Merge pull request #457 from prometheus/beorn7/histogram

Lock-free atomic observations in Histograms!
This commit is contained in:
Björn Rabenstein 2018-09-12 15:04:00 +02:00 committed by GitHub
commit b5bfa0eb2c
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2 changed files with 164 additions and 19 deletions

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@ -16,7 +16,9 @@ package prometheus
import (
"fmt"
"math"
"runtime"
"sort"
"sync"
"sync/atomic"
"github.com/golang/protobuf/proto"
@ -200,28 +202,49 @@ func newHistogram(desc *Desc, opts HistogramOpts, labelValues ...string) Histogr
}
}
}
// Finally we know the final length of h.upperBounds and can make counts.
h.counts = make([]uint64, len(h.upperBounds))
// Finally we know the final length of h.upperBounds and can make counts
// for both states:
h.counts[0].buckets = make([]uint64, len(h.upperBounds))
h.counts[1].buckets = make([]uint64, len(h.upperBounds))
h.init(h) // Init self-collection.
return h
}
type histogram struct {
type histogramCounts struct {
// sumBits contains the bits of the float64 representing the sum of all
// observations. sumBits and count have to go first in the struct to
// guarantee alignment for atomic operations.
// http://golang.org/pkg/sync/atomic/#pkg-note-BUG
sumBits uint64
count uint64
buckets []uint64
}
type histogram struct {
selfCollector
// Note that there is no mutex required.
desc *Desc
writeMtx sync.Mutex // Only used in the Write method.
upperBounds []float64
counts []uint64
// Two counts, one is "hot" for lock-free observations, the other is
// "cold" for writing out a dto.Metric.
counts [2]histogramCounts
hotIdx int // Index of currently-hot counts. Only used within Write.
// This is a complicated one. For lock-free yet atomic observations, we
// need to save the total count of observations again, combined with the
// index of the currently-hot counts struct, so that we can perform the
// operation on both values atomically. The least significant bit
// defines the hot counts struct. The remaining 63 bits represent the
// total count of observations. This happens under the assumption that
// the 63bit count will never overflow. Rationale: An observations takes
// about 30ns. Let's assume it could happen in 10ns. Overflowing the
// counter will then take at least (2^63)*10ns, which is about 3000
// years.
countAndHotIdx uint64
labelPairs []*dto.LabelPair
}
@ -241,36 +264,113 @@ func (h *histogram) Observe(v float64) {
// 100 buckets: 78.1 ns/op linear - binary 54.9 ns/op
// 300 buckets: 154 ns/op linear - binary 61.6 ns/op
i := sort.SearchFloat64s(h.upperBounds, v)
if i < len(h.counts) {
atomic.AddUint64(&h.counts[i], 1)
// We increment h.countAndHotIdx by 2 so that the counter in the upper
// 63 bits gets incremented by 1. At the same time, we get the new value
// back, which we can use to find the currently-hot counts.
n := atomic.AddUint64(&h.countAndHotIdx, 2)
hotCounts := &h.counts[n%2]
if i < len(h.upperBounds) {
atomic.AddUint64(&hotCounts.buckets[i], 1)
}
atomic.AddUint64(&h.count, 1)
for {
oldBits := atomic.LoadUint64(&h.sumBits)
oldBits := atomic.LoadUint64(&hotCounts.sumBits)
newBits := math.Float64bits(math.Float64frombits(oldBits) + v)
if atomic.CompareAndSwapUint64(&h.sumBits, oldBits, newBits) {
if atomic.CompareAndSwapUint64(&hotCounts.sumBits, oldBits, newBits) {
break
}
}
// Increment count last as we take it as a signal that the observation
// is complete.
atomic.AddUint64(&hotCounts.count, 1)
}
func (h *histogram) Write(out *dto.Metric) error {
his := &dto.Histogram{}
buckets := make([]*dto.Bucket, len(h.upperBounds))
var (
his = &dto.Histogram{}
buckets = make([]*dto.Bucket, len(h.upperBounds))
hotCounts, coldCounts *histogramCounts
count uint64
)
his.SampleSum = proto.Float64(math.Float64frombits(atomic.LoadUint64(&h.sumBits)))
his.SampleCount = proto.Uint64(atomic.LoadUint64(&h.count))
var count uint64
// For simplicity, we mutex the rest of this method. It is not in the
// hot path, i.e. Observe is called much more often than Write. The
// complication of making Write lock-free isn't worth it.
h.writeMtx.Lock()
defer h.writeMtx.Unlock()
// This is a bit arcane, which is why the following spells out this if
// clause in English:
//
// If the currently-hot counts struct is #0, we atomically increment
// h.countAndHotIdx by 1 so that from now on Observe will use the counts
// struct #1. Furthermore, the atomic increment gives us the new value,
// which, in its most significant 63 bits, tells us the count of
// observations done so far up to and including currently ongoing
// observations still using the counts struct just changed from hot to
// cold. To have a normal uint64 for the count, we bitshift by 1 and
// save the result in count. We also set h.hotIdx to 1 for the next
// Write call, and we will refer to counts #1 as hotCounts and to counts
// #0 as coldCounts.
//
// If the currently-hot counts struct is #1, we do the corresponding
// things the other way round. We have to _decrement_ h.countAndHotIdx
// (which is a bit arcane in itself, as we have to express -1 with an
// unsigned int...).
if h.hotIdx == 0 {
count = atomic.AddUint64(&h.countAndHotIdx, 1) >> 1
h.hotIdx = 1
hotCounts = &h.counts[1]
coldCounts = &h.counts[0]
} else {
count = atomic.AddUint64(&h.countAndHotIdx, ^uint64(0)) >> 1 // Decrement.
h.hotIdx = 0
hotCounts = &h.counts[0]
coldCounts = &h.counts[1]
}
// Now we have to wait for the now-declared-cold counts to actually cool
// down, i.e. wait for all observations still using it to finish. That's
// the case once the count in the cold counts struct is the same as the
// one atomically retrieved from the upper 63bits of h.countAndHotIdx.
for {
if count == atomic.LoadUint64(&coldCounts.count) {
break
}
runtime.Gosched() // Let observations get work done.
}
his.SampleCount = proto.Uint64(count)
his.SampleSum = proto.Float64(math.Float64frombits(atomic.LoadUint64(&coldCounts.sumBits)))
var cumCount uint64
for i, upperBound := range h.upperBounds {
count += atomic.LoadUint64(&h.counts[i])
cumCount += atomic.LoadUint64(&coldCounts.buckets[i])
buckets[i] = &dto.Bucket{
CumulativeCount: proto.Uint64(count),
CumulativeCount: proto.Uint64(cumCount),
UpperBound: proto.Float64(upperBound),
}
}
his.Bucket = buckets
out.Histogram = his
out.Label = h.labelPairs
// Finally add all the cold counts to the new hot counts and reset the cold counts.
atomic.AddUint64(&hotCounts.count, count)
atomic.StoreUint64(&coldCounts.count, 0)
for {
oldBits := atomic.LoadUint64(&hotCounts.sumBits)
newBits := math.Float64bits(math.Float64frombits(oldBits) + his.GetSampleSum())
if atomic.CompareAndSwapUint64(&hotCounts.sumBits, oldBits, newBits) {
atomic.StoreUint64(&coldCounts.sumBits, 0)
break
}
}
for i := range h.upperBounds {
atomic.AddUint64(&hotCounts.buckets[i], atomic.LoadUint64(&coldCounts.buckets[i]))
atomic.StoreUint64(&coldCounts.buckets[i], 0)
}
return nil
}

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@ -17,6 +17,7 @@ import (
"math"
"math/rand"
"reflect"
"runtime"
"sort"
"sync"
"testing"
@ -346,3 +347,47 @@ func TestBuckets(t *testing.T) {
t.Errorf("linear buckets: got %v, want %v", got, want)
}
}
func TestHistogramAtomicObserve(t *testing.T) {
var (
quit = make(chan struct{})
his = NewHistogram(HistogramOpts{
Buckets: []float64{0.5, 10, 20},
})
)
defer func() { close(quit) }()
observe := func() {
for {
select {
case <-quit:
return
default:
his.Observe(1)
}
}
}
go observe()
go observe()
go observe()
for i := 0; i < 100; i++ {
m := &dto.Metric{}
if err := his.Write(m); err != nil {
t.Fatal("unexpected error writing histogram:", err)
}
h := m.GetHistogram()
if h.GetSampleCount() != uint64(h.GetSampleSum()) ||
h.GetSampleCount() != h.GetBucket()[1].GetCumulativeCount() ||
h.GetSampleCount() != h.GetBucket()[2].GetCumulativeCount() {
t.Fatalf(
"inconsistent counts in histogram: count=%d sum=%f buckets=[%d, %d]",
h.GetSampleCount(), h.GetSampleSum(),
h.GetBucket()[1].GetCumulativeCount(), h.GetBucket()[2].GetCumulativeCount(),
)
}
runtime.Gosched()
}
}