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runtime,runtime/metrics: use explicit histogram boundaries

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This change modifies the semantics of
runtime/metrics.Float64Histogram.Buckets to remove implicit buckets to
that extend to positive and negative infinity and instead defines all
bucket boundaries as explicitly listed.

Bucket boundaries remain the same as before except
/gc/heap/allocs-by-size:objects and /gc/heap/frees-by-size:objects no
longer have a bucket that extends to negative infinity.

This change simplifies the Float64Histogram API, making it both easier
to understand and easier to use.

Also, add a test for allocs-by-size and frees-by-size that checks them
against MemStats.

Fixes #43443.

Change-Id: I5620f15bd084562dadf288f733c4a8cace21910c
Reviewed-on: https://go-review.googlesource.com/c/go/+/281238
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Trust: Michael Knyszek <mknyszek@google.com>
This commit is contained in:
Michael Anthony Knyszek 2021-01-06 23:05:22 +00:00 committed by Michael Knyszek
parent a9ccd2d795
commit ae97717133
4 changed files with 99 additions and 30 deletions
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32
src/runtime/histogram.go
View File

@ -7,6 +7,7 @@ package runtime
import (
"runtime/internal/atomic"
"runtime/internal/sys"
"unsafe"
)
const (
@ -69,7 +70,13 @@ const (
// for concurrent use. It is also safe to read all the values
// atomically.
type timeHistogram struct {
counts [timeHistNumSuperBuckets * timeHistNumSubBuckets]uint64
counts [timeHistNumSuperBuckets * timeHistNumSubBuckets]uint64
// underflow counts all the times we got a negative duration
// sample. Because of how time works on some platforms, it's
// possible to measure negative durations. We could ignore them,
// but we record them anyway because it's better to have some
// signal that it's happening than just missing samples.
underflow uint64
}
@ -107,14 +114,30 @@ func (h *timeHistogram) record(duration int64) {
atomic.Xadd64(&h.counts[superBucket*timeHistNumSubBuckets+subBucket], 1)
}
const (
fInf = 0x7FF0000000000000
fNegInf = 0xFFF0000000000000
)
func float64Inf() float64 {
inf := uint64(fInf)
return *(*float64)(unsafe.Pointer(&inf))
}
func float64NegInf() float64 {
inf := uint64(fNegInf)
return *(*float64)(unsafe.Pointer(&inf))
}
// timeHistogramMetricsBuckets generates a slice of boundaries for
// the timeHistogram. These boundaries are represented in seconds,
// not nanoseconds like the timeHistogram represents durations.
func timeHistogramMetricsBuckets() []float64 {
b := make([]float64, timeHistTotalBuckets-1)
b := make([]float64, timeHistTotalBuckets+1)
b[0] = float64NegInf()
for i := 0; i < timeHistNumSuperBuckets; i++ {
superBucketMin := uint64(0)
// The (inclusive) minimum for the first bucket is 0.
// The (inclusive) minimum for the first non-negative bucket is 0.
if i > 0 {
// The minimum for the second bucket will be
// 1 << timeHistSubBucketBits, indicating that all
@ -141,8 +164,9 @@ func timeHistogramMetricsBuckets() []float64 {
// Convert the subBucketMin which is in nanoseconds to a float64 seconds value.
// These values will all be exactly representable by a float64.
b[i*timeHistNumSubBuckets+j] = float64(subBucketMin) / 1e9
b[i*timeHistNumSubBuckets+j+1] = float64(subBucketMin) / 1e9
}
}
b[len(b)-1] = float64Inf()
return b
}

32
src/runtime/metrics.go
View File

@ -41,8 +41,13 @@ func initMetrics() {
if metricsInit {
return
}
sizeClassBuckets = make([]float64, _NumSizeClasses)
for i := range sizeClassBuckets {
sizeClassBuckets = make([]float64, _NumSizeClasses, _NumSizeClasses+1)
// Skip size class 0 which is a stand-in for large objects, but large
// objects are tracked separately (and they actually get placed in
// the last bucket, not the first).
sizeClassBuckets[0] = 1 // The smallest allocation is 1 byte in size.
for i := 1; i < _NumSizeClasses; i++ {
// Size classes have an inclusive upper-bound
// and exclusive lower bound (e.g. 48-byte size class is
// (32, 48]) whereas we want and inclusive lower-bound
@ -56,6 +61,8 @@ func initMetrics() {
// boundaries.
sizeClassBuckets[i] = float64(class_to_size[i] + 1)
}
sizeClassBuckets = append(sizeClassBuckets, float64Inf())
timeHistBuckets = timeHistogramMetricsBuckets()
metrics = map[string]metricData{
"/gc/cycles/automatic:gc-cycles": {
@ -84,8 +91,10 @@ func initMetrics() {
compute: func(in *statAggregate, out *metricValue) {
hist := out.float64HistOrInit(sizeClassBuckets)
hist.counts[len(hist.counts)-1] = uint64(in.heapStats.largeAllocCount)
for i := range hist.buckets {
hist.counts[i] = uint64(in.heapStats.smallAllocCount[i])
// Cut off the first index which is ostensibly for size class 0,
// but large objects are tracked separately so it's actually unused.
for i, count := range in.heapStats.smallAllocCount[1:] {
hist.counts[i] = uint64(count)
}
},
},
@ -94,8 +103,10 @@ func initMetrics() {
compute: func(in *statAggregate, out *metricValue) {
hist := out.float64HistOrInit(sizeClassBuckets)
hist.counts[len(hist.counts)-1] = uint64(in.heapStats.largeFreeCount)
for i := range hist.buckets {
hist.counts[i] = uint64(in.heapStats.smallFreeCount[i])
// Cut off the first index which is ostensibly for size class 0,
// but large objects are tracked separately so it's actually unused.
for i, count := range in.heapStats.smallFreeCount[1:] {
hist.counts[i] = uint64(count)
}
},
},
@ -116,8 +127,11 @@ func initMetrics() {
"/gc/pauses:seconds": {
compute: func(_ *statAggregate, out *metricValue) {
hist := out.float64HistOrInit(timeHistBuckets)
// The bottom-most bucket, containing negative values, is tracked
// as a separately as underflow, so fill that in manually and then
// iterate over the rest.
hist.counts[0] = atomic.Load64(&memstats.gcPauseDist.underflow)
for i := range hist.buckets {
for i := range memstats.gcPauseDist.counts {
hist.counts[i+1] = atomic.Load64(&memstats.gcPauseDist.counts[i])
}
},
@ -437,8 +451,8 @@ func (v *metricValue) float64HistOrInit(buckets []float64) *metricFloat64Histogr
v.pointer = unsafe.Pointer(hist)
}
hist.buckets = buckets
if len(hist.counts) != len(hist.buckets)+1 {
hist.counts = make([]uint64, len(buckets)+1)
if len(hist.counts) != len(hist.buckets)-1 {
hist.counts = make([]uint64, len(buckets)-1)
}
return hist
}

29
src/runtime/metrics/histogram.go
View File

@ -6,25 +6,28 @@ package metrics
// Float64Histogram represents a distribution of float64 values.
type Float64Histogram struct {
// Counts contains the weights for each histogram bucket. The length of
// Counts is equal to the length of Buckets (in the metric description)
// plus one to account for the implicit minimum bucket.
// Counts contains the weights for each histogram bucket.
//
// Given N buckets, the following is the mathematical relationship between
// Counts and Buckets.
// count[0] is the weight of the range (-inf, bucket[0])
// count[n] is the weight of the range [bucket[n], bucket[n+1]), for 0 < n < N-1
// count[N-1] is the weight of the range [bucket[N-1], inf)
// Given N buckets, Count[n] is the weight of the range
// [bucket[n], bucket[n+1]), for 0 <= n < N.
Counts []uint64
// Buckets contains the boundaries between histogram buckets, in increasing order.
// Buckets contains the boundaries of the histogram buckets, in increasing order.
//
// Because this slice contains boundaries, there are len(Buckets)+1 counts:
// a count for all values less than the first boundary, a count covering each
// [slice[i], slice[i+1]) interval, and a count for all values greater than or
// equal to the last boundary.
// Buckets[0] is the inclusive lower bound of the minimum bucket while
// Buckets[len(Buckets)-1] is the exclusive upper bound of the maximum bucket.
// Hence, there are len(Buckets)-1 counts. Furthermore, len(Buckets) != 1, always,
// since at least two boundaries are required to describe one bucket (and 0
// boundaries are used to describe 0 buckets).
//
// Buckets[0] is permitted to have value -Inf and Buckets[len(Buckets)-1] is
// permitted to have value Inf.
//
// For a given metric name, the value of Buckets is guaranteed not to change
// between calls until program exit.
//
// This slice value is permitted to alias with other Float64Histograms' Buckets
// fields, so the values within should only ever be read. If they need to be
// modified, the user must make a copy.
Buckets []float64
}

36
src/runtime/metrics_test.go
View File

@ -70,6 +70,34 @@ func TestReadMetrics(t *testing.T) {
checkUint64(t, name, samples[i].Value.Uint64(), mstats.BuckHashSys)
case "/memory/classes/total:bytes":
checkUint64(t, name, samples[i].Value.Uint64(), mstats.Sys)
case "/gc/heap/allocs-by-size:objects":
hist := samples[i].Value.Float64Histogram()
// Skip size class 0 in BySize, because it's always empty and not represented
// in the histogram.
for i, sc := range mstats.BySize[1:] {
if b, s := hist.Buckets[i+1], float64(sc.Size+1); b != s {
t.Errorf("bucket does not match size class: got %f, want %f", b, s)
// The rest of the checks aren't expected to work anyway.
continue
}
if c, m := hist.Counts[i], sc.Mallocs; c != m {
t.Errorf("histogram counts do not much BySize for class %d: got %d, want %d", i, c, m)
}
}
case "/gc/heap/frees-by-size:objects":
hist := samples[i].Value.Float64Histogram()
// Skip size class 0 in BySize, because it's always empty and not represented
// in the histogram.
for i, sc := range mstats.BySize[1:] {
if b, s := hist.Buckets[i+1], float64(sc.Size+1); b != s {
t.Errorf("bucket does not match size class: got %f, want %f", b, s)
// The rest of the checks aren't expected to work anyway.
continue
}
if c, f := hist.Counts[i], sc.Frees; c != f {
t.Errorf("histogram counts do not much BySize for class %d: got %d, want %d", i, c, f)
}
}
case "/gc/heap/objects:objects":
checkUint64(t, name, samples[i].Value.Uint64(), mstats.HeapObjects)
case "/gc/heap/goal:bytes":
@ -154,11 +182,11 @@ func TestReadMetricsConsistency(t *testing.T) {
if totalVirtual.got != totalVirtual.want {
t.Errorf(`"/memory/classes/total:bytes" does not match sum of /memory/classes/**: got %d, want %d`, totalVirtual.got, totalVirtual.want)
}
if objects.alloc.Counts[0] > 0 {
t.Error("found counts for objects of non-positive size in allocs-by-size")
if b, c := len(objects.alloc.Buckets), len(objects.alloc.Counts); b != c+1 {
t.Errorf("allocs-by-size has wrong bucket or counts length: %d buckets, %d counts", b, c)
}
if objects.free.Counts[0] > 0 {
t.Error("found counts for objects of non-positive size in frees-by-size")
if b, c := len(objects.free.Buckets), len(objects.free.Counts); b != c+1 {
t.Errorf("frees-by-size has wrong bucket or counts length: %d buckets, %d counts", b, c)
}
if len(objects.alloc.Buckets) != len(objects.free.Buckets) {
t.Error("allocs-by-size and frees-by-size buckets don't match in length")