From: Michael Anthony Knyszek Date: Thu, 6 Aug 2020 20:36:49 +0000 (+0000) Subject: runtime: add timeHistogram type X-Git-Tag: go1.16beta1~506 X-Git-Url: http://www.git.cypherpunks.su/?a=commitdiff_plain;h=36c5edd8d9e6c13af26733e5c820eae0598203fe;p=gostls13.git runtime: add timeHistogram type This change adds a concurrent HDR time histogram to the runtime with tests. It also adds a function to generate boundaries for use by the metrics package. For #37112. Change-Id: Ifbef8ddce8e3a965a0dcd58ccd4915c282ae2098 Reviewed-on: https://go-review.googlesource.com/c/go/+/247046 Run-TryBot: Michael Knyszek TryBot-Result: Go Bot Trust: Michael Knyszek Reviewed-by: Michael Pratt --- diff --git a/src/runtime/export_test.go b/src/runtime/export_test.go index d043fe3ee5..4ca0420d2a 100644 --- a/src/runtime/export_test.go +++ b/src/runtime/export_test.go @@ -1141,3 +1141,27 @@ func MSpanCountAlloc(ms *MSpan, bits []byte) int { s.gcmarkBits = nil return result } + +const ( + TimeHistSubBucketBits = timeHistSubBucketBits + TimeHistNumSubBuckets = timeHistNumSubBuckets + TimeHistNumSuperBuckets = timeHistNumSuperBuckets +) + +type TimeHistogram timeHistogram + +// Counts returns the counts for the given bucket, subBucket indices. +// Returns true if the bucket was valid, otherwise returns the counts +// for the overflow bucket and false. +func (th *TimeHistogram) Count(bucket, subBucket uint) (uint64, bool) { + t := (*timeHistogram)(th) + i := bucket*TimeHistNumSubBuckets + subBucket + if i >= uint(len(t.counts)) { + return t.overflow, false + } + return t.counts[i], true +} + +func (th *TimeHistogram) Record(duration int64) { + (*timeHistogram)(th).record(duration) +} diff --git a/src/runtime/histogram.go b/src/runtime/histogram.go new file mode 100644 index 0000000000..4020969eb9 --- /dev/null +++ b/src/runtime/histogram.go @@ -0,0 +1,148 @@ +// Copyright 2020 The Go Authors. All rights reserved. +// Use of this source code is governed by a BSD-style +// license that can be found in the LICENSE file. + +package runtime + +import ( + "runtime/internal/atomic" + "runtime/internal/sys" +) + +const ( + // For the time histogram type, we use an HDR histogram. + // Values are placed in super-buckets based solely on the most + // significant set bit. Thus, super-buckets are power-of-2 sized. + // Values are then placed into sub-buckets based on the value of + // the next timeHistSubBucketBits most significant bits. Thus, + // sub-buckets are linear within a super-bucket. + // + // Therefore, the number of sub-buckets (timeHistNumSubBuckets) + // defines the error. This error may be computed as + // 1/timeHistNumSubBuckets*100%. For example, for 16 sub-buckets + // per super-bucket the error is approximately 6%. + // + // The number of super-buckets (timeHistNumSuperBuckets), on the + // other hand, defines the range. To reserve room for sub-buckets, + // bit timeHistSubBucketBits is the first bit considered for + // super-buckets, so super-bucket indicies are adjusted accordingly. + // + // As an example, consider 45 super-buckets with 16 sub-buckets. + // + // 00110 + // ^---- + // │ ^ + // │ └---- Lowest 4 bits -> sub-bucket 6 + // └------- Bit 4 unset -> super-bucket 0 + // + // 10110 + // ^---- + // │ ^ + // │ └---- Next 4 bits -> sub-bucket 6 + // └------- Bit 4 set -> super-bucket 1 + // 100010 + // ^----^ + // │ ^ └-- Lower bits ignored + // │ └---- Next 4 bits -> sub-bucket 1 + // └------- Bit 5 set -> super-bucket 2 + // + // Following this pattern, bucket 45 will have the bit 48 set. We don't + // have any buckets for higher values, so the highest sub-bucket will + // contain values of 2^48-1 nanoseconds or approx. 3 days. This range is + // more than enough to handle durations produced by the runtime. + timeHistSubBucketBits = 4 + timeHistNumSubBuckets = 1 << timeHistSubBucketBits + timeHistNumSuperBuckets = 45 + timeHistTotalBuckets = timeHistNumSuperBuckets*timeHistNumSubBuckets + 1 +) + +// timeHistogram represents a distribution of durations in +// nanoseconds. +// +// The accuracy and range of the histogram is defined by the +// timeHistSubBucketBits and timeHistNumSuperBuckets constants. +// +// It is an HDR histogram with exponentially-distributed +// buckets and linearly distributed sub-buckets. +// +// Counts in the histogram are updated atomically, so it is safe +// for concurrent use. It is also safe to read all the values +// atomically. +type timeHistogram struct { + counts [timeHistNumSuperBuckets * timeHistNumSubBuckets]uint64 + overflow uint64 +} + +// record adds the given duration to the distribution. +// +// Although the duration is an int64 to facilitate ease-of-use +// with e.g. nanotime, the duration must be non-negative. +func (h *timeHistogram) record(duration int64) { + if duration < 0 { + throw("timeHistogram encountered negative duration") + } + // The index of the exponential bucket is just the index + // of the highest set bit adjusted for how many bits we + // use for the subbucket. Note that it's timeHistSubBucketsBits-1 + // because we use the 0th bucket to hold values < timeHistNumSubBuckets. + var superBucket, subBucket uint + if duration >= timeHistNumSubBuckets { + // At this point, we know the duration value will always be + // at least timeHistSubBucketsBits long. + superBucket = uint(sys.Len64(uint64(duration))) - timeHistSubBucketBits + if superBucket*timeHistNumSubBuckets >= uint(len(h.counts)) { + // The bucket index we got is larger than what we support, so + // add into the special overflow bucket. + atomic.Xadd64(&h.overflow, 1) + return + } + // The linear subbucket index is just the timeHistSubBucketsBits + // bits after the top bit. To extract that value, shift down + // the duration such that we leave the top bit and the next bits + // intact, then extract the index. + subBucket = uint((duration >> (superBucket - 1)) % timeHistNumSubBuckets) + } else { + subBucket = uint(duration) + } + atomic.Xadd64(&h.counts[superBucket*timeHistNumSubBuckets+subBucket], 1) +} + +// 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) + for i := 0; i < timeHistNumSuperBuckets; i++ { + superBucketMin := uint64(0) + // The (inclusive) minimum for the first bucket is 0. + if i > 0 { + // The minimum for the second bucket will be + // 1 << timeHistSubBucketBits, indicating that all + // sub-buckets are represented by the next timeHistSubBucketBits + // bits. + // Thereafter, we shift up by 1 each time, so we can represent + // this pattern as (i-1)+timeHistSubBucketBits. + superBucketMin = uint64(1) << uint(i-1+timeHistSubBucketBits) + } + // subBucketShift is the amount that we need to shift the sub-bucket + // index to combine it with the bucketMin. + subBucketShift := uint(0) + if i > 1 { + // The first two buckets are exact with respect to integers, + // so we'll never have to shift the sub-bucket index. Thereafter, + // we shift up by 1 with each subsequent bucket. + subBucketShift = uint(i - 2) + } + for j := 0; j < timeHistNumSubBuckets; j++ { + // j is the sub-bucket index. By shifting the index into position to + // combine with the bucket minimum, we obtain the minimum value for that + // sub-bucket. + subBucketMin := superBucketMin + (uint64(j) << subBucketShift) + + // 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 + } + } + return b +} diff --git a/src/runtime/histogram_test.go b/src/runtime/histogram_test.go new file mode 100644 index 0000000000..5f5b28f784 --- /dev/null +++ b/src/runtime/histogram_test.go @@ -0,0 +1,58 @@ +// Copyright 2020 The Go Authors. All rights reserved. +// Use of this source code is governed by a BSD-style +// license that can be found in the LICENSE file. + +package runtime_test + +import ( + . "runtime" + "testing" +) + +var dummyTimeHistogram TimeHistogram + +func TestTimeHistogram(t *testing.T) { + // We need to use a global dummy because this + // could get stack-allocated with a non-8-byte alignment. + // The result of this bad alignment is a segfault on + // 32-bit platforms when calling Record. + h := &dummyTimeHistogram + + // Record exactly one sample in each bucket. + for i := 0; i < TimeHistNumSuperBuckets; i++ { + var base int64 + if i > 0 { + base = int64(1) << (i + TimeHistSubBucketBits - 1) + } + for j := 0; j < TimeHistNumSubBuckets; j++ { + v := int64(j) + if i > 0 { + v <<= i - 1 + } + h.Record(base + v) + } + } + // Hit the overflow bucket. + h.Record(int64(^uint64(0) >> 1)) + + // Check to make sure there's exactly one count in each + // bucket. + for i := uint(0); i < TimeHistNumSuperBuckets; i++ { + for j := uint(0); j < TimeHistNumSubBuckets; j++ { + c, ok := h.Count(i, j) + if !ok { + t.Errorf("hit overflow bucket unexpectedly: (%d, %d)", i, j) + } else if c != 1 { + t.Errorf("bucket (%d, %d) has count that is not 1: %d", i, j, c) + } + } + } + c, ok := h.Count(TimeHistNumSuperBuckets, 0) + if ok { + t.Errorf("expected to hit overflow bucket: (%d, %d)", TimeHistNumSuperBuckets, 0) + } + if c != 1 { + t.Errorf("overflow bucket has count that is not 1: %d", c) + } + dummyTimeHistogram = TimeHistogram{} +} diff --git a/src/runtime/metrics.go b/src/runtime/metrics.go index 32d8ab461c..2be38ccaaa 100644 --- a/src/runtime/metrics.go +++ b/src/runtime/metrics.go @@ -20,6 +20,7 @@ var ( metrics map[string]metricData sizeClassBuckets []float64 + timeHistBuckets []float64 ) type metricData struct { @@ -44,6 +45,7 @@ func initMetrics() { for i := range sizeClassBuckets { sizeClassBuckets[i] = float64(class_to_size[i]) } + timeHistBuckets = timeHistogramMetricsBuckets() metrics = map[string]metricData{ "/gc/cycles/automatic:gc-cycles": { deps: makeStatDepSet(sysStatsDep),