in terms of the design 493 // document. 494 H_m_prev := c.heapMarked 495 h_t := c.triggerRatio 496 H_T := c.trigger 497 h_a := actualGrowthRatio 498 H_a := c.heapLive 499 h_g := goalGrowthRatio 500 H_g := int64(float64(H_m_prev) * (1 + h_g)) 501 u_a := utilization 502 u_g := gcGoalUtilization 503 W_a := c.scanWork 504 print("pacer: H_m_prev=", H_m_prev, 505 " h_t=", h_t, " H_T=", H_T, 506 " h_a=", h_a, " H_a=", H_a, 507 " h_g=", h_g, " H_g=", H_g, 508 " u_a=", u_a, " u_g=", u_g, 509 " W_a=", W_a, 510 " goalΔ=", goalGrowthRatio-h_t, 511 " actualΔ=", h_a-h_t, 512 " u_a/u_g=", u_a/u_g, 513 "\n") 514 } 515 516 return triggerRatio 517 } 518 519 // enlistWorker encourages another dedicated mark worker to start on 520 // another P if there are spare worker slots. It is used by putfull 521 // when more work is made available. 522 // 523 //go:nowritebarrier 524 func (c *gcControllerState) enlistWorker() { 525 // If there are idle Ps, wake one so it will run an idle worker. 526 // NOTE: This is suspected of causing deadlocks. See golang.org/issue/19112. 527 // 528 // if atomic.Load(&sched.npidle) != 0 && atomic.Load(&sched.nmspinning) == 0 { 529 // wakep() 530 // return 531 // } 532 533 // There are no idle Ps. If we need more dedicated workers, 534 // try to preempt a running P so it will switch to a worker. 535 if c.dedicatedMarkWorkersNeeded <= 0 { 536 return 537 } 538 // Pick a random other P to preempt. 539 if gomaxprocs <= 1 { 540 return 541 } 542 gp := getg() 543 if gp == nil || gp.m == nil || gp.m.p == 0 { 544 return 545 } 546 myID := gp.m.p.ptr().id 547 for tries := 0; tries < 5; tries++ { 548 id := int32(fastrandn(uint32(gomaxprocs - 1))) 549 if id >= myID { 550 id++ 551 } 552 p := allp[id] 553 if p.status != _Prunning { 554 continue 555 } 556 if preemptone(p) { 557 return 558 } 559 } 560 } 561 562 // findRunnableGCWorker returns a background mark worker for _p_ if it 563 // should be run. This must only be called when gcBlackenEnabled != 0. 564 func (c *gcControllerState) findRunnableGCWorker(_p_ *p) *g { 565 if gcBlackenEnabled == 0 { 566 throw("gcControllerState.findRunnable: blackening not enabled") 567 } 568 569 if !gcMarkWorkAvailable(_p_) { 570 // No work to be done right now. This can happen at 571 // the end of the mark phase when there are still 572 // assists tapering off. Don't bother running a worker 573 // now because it'll just return immediately. 574 return nil 575 } 576 577 // Grab a worker before we commit to running below. 578 node := (*gcBgMarkWorkerNode)(gcBgMarkWorkerPool.pop()) 579 if node == nil { 580 // There is at least one worker per P, so normally there are 581 // enough workers to run on all Ps, if necessary. However, once 582 // a worker enters gcMarkDone it may park without rejoining the 583 // pool, thus freeing a P with no corresponding worker. 584 // gcMarkDone never depends on another worker doing work, so it 585 // is safe to simply do nothing here. 586 // 587 // If gcMarkDone bails out without completing the mark phase, 588 // it will always do so with queued global work. Thus, that P 589 // will be immediately eligible to re-run the worker G it was 590 // just using, ensuring work can complete. 591 return nil 592 } 593 594 decIfPositive := func(ptr *int64) bool { 595 for { 596 v := atomic.Loadint64(ptr) 597 if v <= 0 { 598 return false 599 } 600 601 if atomic.Casint64(ptr, v, v-1) { 602 return true 603 } 604 } 605 } 606 607 if decIfPositive(&c.dedicatedMarkWorkersNeeded) { 608 // This P is now dedicated to marking until the end of 609 // the concurrent mark phase. 610 _p_.gcMarkWorkerMode = gcMarkWorkerDedicatedMode 611 } else if c.fractionalUtilizationGoal == 0 { 612 // No need for fractional workers. 613 gcBgMarkWorkerPool.push(&node.node) 614 return nil 615 } else { 616 // Is this P behind on the fractional utilization 617 // goal? 618 // 619 // This should be kept in sync with pollFractionalWorkerExit. 620 delta := nanotime() - c.markStartTime 621 if delta > 0 && float64(_p_.gcFractionalMarkTime)/float64(delta) > c.fractionalUtilizationGoal { 622 // Nope. No need to run a fractional worker. 623 gcBgMarkWorkerPool.push(&node.node) 624 return nil 625 } 626 // Run a fractional worker. 627 _p_.gcMarkWorkerMode = gcMarkWorkerFractionalMode 628 } 629 630 // Run the background mark worker. 631 gp := node.gp.ptr() 632 casgstatus(gp, _Gwaiting, _Grunnable) 633 if trace.enabled { 634 traceGoUnpark(gp, 0) 635 } 636 return gp 637 } 638 639 // commit sets the trigger ratio and updates everything 640 // derived from it: the absolute trigger, the heap goal, mark pacing, 641 // and sweep pacing. 642 // 643 // This can be called any time. If GC is the in the middle of a 644 // concurrent phase, it will adjust the pacing of that phase. 645 // 646 // This depends on gcPercent, gcController.heapMarked, and 647 // gcController.heapLive. These must be up to date. 648 // 649 // mheap_.lock must be held or the world must be stopped. 650 func (c *gcControllerState) commit(triggerRatio float64) { 651 assertWorldStoppedOrLockHeld(&mheap_.lock) 652 653 // Compute the next GC goal, which is when the allocated heap 654 // has grown by GOGC/100 over the heap marked by the last 655 // cycle. 656 goal := ^uint64(0) 657 if c.gcPercent >= 0 { 658 goal = c.heapMarked + c.heapMarked*uint64(c.gcPercent)/100 659 } 660 661 // Set the trigger ratio, capped to reasonable bounds. 662 if c.gcPercent >= 0 { 663 scalingFactor := float64(c.gcPercent) / 100 664 // Ensure there's always a little margin so that the 665 // mutator assist ratio isn't infinity. 666 maxTriggerRatio := 0.95 * scalingFactor 667 if triggerRatio > maxTriggerRatio { 668 triggerRatio = maxTriggerRatio 669 } 670 671 // If we let triggerRatio go too low, then if the application 672 // is allocating very rapidly we might end up in a situation 673 // where we're allocating black during a nearly always-on GC. 674 // The result of this is a growing heap and ultimately an 675 // increase in RSS. By capping us at a point >0, we're essentially 676 // saying that we're OK using more CPU during the GC to prevent 677 // this growth in RSS. 678 // 679 // The current constant was chosen empirically: given a sufficiently 680 // fast/scalable allocator with 48 Ps that could drive the trigger ratio 681 // to <0.05, this constant causes applications to retain the same peak 682 // RSS compared to not having this allocator. 683 minTriggerRatio := 0.6 * scalingFactor 684 if triggerRatio < minTriggerRatio { 685 triggerRatio = minTriggerRatio 686 } 687 } else if triggerRatio < 0 { 688 // gcPercent < 0, so just make sure we're not getting a negative 689 // triggerRatio. This case isn't expected to happen in practice, 690 // and doesn't really matter because if gcPercent < 0 then we won't 691 // ever consume triggerRatio further on in this function, but let's 692 // just be defensive here; the triggerRatio being negative is almost 693 // certainly undesirable. 694 triggerRatio = 0 695 } 696 c.triggerRatio = triggerRatio 697 698 // Compute the absolute GC trigger from the trigger ratio. 699 // 700 // We trigger the next GC cycle when the allocated heap has 701 // grown by the trigger ratio over the marked heap size. 702 trigger := ^uint64(0) 703 if c.gcPercent >= 0 { 704 trigger = uint64(float64(c.heapMarked) * (1 + triggerRatio)) 705 // Don't trigger below the minimum heap size. 706 minTrigger := c.heapMinimum 707 if !isSweepDone() { 708 // Concurrent sweep happens in the heap growth 709 // from gcController.heapLive to trigger, so ensure 710 // that concurrent sweep has some heap growth 711 // in which to perform sweeping before we 712 // start the next GC cycle. 713 sweepMin := atomic.Load64(&c.heapLive) + sweepMinHeapDistance 714 if sweepMin > minTrigger { 715 minTrigger = sweepMin 716 } 717 } 718 if trigger < minTrigger { 719 trigger = minTrigger 720 } 721 if int64(trigger) < 0 { 722 print("runtime: heapGoal=", c.heapGoal, " heapMarked=", c.heapMarked, " gcController.heapLive=", c.heapLive, " initialHeapLive=", work.initialHeapLive, "triggerRatio=", triggerRatio, " minTrigger=", minTrigger, "\n") 723 throw("trigger underflow") 724 } 725 if trigger > goal { 726 // The trigger ratio is always less than GOGC/100, but 727 // other bounds on the trigger may have raised it. 728 // Push up the goal, too. 729 goal = trigger 730 } 731 } 732 733 // Commit to the trigger and goal. 734 c.trigger = trigger 735 atomic.Store64(&c.heapGoal, goal) 736 if trace.enabled { 737 traceHeapGoal() 738 } 739 740 // Update mark pacing. 741 if gcphase != _GCoff { 742 c.revise() 743 } 744 745 // Update sweep pacing. 746 if isSweepDone() { 747 mheap_.sweepPagesPerByte = 0 748 } else { 749 // Concurrent sweep needs to sweep all of the in-use 750 // pages by the time the allocated heap reaches the GC 751 // trigger. Compute the ratio of in-use pages to sweep 752 // per byte allocated, accounting for the fact that 753 // some might already be swept. 754 heapLiveBasis := atomic.Load64(&c.heapLive) 755 heapDistance := int64(trigger) - int64(heapLiveBasis) 756 // Add a little margin so rounding errors and 757 // concurrent sweep are less likely to leave pages 758 // unswept when GC starts. 759 heapDistance -= 1024 * 1024 760 if heapDistance < _PageSize { 761 // Avoid setting the sweep ratio extremely high 762 heapDistance = _PageSize 763 } 764 pagesSwept := atomic.Load64(&mheap_.pagesSwept) 765 pagesInUse := atomic.Load64(&mheap_.pagesInUse) 766 sweepDistancePages := int64(pagesInUse) - int64(pagesSwept) 767 if sweepDistancePages <= 0 { 768 mheap_.sweepPagesPerByte = 0 769 } else { 770 mheap_.sweepPagesPerByte = float64(sweepDistancePages) / float64(heapDistance) 771 mheap_.sweepHeapLiveBasis = heapLiveBasis 772 // Write pagesSweptBasis last, since this 773 // signals concurrent sweeps to recompute 774 // their debt. 775 atomic.Store64(&mheap_.pagesSweptBasis, pagesSwept) 776 } 777 } 778 779 gcPaceScavenger() 780 } 781 782 // effectiveGrowthRatio returns the current effective heap growth 783 // ratio (GOGC/100) based on heapMarked from the previous GC and 784 // heapGoal for the current GC. 785 // 786 // This may differ from gcPercent/100 because of various upper and 787 // lower bounds on gcPercent. For example, if the heap is smaller than 788 // heapMinimum, this can be higher than gcPercent/100. 789 // 790 // mheap_.lock must be held or the world must be stopped. 791 func (c *gcControllerState) effectiveGrowthRatio() float64 { 792 assertWorldStoppedOrLockHeld(&mheap_.lock) 793 794 egogc := float64(atomic.Load64(&c.heapGoal)-c.heapMarked) / float64(c.heapMarked) 795 if egogc < 0 { 796 // Shouldn't happen, but just in case. 797 egogc = 0 798 } 799 return egogc 800 } 801 802 // setGCPercent updates gcPercent and all related pacer state. 803 // Returns the old value of gcPercent. 804 // 805 // The world must be stopped, or mheap_.lock must be held. 806 func (c *gcControllerState) setGCPercent(in int32) int32 { 807 assertWorldStoppedOrLockHeld(&mheap_.lock) 808 809 out := c.gcPercent 810 if in < 0 { 811 in = -1 812 } 813 c.gcPercent = in 814 c.heapMinimum = defaultHeapMinimum * uint64(c.gcPercent) / 100 815 // Update pacing in response to gcPercent change. 816 c.commit(c.triggerRatio) 817 818 return out 819 } 820 821 //go:linkname setGCPercent runtime/debug.setGCPercent 822 func setGCPercent(in int32) (out int32) { 823 // Run on the system stack since we grab the heap lock. 824 systemstack(func() { 825 lock(&mheap_.lock) 826 out = gcController.setGCPercent(in) 827 unlock(&mheap_.lock) 828 }) 829 830 // If we just disabled GC, wait for any concurrent GC mark to 831 // finish so we always return with no GC running. 832 if in < 0 { 833 gcWaitOnMark(atomic.Load(&work.cycles)) 834 } 835 836 return out 837 } 838 839 func readGOGC() int32 { 840 p := gogetenv("GOGC") 841 if p == "off" { 842 return -1 843 } 844 if n, ok := atoi32(p); ok { 845 return n 846 } 847 return 100 848 } 849