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 "runtime/internal/atomic" 11 "unsafe" 12 ) 13 14 const ( 15 // gcGoalUtilization is the goal CPU utilization for 16 // marking as a fraction of GOMAXPROCS. 17 gcGoalUtilization = goexperiment.PacerRedesignInt*gcBackgroundUtilization + 18 (1-goexperiment.PacerRedesignInt)*(gcBackgroundUtilization+0.05) 19 20 // gcBackgroundUtilization is the fixed CPU utilization for background 21 // marking. It must be <= gcGoalUtilization. The difference between 22 // gcGoalUtilization and gcBackgroundUtilization will be made up by 23 // mark assists. The scheduler will aim to use within 50% of this 24 // goal. 25 // 26 // Setting this to < gcGoalUtilization avoids saturating the trigger 27 // feedback controller when there are no assists, which allows it to 28 // better control CPU and heap growth. However, the larger the gap, 29 // the more mutator assists are expected to happen, which impact 30 // mutator latency. 31 // 32 // If goexperiment.PacerRedesign, the trigger feedback controller 33 // is replaced with an estimate of the mark/cons ratio that doesn't 34 // have the same saturation issues, so this is set equal to 35 // gcGoalUtilization. 36 gcBackgroundUtilization = 0.25 37 38 // gcCreditSlack is the amount of scan work credit that can 39 // accumulate locally before updating gcController.heapScanWork and, 40 // optionally, gcController.bgScanCredit. Lower values give a more 41 // accurate assist ratio and make it more likely that assists will 42 // successfully steal background credit. Higher values reduce memory 43 // contention. 44 gcCreditSlack = 2000 45 46 // gcAssistTimeSlack is the nanoseconds of mutator assist time that 47 // can accumulate on a P before updating gcController.assistTime. 48 gcAssistTimeSlack = 5000 49 50 // gcOverAssistWork determines how many extra units of scan work a GC 51 // assist does when an assist happens. This amortizes the cost of an 52 // assist by pre-paying for this many bytes of future allocations. 53 gcOverAssistWork = 64 << 10 54 55 // defaultHeapMinimum is the value of heapMinimum for GOGC==100. 56 defaultHeapMinimum = (goexperiment.HeapMinimum512KiBInt)*(512<<10) + 57 (1-goexperiment.HeapMinimum512KiBInt)*(4<<20) 58 59 // scannableStackSizeSlack is the bytes of stack space allocated or freed 60 // that can accumulate on a P before updating gcController.stackSize. 61 scannableStackSizeSlack = 8 << 10 62 ) 63 64 func init() { 65 if offset := unsafe.Offsetof(gcController.heapLive); offset%8 != 0 { 66 println(offset) 67 throw("gcController.heapLive not aligned to 8 bytes") 68 } 69 } 70 71 // gcController implements the GC pacing controller that determines 72 // when to trigger concurrent garbage collection and how much marking 73 // work to do in mutator assists and background marking. 74 // 75 // It uses a feedback control algorithm to adjust the gcController.trigger 76 // trigger based on the heap growth and GC CPU utilization each cycle. 77 // This algorithm optimizes for heap growth to match GOGC and for CPU 78 // utilization between assist and background marking to be 25% of 79 // GOMAXPROCS. The high-level design of this algorithm is documented 80 // at https://golang.org/s/go15gcpacing. 81 // 82 // All fields of gcController are used only during a single mark 83 // cycle. 84 var gcController gcControllerState 85 86 type gcControllerState struct { 87 88 // Initialized from GOGC. GOGC=off means no GC. 89 gcPercent atomic.Int32 90 91 _ uint32 // padding so following 64-bit values are 8-byte aligned 92 93 // heapMinimum is the minimum heap size at which to trigger GC. 94 // For small heaps, this overrides the usual GOGC*live set rule. 95 // 96 // When there is a very small live set but a lot of allocation, simply 97 // collecting when the heap reaches GOGC*live results in many GC 98 // cycles and high total per-GC overhead. This minimum amortizes this 99 // per-GC overhead while keeping the heap reasonably small. 100 // 101 // During initialization this is set to 4MB*GOGC/100. In the case of 102 // GOGC==0, this will set heapMinimum to 0, resulting in constant 103 // collection even when the heap size is small, which is useful for 104 // debugging. 105 heapMinimum uint64 106 107 // triggerRatio is the heap growth ratio that triggers marking. 108 // 109 // E.g., if this is 0.6, then GC should start when the live 110 // heap has reached 1.6 times the heap size marked by the 111 // previous cycle. This should be ≤ GOGC/100 so the trigger 112 // heap size is less than the goal heap size. This is set 113 // during mark termination for the next cycle's trigger. 114 // 115 // Protected by mheap_.lock or a STW. 116 // 117 // Used if !goexperiment.PacerRedesign. 118 triggerRatio float64 119 120 // trigger is the heap size that triggers marking. 121 // 122 // When heapLive ≥ trigger, the mark phase will start. 123 // This is also the heap size by which proportional sweeping 124 // must be complete. 125 // 126 // This is computed from triggerRatio during mark termination 127 // for the next cycle's trigger. 128 // 129 // Protected by mheap_.lock or a STW. 130 trigger uint64 131 132 // consMark is the estimated per-CPU consMark ratio for the application. 133 // 134 // It represents the ratio between the application's allocation 135 // rate, as bytes allocated per CPU-time, and the GC's scan rate, 136 // as bytes scanned per CPU-time. 137 // The units of this ratio are (B / cpu-ns) / (B / cpu-ns). 138 // 139 // At a high level, this value is computed as the bytes of memory 140 // allocated (cons) per unit of scan work completed (mark) in a GC 141 // cycle, divided by the CPU time spent on each activity. 142 // 143 // Updated at the end of each GC cycle, in endCycle. 144 // 145 // For goexperiment.PacerRedesign. 146 consMark float64 147 148 // consMarkController holds the state for the mark-cons ratio 149 // estimation over time. 150 // 151 // Its purpose is to smooth out noisiness in the computation of 152 // consMark; see consMark for details. 153 // 154 // For goexperiment.PacerRedesign. 155 consMarkController piController 156 157 _ uint32 // Padding for atomics on 32-bit platforms. 158 159 // heapGoal is the goal heapLive for when next GC ends. 160 // Set to ^uint64(0) if disabled. 161 // 162 // Read and written atomically, unless the world is stopped. 163 heapGoal uint64 164 165 // lastHeapGoal is the value of heapGoal for the previous GC. 166 // Note that this is distinct from the last value heapGoal had, 167 // because it could change if e.g. gcPercent changes. 168 // 169 // Read and written with the world stopped or with mheap_.lock held. 170 lastHeapGoal uint64 171 172 // heapLive is the number of bytes considered live by the GC. 173 // That is: retained by the most recent GC plus allocated 174 // since then. heapLive ≤ memstats.heapAlloc, since heapAlloc includes 175 // unmarked objects that have not yet been swept (and hence goes up as we 176 // allocate and down as we sweep) while heapLive excludes these 177 // objects (and hence only goes up between GCs). 178 // 179 // This is updated atomically without locking. To reduce 180 // contention, this is updated only when obtaining a span from 181 // an mcentral and at this point it counts all of the 182 // unallocated slots in that span (which will be allocated 183 // before that mcache obtains another span from that 184 // mcentral). Hence, it slightly overestimates the "true" live 185 // heap size. It's better to overestimate than to 186 // underestimate because 1) this triggers the GC earlier than 187 // necessary rather than potentially too late and 2) this 188 // leads to a conservative GC rate rather than a GC rate that 189 // is potentially too low. 190 // 191 // Reads should likewise be atomic (or during STW). 192 // 193 // Whenever this is updated, call traceHeapAlloc() and 194 // this gcControllerState's revise() method. 195 heapLive uint64 196 197 // heapScan is the number of bytes of "scannable" heap. This 198 // is the live heap (as counted by heapLive), but omitting 199 // no-scan objects and no-scan tails of objects. 200 // 201 // For !goexperiment.PacerRedesign: Whenever this is updated, 202 // call this gcControllerState's revise() method. It is read 203 // and written atomically or with the world stopped. 204 // 205 // For goexperiment.PacerRedesign: This value is fixed at the 206 // start of a GC cycle, so during a GC cycle it is safe to 207 // read without atomics, and it represents the maximum scannable 208 // heap. 209 heapScan uint64 210 211 // lastHeapScan is the number of bytes of heap that were scanned 212 // last GC cycle. It is the same as heapMarked, but only 213 // includes the "scannable" parts of objects. 214 // 215 // Updated when the world is stopped. 216 lastHeapScan uint64 217 218 // stackScan is a snapshot of scannableStackSize taken at each GC 219 // STW pause and is used in pacing decisions. 220 // 221 // Updated only while the world is stopped. 222 stackScan uint64 223 224 // scannableStackSize is the amount of allocated goroutine stack space in 225 // use by goroutines. 226 // 227 // This number tracks allocated goroutine stack space rather than used 228 // goroutine stack space (i.e. what is actually scanned) because used 229 // goroutine stack space is much harder to measure cheaply. By using 230 // allocated space, we make an overestimate; this is OK, it's better 231 // to conservatively overcount than undercount. 232 // 233 // Read and updated atomically. 234 scannableStackSize uint64 235 236 // globalsScan is the total amount of global variable space 237 // that is scannable. 238 // 239 // Read and updated atomically. 240 globalsScan uint64 241 242 // heapMarked is the number of bytes marked by the previous 243 // GC. After mark termination, heapLive == heapMarked, but 244 // unlike heapLive, heapMarked does not change until the 245 // next mark termination. 246 heapMarked uint64 247 248 // heapScanWork is the total heap scan work performed this cycle. 249 // stackScanWork is the total stack scan work performed this cycle. 250 // globalsScanWork is the total globals scan work performed this cycle. 251 // 252 // These are updated atomically during the cycle. Updates occur in 253 // bounded batches, since they are both written and read 254 // throughout the cycle. At the end of the cycle, heapScanWork is how 255 // much of the retained heap is scannable. 256 // 257 // Currently these are measured in bytes. For most uses, this is an 258 // opaque unit of work, but for estimation the definition is important. 259 // 260 // Note that stackScanWork includes all allocated space, not just the 261 // size of the stack itself, mirroring stackSize. 262 // 263 // For !goexperiment.PacerRedesign, stackScanWork and globalsScanWork 264 // are always zero. 265 heapScanWork atomic.Int64 266 stackScanWork atomic.Int64 267 globalsScanWork atomic.Int64 268 269 // bgScanCredit is the scan work credit accumulated by the 270 // concurrent background scan. This credit is accumulated by 271 // the background scan and stolen by mutator assists. This is 272 // updated atomically. Updates occur in bounded batches, since 273 // it is both written and read throughout the cycle. 274 bgScanCredit int64 275 276 // assistTime is the nanoseconds spent in mutator assists 277 // during this cycle. This is updated atomically. Updates 278 // occur in bounded batches, since it is both written and read 279 // throughout the cycle. 280 assistTime int64 281 282 // dedicatedMarkTime is the nanoseconds spent in dedicated 283 // mark workers during this cycle. This is updated atomically 284 // at the end of the concurrent mark phase. 285 dedicatedMarkTime int64 286 287 // fractionalMarkTime is the nanoseconds spent in the 288 // fractional mark worker during this cycle. This is updated 289 // atomically throughout the cycle and will be up-to-date if 290 // the fractional mark worker is not currently running. 291 fractionalMarkTime int64 292 293 // idleMarkTime is the nanoseconds spent in idle marking 294 // during this cycle. This is updated atomically throughout 295 // the cycle. 296 idleMarkTime int64 297 298 // markStartTime is the absolute start time in nanoseconds 299 // that assists and background mark workers started. 300 markStartTime int64 301 302 // dedicatedMarkWorkersNeeded is the number of dedicated mark 303 // workers that need to be started. This is computed at the 304 // beginning of each cycle and decremented atomically as 305 // dedicated mark workers get started. 306 dedicatedMarkWorkersNeeded int64 307 308 // assistWorkPerByte is the ratio of scan work to allocated 309 // bytes that should be performed by mutator assists. This is 310 // computed at the beginning of each cycle and updated every 311 // time heapScan is updated. 312 assistWorkPerByte atomic.Float64 313 314 // assistBytesPerWork is 1/assistWorkPerByte. 315 // 316 // Note that because this is read and written independently 317 // from assistWorkPerByte users may notice a skew between 318 // the two values, and such a state should be safe. 319 assistBytesPerWork atomic.Float64 320 321 // fractionalUtilizationGoal is the fraction of wall clock 322 // time that should be spent in the fractional mark worker on 323 // each P that isn't running a dedicated worker. 324 // 325 // For example, if the utilization goal is 25% and there are 326 // no dedicated workers, this will be 0.25. If the goal is 327 // 25%, there is one dedicated worker, and GOMAXPROCS is 5, 328 // this will be 0.05 to make up the missing 5%. 329 // 330 // If this is zero, no fractional workers are needed. 331 fractionalUtilizationGoal float64 332 333 // test indicates that this is a test-only copy of gcControllerState. 334 test bool 335 336 _ cpu.CacheLinePad 337 } 338 339 func (c *gcControllerState) init(gcPercent int32) { 340 c.heapMinimum = defaultHeapMinimum 341 342 if goexperiment.PacerRedesign { 343 c.consMarkController = piController{ 344 // Tuned first via the Ziegler-Nichols process in simulation, 345 // then the integral time was manually tuned against real-world 346 // applications to deal with noisiness in the measured cons/mark 347 // ratio. 348 kp: 0.9, 349 ti: 4.0, 350 351 // Set a high reset time in GC cycles. 352 // This is inversely proportional to the rate at which we 353 // accumulate error from clipping. By making this very high 354 // we make the accumulation slow. In general, clipping is 355 // OK in our situation, hence the choice. 356 // 357 // Tune this if we get unintended effects from clipping for 358 // a long time. 359 tt: 1000, 360 min: -1000, 361 max: 1000, 362 } 363 } else { 364 // Set a reasonable initial GC trigger. 365 c.triggerRatio = 7 / 8.0 366 367 // Fake a heapMarked value so it looks like a trigger at 368 // heapMinimum is the appropriate growth from heapMarked. 369 // This will go into computing the initial GC goal. 370 c.heapMarked = uint64(float64(c.heapMinimum) / (1 + c.triggerRatio)) 371 } 372 373 // This will also compute and set the GC trigger and goal. 374 c.setGCPercent(gcPercent) 375 } 376 377 // startCycle resets the GC controller's state and computes estimates 378 // for a new GC cycle. The caller must hold worldsema and the world 379 // must be stopped. 380 func (c *gcControllerState) startCycle(markStartTime int64, procs int) { 381 c.heapScanWork.Store(0) 382 c.stackScanWork.Store(0) 383 c.globalsScanWork.Store(0) 384 c.bgScanCredit = 0 385 c.assistTime = 0 386 c.dedicatedMarkTime = 0 387 c.fractionalMarkTime = 0 388 c.idleMarkTime = 0 389 c.markStartTime = markStartTime 390 c.stackScan = atomic.Load64(&c.scannableStackSize) 391 392 // Ensure that the heap goal is at least a little larger than 393 // the current live heap size. This may not be the case if GC 394 // start is delayed or if the allocation that pushed gcController.heapLive 395 // over trigger is large or if the trigger is really close to 396 // GOGC. Assist is proportional to this distance, so enforce a 397 // minimum distance, even if it means going over the GOGC goal 398 // by a tiny bit. 399 if goexperiment.PacerRedesign { 400 if c.heapGoal < c.heapLive+64<<10 { 401 c.heapGoal = c.heapLive + 64<<10 402 } 403 } else { 404 if c.heapGoal < c.heapLive+1<<20 { 405 c.heapGoal = c.heapLive + 1<<20 406 } 407 } 408 409 // Compute the background mark utilization goal. In general, 410 // this may not come out exactly. We round the number of 411 // dedicated workers so that the utilization is closest to 412 // 25%. For small GOMAXPROCS, this would introduce too much 413 // error, so we add fractional workers in that case. 414 totalUtilizationGoal := float64(procs) * gcBackgroundUtilization 415 c.dedicatedMarkWorkersNeeded = int64(totalUtilizationGoal + 0.5) 416 utilError := float64(c.dedicatedMarkWorkersNeeded)/totalUtilizationGoal - 1 417 const maxUtilError = 0.3 418 if utilError < -maxUtilError || utilError > maxUtilError { 419 // Rounding put us more than 30% off our goal. With 420 // gcBackgroundUtilization of 25%, this happens for 421 // GOMAXPROCS<=3 or GOMAXPROCS=6. Enable fractional 422 // workers to compensate. 423 if float64(c.dedicatedMarkWorkersNeeded) > totalUtilizationGoal { 424 // Too many dedicated workers. 425 c.dedicatedMarkWorkersNeeded-- 426 } 427 c.fractionalUtilizationGoal = (totalUtilizationGoal - float64(c.dedicatedMarkWorkersNeeded)) / float64(procs) 428 } else { 429 c.fractionalUtilizationGoal = 0 430 } 431 432 // In STW mode, we just want dedicated workers. 433 if debug.gcstoptheworld > 0 { 434 c.dedicatedMarkWorkersNeeded = int64(procs) 435 c.fractionalUtilizationGoal = 0 436 } 437 438 // Clear per-P state 439 for _, p := range allp { 440 p.gcAssistTime = 0 441 p.gcFractionalMarkTime = 0 442 } 443 444 // Compute initial values for controls that are updated 445 // throughout the cycle. 446 c.revise() 447 448 if debug.gcpacertrace > 0 { 449 assistRatio := c.assistWorkPerByte.Load() 450 print("pacer: assist ratio=", assistRatio, 451 " (scan ", gcController.heapScan>>20, " MB in ", 452 work.initialHeapLive>>20, "->", 453 c.heapGoal>>20, " MB)", 454 " workers=", c.dedicatedMarkWorkersNeeded, 455 "+", c.fractionalUtilizationGoal, "\n") 456 } 457 } 458 459 // revise updates the assist ratio during the GC cycle to account for 460 // improved estimates. This should be called whenever gcController.heapScan, 461 // gcController.heapLive, or gcController.heapGoal is updated. It is safe to 462 // call concurrently, but it may race with other calls to revise. 463 // 464 // The result of this race is that the two assist ratio values may not line 465 // up or may be stale. In practice this is OK because the assist ratio 466 // moves slowly throughout a GC cycle, and the assist ratio is a best-effort 467 // heuristic anyway. Furthermore, no part of the heuristic depends on 468 // the two assist ratio values being exact reciprocals of one another, since 469 // the two values are used to convert values from different sources. 470 // 471 // The worst case result of this raciness is that we may miss a larger shift 472 // in the ratio (say, if we decide to pace more aggressively against the 473 // hard heap goal) but even this "hard goal" is best-effort (see #40460). 474 // The dedicated GC should ensure we don't exceed the hard goal by too much 475 // in the rare case we do exceed it. 476 // 477 // It should only be called when gcBlackenEnabled != 0 (because this 478 // is when assists are enabled and the necessary statistics are 479 // available). 480 func (c *gcControllerState) revise() { 481 gcPercent := c.gcPercent.Load() 482 if gcPercent < 0 { 483 // If GC is disabled but we're running a forced GC, 484 // act like GOGC is huge for the below calculations. 485 gcPercent = 100000 486 } 487 live := atomic.Load64(&c.heapLive) 488 scan := atomic.Load64(&c.heapScan) 489 work := c.heapScanWork.Load() + c.stackScanWork.Load() + c.globalsScanWork.Load() 490 491 // Assume we're under the soft goal. Pace GC to complete at 492 // heapGoal assuming the heap is in steady-state. 493 heapGoal := int64(atomic.Load64(&c.heapGoal)) 494 495 var scanWorkExpected int64 496 if goexperiment.PacerRedesign { 497 // The expected scan work is computed as the amount of bytes scanned last 498 // GC cycle, plus our estimate of stacks and globals work for this cycle. 499 scanWorkExpected = int64(c.lastHeapScan + c.stackScan + c.globalsScan) 500 501 // maxScanWork is a worst-case estimate of the amount of scan work that 502 // needs to be performed in this GC cycle. Specifically, it represents 503 // the case where *all* scannable memory turns out to be live. 504 maxScanWork := int64(scan + c.stackScan + c.globalsScan) 505 if work > scanWorkExpected { 506 // We've already done more scan work than expected. Because our expectation 507 // is based on a steady-state scannable heap size, we assume this means our 508 // heap is growing. Compute a new heap goal that takes our existing runway 509 // computed for scanWorkExpected and extrapolates it to maxScanWork, the worst-case 510 // scan work. This keeps our assist ratio stable if the heap continues to grow. 511 // 512 // The effect of this mechanism is that assists stay flat in the face of heap 513 // growths. It's OK to use more memory this cycle to scan all the live heap, 514 // because the next GC cycle is inevitably going to use *at least* that much 515 // memory anyway. 516 extHeapGoal := int64(float64(heapGoal-int64(c.trigger))/float64(scanWorkExpected)*float64(maxScanWork)) + int64(c.trigger) 517 scanWorkExpected = maxScanWork 518 519 // hardGoal is a hard limit on the amount that we're willing to push back the 520 // heap goal, and that's twice the heap goal (i.e. if GOGC=100 and the heap and/or 521 // stacks and/or globals grow to twice their size, this limits the current GC cycle's 522 // growth to 4x the original live heap's size). 523 // 524 // This maintains the invariant that we use no more memory than the next GC cycle 525 // will anyway. 526 hardGoal := int64((1.0 + float64(gcPercent)/100.0) * float64(heapGoal)) 527 if extHeapGoal > hardGoal { 528 extHeapGoal = hardGoal 529 } 530 heapGoal = extHeapGoal 531 } 532 if int64(live) > heapGoal { 533 // We're already past our heap goal, even the extrapolated one. 534 // Leave ourselves some extra runway, so in the worst case we 535 // finish by that point. 536 const maxOvershoot = 1.1 537 heapGoal = int64(float64(heapGoal) * maxOvershoot) 538 539 // Compute the upper bound on the scan work remaining. 540 scanWorkExpected = maxScanWork 541 } 542 } else { 543 // Compute the expected scan work remaining. 544 // 545 // This is estimated based on the expected 546 // steady-state scannable heap. For example, with 547 // GOGC=100, only half of the scannable heap is 548 // expected to be live, so that's what we target. 549 // 550 // (This is a float calculation to avoid overflowing on 551 // 100*heapScan.) 552 scanWorkExpected = int64(float64(scan) * 100 / float64(100+gcPercent)) 553 if int64(live) > heapGoal || work > scanWorkExpected { 554 // We're past the soft goal, or we've already done more scan 555 // work than we expected. Pace GC so that in the worst case it 556 // will complete by the hard goal. 557 const maxOvershoot = 1.1 558 heapGoal = int64(float64(heapGoal) * maxOvershoot) 559 560 // Compute the upper bound on the scan work remaining. 561 scanWorkExpected = int64(scan) 562 } 563 } 564 565 // Compute the remaining scan work estimate. 566 // 567 // Note that we currently count allocations during GC as both 568 // scannable heap (heapScan) and scan work completed 569 // (scanWork), so allocation will change this difference 570 // slowly in the soft regime and not at all in the hard 571 // regime. 572 scanWorkRemaining := scanWorkExpected - work 573 if scanWorkRemaining < 1000 { 574 // We set a somewhat arbitrary lower bound on 575 // remaining scan work since if we aim a little high, 576 // we can miss by a little. 577 // 578 // We *do* need to enforce that this is at least 1, 579 // since marking is racy and double-scanning objects 580 // may legitimately make the remaining scan work 581 // negative, even in the hard goal regime. 582 scanWorkRemaining = 1000 583 } 584 585 // Compute the heap distance remaining. 586 heapRemaining := heapGoal - int64(live) 587 if heapRemaining <= 0 { 588 // This shouldn't happen, but if it does, avoid 589 // dividing by zero or setting the assist negative. 590 heapRemaining = 1 591 } 592 593 // Compute the mutator assist ratio so by the time the mutator 594 // allocates the remaining heap bytes up to heapGoal, it will 595 // have done (or stolen) the remaining amount of scan work. 596 // Note that the assist ratio values are updated atomically 597 // but not together. This means there may be some degree of 598 // skew between the two values. This is generally OK as the 599 // values shift relatively slowly over the course of a GC 600 // cycle. 601 assistWorkPerByte := float64(scanWorkRemaining) / float64(heapRemaining) 602 assistBytesPerWork := float64(heapRemaining) / float64(scanWorkRemaining) 603 c.assistWorkPerByte.Store(assistWorkPerByte) 604 c.assistBytesPerWork.Store(assistBytesPerWork) 605 } 606 607 // endCycle computes the trigger ratio (!goexperiment.PacerRedesign) 608 // or the consMark estimate (goexperiment.PacerRedesign) for the next cycle. 609 // Returns the trigger ratio if application, or 0 (goexperiment.PacerRedesign). 610 // userForced indicates whether the current GC cycle was forced 611 // by the application. 612 func (c *gcControllerState) endCycle(now int64, procs int, userForced bool) float64 { 613 // Record last heap goal for the scavenger. 614 // We'll be updating the heap goal soon. 615 gcController.lastHeapGoal = gcController.heapGoal 616 617 // Compute the duration of time for which assists were turned on. 618 assistDuration := now - c.markStartTime 619 620 // Assume background mark hit its utilization goal. 621 utilization := gcBackgroundUtilization 622 // Add assist utilization; avoid divide by zero. 623 if assistDuration > 0 { 624 utilization += float64(c.assistTime) / float64(assistDuration*int64(procs)) 625 } 626 627 if goexperiment.PacerRedesign { 628 if c.heapLive <= c.trigger { 629 // Shouldn't happen, but let's be very safe about this in case the 630 // GC is somehow extremely short. 631 // 632 // In this case though, the only reasonable value for c.heapLive-c.trigger 633 // would be 0, which isn't really all that useful, i.e. the GC was so short 634 // that it didn't matter. 635 // 636 // Ignore this case and don't update anything. 637 return 0 638 } 639 idleUtilization := 0.0 640 if assistDuration > 0 { 641 idleUtilization = float64(c.idleMarkTime) / float64(assistDuration*int64(procs)) 642 } 643 // Determine the cons/mark ratio. 644 // 645 // The units we want for the numerator and denominator are both B / cpu-ns. 646 // We get this by taking the bytes allocated or scanned, and divide by the amount of 647 // CPU time it took for those operations. For allocations, that CPU time is 648 // 649 // assistDuration * procs * (1 - utilization) 650 // 651 // Where utilization includes just background GC workers and assists. It does *not* 652 // include idle GC work time, because in theory the mutator is free to take that at 653 // any point. 654 // 655 // For scanning, that CPU time is 656 // 657 // assistDuration * procs * (utilization + idleUtilization) 658 // 659 // In this case, we *include* idle utilization, because that is additional CPU time that the 660 // the GC had available to it. 661 // 662 // In effect, idle GC time is sort of double-counted here, but it's very weird compared 663 // to other kinds of GC work, because of how fluid it is. Namely, because the mutator is 664 // *always* free to take it. 665 // 666 // So this calculation is really: 667 // (heapLive-trigger) / (assistDuration * procs * (1-utilization)) / 668 // (scanWork) / (assistDuration * procs * (utilization+idleUtilization) 669 // 670 // Note that because we only care about the ratio, assistDuration and procs cancel out. 671 scanWork := c.heapScanWork.Load() + c.stackScanWork.Load() + c.globalsScanWork.Load() 672 currentConsMark := (float64(c.heapLive-c.trigger) * (utilization + idleUtilization)) / 673 (float64(scanWork) * (1 - utilization)) 674 675 // Update cons/mark controller. The time period for this is 1 GC cycle. 676 // 677 // This use of a PI controller might seem strange. So, here's an explanation: 678 // 679 // currentConsMark represents the consMark we *should've* had to be perfectly 680 // on-target for this cycle. Given that we assume the next GC will be like this 681 // one in the steady-state, it stands to reason that we should just pick that 682 // as our next consMark. In practice, however, currentConsMark is too noisy: 683 // we're going to be wildly off-target in each GC cycle if we do that. 684 // 685 // What we do instead is make a long-term assumption: there is some steady-state 686 // consMark value, but it's obscured by noise. By constantly shooting for this 687 // noisy-but-perfect consMark value, the controller will bounce around a bit, 688 // but its average behavior, in aggregate, should be less noisy and closer to 689 // the true long-term consMark value, provided its tuned to be slightly overdamped. 690 var ok bool 691 oldConsMark := c.consMark 692 c.consMark, ok = c.consMarkController.next(c.consMark, currentConsMark, 1.0) 693 if !ok { 694 // The error spiraled out of control. This is incredibly unlikely seeing 695 // as this controller is essentially just a smoothing function, but it might 696 // mean that something went very wrong with how currentConsMark was calculated. 697 // Just reset consMark and keep going. 698 c.consMark = 0 699 } 700 701 if debug.gcpacertrace > 0 { 702 printlock() 703 goal := gcGoalUtilization * 100 704 print("pacer: ", int(utilization*100), "% CPU (", int(goal), " exp.) for ") 705 print(c.heapScanWork.Load(), "+", c.stackScanWork.Load(), "+", c.globalsScanWork.Load(), " B work (", c.lastHeapScan+c.stackScan+c.globalsScan, " B exp.) ") 706 print("in ", c.trigger, " B -> ", c.heapLive, " B (∆goal ", int64(c.heapLive)-int64(c.heapGoal), ", cons/mark ", oldConsMark, ")") 707 if !ok { 708 print("[controller reset]") 709 } 710 println() 711 printunlock() 712 } 713 return 0 714 } 715 716 // !goexperiment.PacerRedesign below. 717 718 if userForced { 719 // Forced GC means this cycle didn't start at the 720 // trigger, so where it finished isn't good 721 // information about how to adjust the trigger. 722 // Just leave it where it is. 723 return c.triggerRatio 724 } 725 726 // Proportional response gain for the trigger controller. Must 727 // be in [0, 1]. Lower values smooth out transient effects but 728 // take longer to respond to phase changes. Higher values 729 // react to phase changes quickly, but are more affected by 730 // transient changes. Values near 1 may be unstable. 731 const triggerGain = 0.5 732 733 // Compute next cycle trigger ratio. First, this computes the 734 // "error" for this cycle; that is, how far off the trigger 735 // was from what it should have been, accounting for both heap 736 // growth and GC CPU utilization. We compute the actual heap 737 // growth during this cycle and scale that by how far off from 738 // the goal CPU utilization we were (to estimate the heap 739 // growth if we had the desired CPU utilization). The 740 // difference between this estimate and the GOGC-based goal 741 // heap growth is the error. 742 goalGrowthRatio := c.effectiveGrowthRatio() 743 actualGrowthRatio := float64(c.heapLive)/float64(c.heapMarked) - 1 744 triggerError := goalGrowthRatio - c.triggerRatio - utilization/gcGoalUtilization*(actualGrowthRatio-c.triggerRatio) 745 746 // Finally, we adjust the trigger for next time by this error, 747 // damped by the proportional gain. 748 triggerRatio := c.triggerRatio + triggerGain*triggerError 749 750 if debug.gcpacertrace > 0 { 751 // Print controller state in terms of the design 752 // document. 753 H_m_prev := c.heapMarked 754 h_t := c.triggerRatio 755 H_T := c.trigger 756 h_a := actualGrowthRatio 757 H_a := c.heapLive 758 h_g := goalGrowthRatio 759 H_g := int64(float64(H_m_prev) * (1 + h_g)) 760 u_a := utilization 761 u_g := gcGoalUtilization 762 W_a := c.heapScanWork.Load() 763 print("pacer: H_m_prev=", H_m_prev, 764 " h_t=", h_t, " H_T=", H_T, 765 " h_a=", h_a, " H_a=", H_a, 766 " h_g=", h_g, " H_g=", H_g, 767 " u_a=", u_a, " u_g=", u_g, 768 " W_a=", W_a, 769 " goalΔ=", goalGrowthRatio-h_t, 770 " actualΔ=", h_a-h_t, 771 " u_a/u_g=", u_a/u_g, 772 "\n") 773 } 774 775 return triggerRatio 776 } 777 778 // enlistWorker encourages another dedicated mark worker to start on 779 // another P if there are spare worker slots. It is used by putfull 780 // when more work is made available. 781 // 782 //go:nowritebarrier 783 func (c *gcControllerState) enlistWorker() { 784 // If there are idle Ps, wake one so it will run an idle worker. 785 // NOTE: This is suspected of causing deadlocks. See golang.org/issue/19112. 786 // 787 // if atomic.Load(&sched.npidle) != 0 && atomic.Load(&sched.nmspinning) == 0 { 788 // wakep() 789 // return 790 // } 791 792 // There are no idle Ps. If we need more dedicated workers, 793 // try to preempt a running P so it will switch to a worker. 794 if c.dedicatedMarkWorkersNeeded <= 0 { 795 return 796 } 797 // Pick a random other P to preempt. 798 if gomaxprocs <= 1 { 799 return 800 } 801 gp := getg() 802 if gp == nil || gp.m == nil || gp.m.p == 0 { 803 return 804 } 805 myID := gp.m.p.ptr().id 806 for tries := 0; tries < 5; tries++ { 807 id := int32(fastrandn(uint32(gomaxprocs - 1))) 808 if id >= myID { 809 id++ 810 } 811 p := allp[id] 812 if p.status != _Prunning { 813 continue 814 } 815 if preemptone(p) { 816 return 817 } 818 } 819 } 820 821 // findRunnableGCWorker returns a background mark worker for _p_ if it 822 // should be run. This must only be called when gcBlackenEnabled != 0. 823 func (c *gcControllerState) findRunnableGCWorker(_p_ *p) *g { 824 if gcBlackenEnabled == 0 { 825 throw("gcControllerState.findRunnable: blackening not enabled") 826 } 827 828 if !gcMarkWorkAvailable(_p_) { 829 // No work to be done right now. This can happen at 830 // the end of the mark phase when there are still 831 // assists tapering off. Don't bother running a worker 832 // now because it'll just return immediately. 833 return nil 834 } 835 836 // Grab a worker before we commit to running below. 837 node := (*gcBgMarkWorkerNode)(gcBgMarkWorkerPool.pop()) 838 if node == nil { 839 // There is at least one worker per P, so normally there are 840 // enough workers to run on all Ps, if necessary. However, once 841 // a worker enters gcMarkDone it may park without rejoining the 842 // pool, thus freeing a P with no corresponding worker. 843 // gcMarkDone never depends on another worker doing work, so it 844 // is safe to simply do nothing here. 845 // 846 // If gcMarkDone bails out without completing the mark phase, 847 // it will always do so with queued global work. Thus, that P 848 // will be immediately eligible to re-run the worker G it was 849 // just using, ensuring work can complete. 850 return nil 851 } 852 853 decIfPositive := func(ptr *int64) bool { 854 for { 855 v := atomic.Loadint64(ptr) 856 if v <= 0 { 857 return false 858 } 859 860 if atomic.Casint64(ptr, v, v-1) { 861 return true 862 } 863 } 864 } 865 866 if decIfPositive(&c.dedicatedMarkWorkersNeeded) { 867 // This P is now dedicated to marking until the end of 868 // the concurrent mark phase. 869 _p_.gcMarkWorkerMode = gcMarkWorkerDedicatedMode 870 } else if c.fractionalUtilizationGoal == 0 { 871 // No need for fractional workers. 872 gcBgMarkWorkerPool.push(&node.node) 873 return nil 874 } else { 875 // Is this P behind on the fractional utilization 876 // goal? 877 // 878 // This should be kept in sync with pollFractionalWorkerExit. 879 delta := nanotime() - c.markStartTime 880 if delta > 0 && float64(_p_.gcFractionalMarkTime)/float64(delta) > c.fractionalUtilizationGoal { 881 // Nope. No need to run a fractional worker. 882 gcBgMarkWorkerPool.push(&node.node) 883 return nil 884 } 885 // Run a fractional worker. 886 _p_.gcMarkWorkerMode = gcMarkWorkerFractionalMode 887 } 888 889 // Run the background mark worker. 890 gp := node.gp.ptr() 891 casgstatus(gp, _Gwaiting, _Grunnable) 892 if trace.enabled { 893 traceGoUnpark(gp, 0) 894 } 895 return gp 896 } 897 898 // resetLive sets up the controller state for the next mark phase after the end 899 // of the previous one. Must be called after endCycle and before commit, before 900 // the world is started. 901 // 902 // The world must be stopped. 903 func (c *gcControllerState) resetLive(bytesMarked uint64) { 904 c.heapMarked = bytesMarked 905 c.heapLive = bytesMarked 906 c.heapScan = uint64(c.heapScanWork.Load()) 907 c.lastHeapScan = uint64(c.heapScanWork.Load()) 908 909 // heapLive was updated, so emit a trace event. 910 if trace.enabled { 911 traceHeapAlloc() 912 } 913 } 914 915 // logWorkTime updates mark work accounting in the controller by a duration of 916 // work in nanoseconds. 917 // 918 // Safe to execute at any time. 919 func (c *gcControllerState) logWorkTime(mode gcMarkWorkerMode, duration int64) { 920 switch mode { 921 case gcMarkWorkerDedicatedMode: 922 atomic.Xaddint64(&c.dedicatedMarkTime, duration) 923 atomic.Xaddint64(&c.dedicatedMarkWorkersNeeded, 1) 924 case gcMarkWorkerFractionalMode: 925 atomic.Xaddint64(&c.fractionalMarkTime, duration) 926 case gcMarkWorkerIdleMode: 927 atomic.Xaddint64(&c.idleMarkTime, duration) 928 default: 929 throw("logWorkTime: unknown mark worker mode") 930 } 931 } 932 933 func (c *gcControllerState) update(dHeapLive, dHeapScan int64) { 934 if dHeapLive != 0 { 935 atomic.Xadd64(&gcController.heapLive, dHeapLive) 936 if trace.enabled { 937 // gcController.heapLive changed. 938 traceHeapAlloc() 939 } 940 } 941 // Only update heapScan in the new pacer redesign if we're not 942 // currently in a GC. 943 if !goexperiment.PacerRedesign || gcBlackenEnabled == 0 { 944 if dHeapScan != 0 { 945 atomic.Xadd64(&gcController.heapScan, dHeapScan) 946 } 947 } 948 if gcBlackenEnabled != 0 { 949 // gcController.heapLive and heapScan changed. 950 c.revise() 951 } 952 } 953 954 func (c *gcControllerState) addScannableStack(pp *p, amount int64) { 955 if pp == nil { 956 atomic.Xadd64(&c.scannableStackSize, amount) 957 return 958 } 959 pp.scannableStackSizeDelta += amount 960 if pp.scannableStackSizeDelta >= scannableStackSizeSlack || pp.scannableStackSizeDelta <= -scannableStackSizeSlack { 961 atomic.Xadd64(&c.scannableStackSize, pp.scannableStackSizeDelta) 962 pp.scannableStackSizeDelta = 0 963 } 964 } 965 966 func (c *gcControllerState) addGlobals(amount int64) { 967 atomic.Xadd64(&c.globalsScan, amount) 968 } 969 970 // commit recomputes all pacing parameters from scratch, namely 971 // absolute trigger, the heap goal, mark pacing, and sweep pacing. 972 // 973 // If goexperiment.PacerRedesign is true, triggerRatio is ignored. 974 // 975 // This can be called any time. If GC is the in the middle of a 976 // concurrent phase, it will adjust the pacing of that phase. 977 // 978 // This depends on gcPercent, gcController.heapMarked, and 979 // gcController.heapLive. These must be up to date. 980 // 981 // mheap_.lock must be held or the world must be stopped. 982 func (c *gcControllerState) commit(triggerRatio float64) { 983 if !c.test { 984 assertWorldStoppedOrLockHeld(&mheap_.lock) 985 } 986 987 if !goexperiment.PacerRedesign { 988 c.oldCommit(triggerRatio) 989 return 990 } 991 992 // Compute the next GC goal, which is when the allocated heap 993 // has grown by GOGC/100 over where it started the last cycle, 994 // plus additional runway for non-heap sources of GC work. 995 goal := ^uint64(0) 996 if gcPercent := c.gcPercent.Load(); gcPercent >= 0 { 997 goal = c.heapMarked + (c.heapMarked+atomic.Load64(&c.stackScan)+atomic.Load64(&c.globalsScan))*uint64(gcPercent)/100 998 } 999 1000 // Don't trigger below the minimum heap size. 1001 minTrigger := c.heapMinimum 1002 if !isSweepDone() { 1003 // Concurrent sweep happens in the heap growth 1004 // from gcController.heapLive to trigger, so ensure 1005 // that concurrent sweep has some heap growth 1006 // in which to perform sweeping before we 1007 // start the next GC cycle. 1008 sweepMin := atomic.Load64(&c.heapLive) + sweepMinHeapDistance 1009 if sweepMin > minTrigger { 1010 minTrigger = sweepMin 1011 } 1012 } 1013 1014 // If we let the trigger go too low, then if the application 1015 // is allocating very rapidly we might end up in a situation 1016 // where we're allocating black during a nearly always-on GC. 1017 // The result of this is a growing heap and ultimately an 1018 // increase in RSS. By capping us at a point >0, we're essentially 1019 // saying that we're OK using more CPU during the GC to prevent 1020 // this growth in RSS. 1021 // 1022 // The current constant was chosen empirically: given a sufficiently 1023 // fast/scalable allocator with 48 Ps that could drive the trigger ratio 1024 // to <0.05, this constant causes applications to retain the same peak 1025 // RSS compared to not having this allocator. 1026 if triggerBound := uint64(0.7*float64(goal-c.heapMarked)) + c.heapMarked; minTrigger < triggerBound { 1027 minTrigger = triggerBound 1028 } 1029 1030 // For small heaps, set the max trigger point at 95% of the heap goal. 1031 // This ensures we always have *some* headroom when the GC actually starts. 1032 // For larger heaps, set the max trigger point at the goal, minus the 1033 // minimum heap size. 1034 // This choice follows from the fact that the minimum heap size is chosen 1035 // to reflect the costs of a GC with no work to do. With a large heap but 1036 // very little scan work to perform, this gives us exactly as much runway 1037 // as we would need, in the worst case. 1038 maxRunway := uint64(0.95 * float64(goal-c.heapMarked)) 1039 if largeHeapMaxRunway := goal - c.heapMinimum; goal > c.heapMinimum && maxRunway < largeHeapMaxRunway { 1040 maxRunway = largeHeapMaxRunway 1041 } 1042 maxTrigger := maxRunway + c.heapMarked 1043 if maxTrigger < minTrigger { 1044 maxTrigger = minTrigger 1045 } 1046 1047 // Compute the trigger by using our estimate of the cons/mark ratio. 1048 // 1049 // The idea is to take our expected scan work, and multiply it by 1050 // the cons/mark ratio to determine how long it'll take to complete 1051 // that scan work in terms of bytes allocated. This gives us our GC's 1052 // runway. 1053 // 1054 // However, the cons/mark ratio is a ratio of rates per CPU-second, but 1055 // here we care about the relative rates for some division of CPU 1056 // resources among the mutator and the GC. 1057 // 1058 // To summarize, we have B / cpu-ns, and we want B / ns. We get that 1059 // by multiplying by our desired division of CPU resources. We choose 1060 // to express CPU resources as GOMAPROCS*fraction. Note that because 1061 // we're working with a ratio here, we can omit the number of CPU cores, 1062 // because they'll appear in the numerator and denominator and cancel out. 1063 // As a result, this is basically just "weighing" the cons/mark ratio by 1064 // our desired division of resources. 1065 // 1066 // Furthermore, by setting the trigger so that CPU resources are divided 1067 // this way, assuming that the cons/mark ratio is correct, we make that 1068 // division a reality. 1069 var trigger uint64 1070 runway := uint64((c.consMark * (1 - gcGoalUtilization) / (gcGoalUtilization)) * float64(c.lastHeapScan+c.stackScan+c.globalsScan)) 1071 if runway > goal { 1072 trigger = minTrigger 1073 } else { 1074 trigger = goal - runway 1075 } 1076 if trigger < minTrigger { 1077 trigger = minTrigger 1078 } 1079 if trigger > maxTrigger { 1080 trigger = maxTrigger 1081 } 1082 if trigger > goal { 1083 goal = trigger 1084 } 1085 1086 // Commit to the trigger and goal. 1087 c.trigger = trigger 1088 atomic.Store64(&c.heapGoal, goal) 1089 if trace.enabled { 1090 traceHeapGoal() 1091 } 1092 1093 // Update mark pacing. 1094 if gcphase != _GCoff { 1095 c.revise() 1096 } 1097 } 1098 1099 // oldCommit sets the trigger ratio and updates everything 1100 // derived from it: the absolute trigger, the heap goal, mark pacing, 1101 // and sweep pacing. 1102 // 1103 // This can be called any time. If GC is the in the middle of a 1104 // concurrent phase, it will adjust the pacing of that phase. 1105 // 1106 // This depends on gcPercent, gcController.heapMarked, and 1107 // gcController.heapLive. These must be up to date. 1108 // 1109 // For !goexperiment.PacerRedesign. 1110 func (c *gcControllerState) oldCommit(triggerRatio float64) { 1111 gcPercent := c.gcPercent.Load() 1112 1113 // Compute the next GC goal, which is when the allocated heap 1114 // has grown by GOGC/100 over the heap marked by the last 1115 // cycle. 1116 goal := ^uint64(0) 1117 if gcPercent >= 0 { 1118 goal = c.heapMarked + c.heapMarked*uint64(gcPercent)/100 1119 } 1120 1121 // Set the trigger ratio, capped to reasonable bounds. 1122 if gcPercent >= 0 { 1123 scalingFactor := float64(gcPercent) / 100 1124 // Ensure there's always a little margin so that the 1125 // mutator assist ratio isn't infinity. 1126 maxTriggerRatio := 0.95 * scalingFactor 1127 if triggerRatio > maxTriggerRatio { 1128 triggerRatio = maxTriggerRatio 1129 } 1130 1131 // If we let triggerRatio go too low, then if the application 1132 // is allocating very rapidly we might end up in a situation 1133 // where we're allocating black during a nearly always-on GC. 1134 // The result of this is a growing heap and ultimately an 1135 // increase in RSS. By capping us at a point >0, we're essentially 1136 // saying that we're OK using more CPU during the GC to prevent 1137 // this growth in RSS. 1138 // 1139 // The current constant was chosen empirically: given a sufficiently 1140 // fast/scalable allocator with 48 Ps that could drive the trigger ratio 1141 // to <0.05, this constant causes applications to retain the same peak 1142 // RSS compared to not having this allocator. 1143 minTriggerRatio := 0.6 * scalingFactor 1144 if triggerRatio < minTriggerRatio { 1145 triggerRatio = minTriggerRatio 1146 } 1147 } else if triggerRatio < 0 { 1148 // gcPercent < 0, so just make sure we're not getting a negative 1149 // triggerRatio. This case isn't expected to happen in practice, 1150 // and doesn't really matter because if gcPercent < 0 then we won't 1151 // ever consume triggerRatio further on in this function, but let's 1152 // just be defensive here; the triggerRatio being negative is almost 1153 // certainly undesirable. 1154 triggerRatio = 0 1155 } 1156 c.triggerRatio = triggerRatio 1157 1158 // Compute the absolute GC trigger from the trigger ratio. 1159 // 1160 // We trigger the next GC cycle when the allocated heap has 1161 // grown by the trigger ratio over the marked heap size. 1162 trigger := ^uint64(0) 1163 if gcPercent >= 0 { 1164 trigger = uint64(float64(c.heapMarked) * (1 + triggerRatio)) 1165 // Don't trigger below the minimum heap size. 1166 minTrigger := c.heapMinimum 1167 if !isSweepDone() { 1168 // Concurrent sweep happens in the heap growth 1169 // from gcController.heapLive to trigger, so ensure 1170 // that concurrent sweep has some heap growth 1171 // in which to perform sweeping before we 1172 // start the next GC cycle. 1173 sweepMin := atomic.Load64(&c.heapLive) + sweepMinHeapDistance 1174 if sweepMin > minTrigger { 1175 minTrigger = sweepMin 1176 } 1177 } 1178 if trigger < minTrigger { 1179 trigger = minTrigger 1180 } 1181 if int64(trigger) < 0 { 1182 print("runtime: heapGoal=", c.heapGoal, " heapMarked=", c.heapMarked, " gcController.heapLive=", c.heapLive, " initialHeapLive=", work.initialHeapLive, "triggerRatio=", triggerRatio, " minTrigger=", minTrigger, "\n") 1183 throw("trigger underflow") 1184 } 1185 if trigger > goal { 1186 // The trigger ratio is always less than GOGC/100, but 1187 // other bounds on the trigger may have raised it. 1188 // Push up the goal, too. 1189 goal = trigger 1190 } 1191 } 1192 1193 // Commit to the trigger and goal. 1194 c.trigger = trigger 1195 atomic.Store64(&c.heapGoal, goal) 1196 if trace.enabled { 1197 traceHeapGoal() 1198 } 1199 1200 // Update mark pacing. 1201 if gcphase != _GCoff { 1202 c.revise() 1203 } 1204 } 1205 1206 // effectiveGrowthRatio returns the current effective heap growth 1207 // ratio (GOGC/100) based on heapMarked from the previous GC and 1208 // heapGoal for the current GC. 1209 // 1210 // This may differ from gcPercent/100 because of various upper and 1211 // lower bounds on gcPercent. For example, if the heap is smaller than 1212 // heapMinimum, this can be higher than gcPercent/100. 1213 // 1214 // mheap_.lock must be held or the world must be stopped. 1215 func (c *gcControllerState) effectiveGrowthRatio() float64 { 1216 if !c.test { 1217 assertWorldStoppedOrLockHeld(&mheap_.lock) 1218 } 1219 1220 egogc := float64(atomic.Load64(&c.heapGoal)-c.heapMarked) / float64(c.heapMarked) 1221 if egogc < 0 { 1222 // Shouldn't happen, but just in case. 1223 egogc = 0 1224 } 1225 return egogc 1226 } 1227 1228 // setGCPercent updates gcPercent and all related pacer state. 1229 // Returns the old value of gcPercent. 1230 // 1231 // Calls gcControllerState.commit. 1232 // 1233 // The world must be stopped, or mheap_.lock must be held. 1234 func (c *gcControllerState) setGCPercent(in int32) int32 { 1235 if !c.test { 1236 assertWorldStoppedOrLockHeld(&mheap_.lock) 1237 } 1238 1239 out := c.gcPercent.Load() 1240 if in < 0 { 1241 in = -1 1242 } 1243 c.heapMinimum = defaultHeapMinimum * uint64(in) / 100 1244 c.gcPercent.Store(in) 1245 // Update pacing in response to gcPercent change. 1246 c.commit(c.triggerRatio) 1247 1248 return out 1249 } 1250 1251 //go:linkname setGCPercent runtime/debug.setGCPercent 1252 func setGCPercent(in int32) (out int32) { 1253 // Run on the system stack since we grab the heap lock. 1254 systemstack(func() { 1255 lock(&mheap_.lock) 1256 out = gcController.setGCPercent(in) 1257 gcPaceSweeper(gcController.trigger) 1258 gcPaceScavenger(gcController.heapGoal, gcController.lastHeapGoal) 1259 unlock(&mheap_.lock) 1260 }) 1261 1262 // If we just disabled GC, wait for any concurrent GC mark to 1263 // finish so we always return with no GC running. 1264 if in < 0 { 1265 gcWaitOnMark(atomic.Load(&work.cycles)) 1266 } 1267 1268 return out 1269 } 1270 1271 func readGOGC() int32 { 1272 p := gogetenv("GOGC") 1273 if p == "off" { 1274 return -1 1275 } 1276 if n, ok := atoi32(p); ok { 1277 return n 1278 } 1279 return 100 1280 } 1281 1282 type piController struct { 1283 kp float64 // Proportional constant. 1284 ti float64 // Integral time constant. 1285 tt float64 // Reset time. 1286 1287 min, max float64 // Output boundaries. 1288 1289 // PI controller state. 1290 1291 errIntegral float64 // Integral of the error from t=0 to now. 1292 1293 // Error flags. 1294 errOverflow bool // Set if errIntegral ever overflowed. 1295 inputOverflow bool // Set if an operation with the input overflowed. 1296 } 1297 1298 // next provides a new sample to the controller. 1299 // 1300 // input is the sample, setpoint is the desired point, and period is how much 1301 // time (in whatever unit makes the most sense) has passed since the last sample. 1302 // 1303 // Returns a new value for the variable it's controlling, and whether the operation 1304 // completed successfully. One reason this might fail is if error has been growing 1305 // in an unbounded manner, to the point of overflow. 1306 // 1307 // In the specific case of an error overflow occurs, the errOverflow field will be 1308 // set and the rest of the controller's internal state will be fully reset. 1309 func (c *piController) next(input, setpoint, period float64) (float64, bool) { 1310 // Compute the raw output value. 1311 prop := c.kp * (setpoint - input) 1312 rawOutput := prop + c.errIntegral 1313 1314 // Clamp rawOutput into output. 1315 output := rawOutput 1316 if isInf(output) || isNaN(output) { 1317 // The input had a large enough magnitude that either it was already 1318 // overflowed, or some operation with it overflowed. 1319 // Set a flag and reset. That's the safest thing to do. 1320 c.reset() 1321 c.inputOverflow = true 1322 return c.min, false 1323 } 1324 if output < c.min { 1325 output = c.min 1326 } else if output > c.max { 1327 output = c.max 1328 } 1329 1330 // Update the controller's state. 1331 if c.ti != 0 && c.tt != 0 { 1332 c.errIntegral += (c.kp*period/c.ti)*(setpoint-input) + (period/c.tt)*(output-rawOutput) 1333 if isInf(c.errIntegral) || isNaN(c.errIntegral) { 1334 // So much error has accumulated that we managed to overflow. 1335 // The assumptions around the controller have likely broken down. 1336 // Set a flag and reset. That's the safest thing to do. 1337 c.reset() 1338 c.errOverflow = true 1339 return c.min, false 1340 } 1341 } 1342 return output, true 1343 } 1344 1345 // reset resets the controller state, except for controller error flags. 1346 func (c *piController) reset() { 1347 c.errIntegral = 0 1348 } 1349