base.go

Documentation: cmd/compile/internal/base

     1  // Copyright 2009 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 base
     6  
     7  import (
     8  	"fmt"
     9  	"os"
    10  	"runtime"
    11  	"runtime/debug"
    12  	"runtime/metrics"
    13  )
    14  
    15  var atExitFuncs []func()
    16  
    17  func AtExit(f func()) {
    18  	atExitFuncs = append(atExitFuncs, f)
    19  }
    20  
    21  func Exit(code int) {
    22  	for i := len(atExitFuncs) - 1; i >= 0; i-- {
    23  		f := atExitFuncs[i]
    24  		atExitFuncs = atExitFuncs[:i]
    25  		f()
    26  	}
    27  	os.Exit(code)
    28  }
    29  
    30  // To enable tracing support (-t flag), set EnableTrace to true.
    31  const EnableTrace = false
    32  
    33  // forEachGC calls fn each GC cycle until it returns false.
    34  func forEachGC(fn func() bool) {
    35  	type T [32]byte // large enough to avoid runtime's tiny object allocator
    36  
    37  	var finalizer func(*T)
    38  	finalizer = func(p *T) {
    39  		if fn() {
    40  			runtime.SetFinalizer(p, finalizer)
    41  		}
    42  	}
    43  
    44  	finalizer(new(T))
    45  }
    46  
    47  // AdjustStartingHeap modifies GOGC so that GC should not occur until the heap
    48  // grows to the requested size.  This is intended but not promised, though it
    49  // is true-mostly, depending on when the adjustment occurs and on the
    50  // compiler's input and behavior.  Once this size is approximately reached
    51  // GOGC is reset to 100; subsequent GCs may reduce the heap below the requested
    52  // size, but this function does not affect that.
    53  //
    54  // -d=gcadjust=1 enables logging of GOGC adjustment events.
    55  //
    56  // NOTE: If you think this code would help startup time in your own
    57  // application and you decide to use it, please benchmark first to see if it
    58  // actually works for you (it may not: the Go compiler is not typical), and
    59  // whatever the outcome, please leave a comment on bug #56546.  This code
    60  // uses supported interfaces, but depends more than we like on
    61  // current+observed behavior of the garbage collector, so if many people need
    62  // this feature, we should consider/propose a better way to accomplish it.
    63  func AdjustStartingHeap(requestedHeapGoal uint64) {
    64  	logHeapTweaks := Debug.GCAdjust == 1
    65  	mp := runtime.GOMAXPROCS(0)
    66  	gcConcurrency := Flag.LowerC
    67  
    68  	const (
    69  		goal   = "/gc/heap/goal:bytes"
    70  		count  = "/gc/cycles/total:gc-cycles"
    71  		allocs = "/gc/heap/allocs:bytes"
    72  		frees  = "/gc/heap/frees:bytes"
    73  	)
    74  
    75  	sample := []metrics.Sample{{Name: goal}, {Name: count}, {Name: allocs}, {Name: frees}}
    76  	const (
    77  		GOAL   = 0
    78  		COUNT  = 1
    79  		ALLOCS = 2
    80  		FREES  = 3
    81  	)
    82  
    83  	// Assumptions and observations of Go's garbage collector, as of Go 1.17-1.20:
    84  
    85  	// - the initial heap goal is 4M, by fiat.  It is possible for Go to start
    86  	//   with a heap as small as 512k, so this may change in the future.
    87  
    88  	// - except for the first heap goal, heap goal is a function of
    89  	//   observed-live at the previous GC and current GOGC.  After the first
    90  	//   GC, adjusting GOGC immediately updates GOGC; before the first GC,
    91  	//   adjusting GOGC does not modify goal (but the change takes effect after
    92  	//   the first GC).
    93  
    94  	// - the before/after first GC behavior is not guaranteed anywhere, it's
    95  	//   just behavior, and it's a bad idea to rely on it.
    96  
    97  	// - we don't know exactly when GC will run, even after we adjust GOGC; the
    98  	//   first GC may not have happened yet, may have already happened, or may
    99  	//   be currently in progress, and GCs can start for several reasons.
   100  
   101  	// - forEachGC above will run the provided function at some delay after each
   102  	//   GC's mark phase terminates; finalizers are run after marking as the
   103  	//   spans containing finalizable objects are swept, driven by GC
   104  	//   background activity and allocation demand.
   105  
   106  	// - "live at last GC" is not available through the current metrics
   107  	//    interface. Instead, live is estimated by knowing the adjusted value of
   108  	//    GOGC and the new heap goal following a GC (this requires knowing that
   109  	//    at least one GC has occurred):
   110  	//		  estLive = 100 * newGoal / (100 + currentGogc)
   111  	//    this new value of GOGC
   112  	//		  newGogc = 100*requestedHeapGoal/estLive - 100
   113  	//    will result in the desired goal. The logging code checks that the
   114  	//    resulting goal is correct.
   115  
   116  	// There's a small risk that the finalizer will be slow to run after a GC
   117  	// that expands the goal to a huge value, and that this will lead to
   118  	// out-of-memory.  This doesn't seem to happen; in experiments on a variety
   119  	// of machines with a variety of extra loads to disrupt scheduling, the
   120  	// worst overshoot observed was 50% past requestedHeapGoal.
   121  
   122  	metrics.Read(sample)
   123  	for _, s := range sample {
   124  		if s.Value.Kind() == metrics.KindBad {
   125  			// Just return, a slightly slower compilation is a tolerable outcome.
   126  			if logHeapTweaks {
   127  				fmt.Fprintf(os.Stderr, "GCAdjust: Regret unexpected KindBad for metric %s\n", s.Name)
   128  			}
   129  			return
   130  		}
   131  	}
   132  
   133  	// Tinker with GOGC to make the heap grow rapidly at first.
   134  	currentGoal := sample[GOAL].Value.Uint64() // Believe this will be 4MByte or less, perhaps 512k
   135  	myGogc := 100 * requestedHeapGoal / currentGoal
   136  	if myGogc <= 150 {
   137  		return
   138  	}
   139  
   140  	if logHeapTweaks {
   141  		sample := append([]metrics.Sample(nil), sample...) // avoid races with GC callback
   142  		AtExit(func() {
   143  			metrics.Read(sample)
   144  			goal := sample[GOAL].Value.Uint64()
   145  			count := sample[COUNT].Value.Uint64()
   146  			oldGogc := debug.SetGCPercent(100)
   147  			if oldGogc == 100 {
   148  				fmt.Fprintf(os.Stderr, "GCAdjust: AtExit goal %d gogc %d count %d maxprocs %d gcConcurrency %d\n",
   149  					goal, oldGogc, count, mp, gcConcurrency)
   150  			} else {
   151  				inUse := sample[ALLOCS].Value.Uint64() - sample[FREES].Value.Uint64()
   152  				overPct := 100 * (int(inUse) - int(requestedHeapGoal)) / int(requestedHeapGoal)
   153  				fmt.Fprintf(os.Stderr, "GCAdjust: AtExit goal %d gogc %d count %d maxprocs %d gcConcurrency %d overPct %d\n",
   154  					goal, oldGogc, count, mp, gcConcurrency, overPct)
   155  
   156  			}
   157  		})
   158  	}
   159  
   160  	debug.SetGCPercent(int(myGogc))
   161  
   162  	adjustFunc := func() bool {
   163  
   164  		metrics.Read(sample)
   165  		goal := sample[GOAL].Value.Uint64()
   166  		count := sample[COUNT].Value.Uint64()
   167  
   168  		if goal <= requestedHeapGoal { // Stay the course
   169  			if logHeapTweaks {
   170  				fmt.Fprintf(os.Stderr, "GCAdjust: Reuse GOGC adjust, current goal %d, count is %d, current gogc %d\n",
   171  					goal, count, myGogc)
   172  			}
   173  			return true
   174  		}
   175  
   176  		// Believe goal has been adjusted upwards, else it would be less-than-or-equal than requestedHeapGoal
   177  		calcLive := 100 * goal / (100 + myGogc)
   178  
   179  		if 2*calcLive < requestedHeapGoal { // calcLive can exceed requestedHeapGoal!
   180  			myGogc = 100*requestedHeapGoal/calcLive - 100
   181  
   182  			if myGogc > 125 {
   183  				// Not done growing the heap.
   184  				oldGogc := debug.SetGCPercent(int(myGogc))
   185  
   186  				if logHeapTweaks {
   187  					// Check that the new goal looks right
   188  					inUse := sample[ALLOCS].Value.Uint64() - sample[FREES].Value.Uint64()
   189  					metrics.Read(sample)
   190  					newGoal := sample[GOAL].Value.Uint64()
   191  					pctOff := 100 * (int64(newGoal) - int64(requestedHeapGoal)) / int64(requestedHeapGoal)
   192  					// Check that the new goal is close to requested.  3% of make.bash fails this test.  Why, TBD.
   193  					if pctOff < 2 {
   194  						fmt.Fprintf(os.Stderr, "GCAdjust: Retry GOGC adjust, current goal %d, count is %d, gogc was %d, is now %d, calcLive %d pctOff %d\n",
   195  							goal, count, oldGogc, myGogc, calcLive, pctOff)
   196  					} else {
   197  						// The GC is being annoying and not giving us the goal that we requested, say more to help understand when/why.
   198  						fmt.Fprintf(os.Stderr, "GCAdjust: Retry GOGC adjust, current goal %d, count is %d, gogc was %d, is now %d, calcLive %d pctOff %d inUse %d\n",
   199  							goal, count, oldGogc, myGogc, calcLive, pctOff, inUse)
   200  					}
   201  				}
   202  				return true
   203  			}
   204  		}
   205  
   206  		// In this case we're done boosting GOGC, set it to 100 and don't set a new finalizer.
   207  		oldGogc := debug.SetGCPercent(100)
   208  		// inUse helps estimate how late the finalizer ran; at the instant the previous GC ended,
   209  		// it was (in theory) equal to the previous GC's heap goal.  In a growing heap it is
   210  		// expected to grow to the new heap goal.
   211  		inUse := sample[ALLOCS].Value.Uint64() - sample[FREES].Value.Uint64()
   212  		overPct := 100 * (int(inUse) - int(requestedHeapGoal)) / int(requestedHeapGoal)
   213  		if logHeapTweaks {
   214  			fmt.Fprintf(os.Stderr, "GCAdjust: Reset GOGC adjust, old goal %d, count is %d, gogc was %d, calcLive %d inUse %d overPct %d\n",
   215  				goal, count, oldGogc, calcLive, inUse, overPct)
   216  		}
   217  		return false
   218  	}
   219  
   220  	forEachGC(adjustFunc)
   221  }
   222
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