From 1ba25fa28871cd3862f8d539155a60e004b42d56 Mon Sep 17 00:00:00 2001 From: Michael Anthony Knyszek Date: Fri, 21 Jan 2022 06:52:43 +0000 Subject: [PATCH] [release-branch.go1.17] runtime: simplify histogram buckets considerably There was an off-by-one error in the time histogram buckets calculation that caused the linear sub-buckets distances to be off by 2x. The fix was trivial, but in writing tests I realized there was a much simpler way to express the calculation for the histogram buckets, and took the opportunity to do that here. The new bucket calculation also fixes the bug. For #50732. Fixes #50734. Change-Id: Idae89986de1c415ee4e148f778e0e101ca003ade Reviewed-on: https://go-review.googlesource.com/c/go/+/380094 Reviewed-by: Michael Pratt Reviewed-by: Emmanuel Odeke Trust: Michael Knyszek Run-TryBot: Michael Knyszek (cherry picked from commit 2e9dcb508647dc473a37ecfa244d2bc4a1843ab4) Reviewed-on: https://go-review.googlesource.com/c/go/+/384621 TryBot-Result: Gopher Robot --- src/runtime/export_test.go | 2 ++ src/runtime/histogram.go | 52 ++++++++++++++++------------------- src/runtime/histogram_test.go | 40 +++++++++++++++++++++++++++ 3 files changed, 65 insertions(+), 29 deletions(-) diff --git a/src/runtime/export_test.go b/src/runtime/export_test.go index b49bc50f97..03ccfe10a8 100644 --- a/src/runtime/export_test.go +++ b/src/runtime/export_test.go @@ -1217,6 +1217,8 @@ func (th *TimeHistogram) Record(duration int64) { (*timeHistogram)(th).record(duration) } +var TimeHistogramMetricsBuckets = timeHistogramMetricsBuckets + func SetIntArgRegs(a int) int { lock(&finlock) old := intArgRegs diff --git a/src/runtime/histogram.go b/src/runtime/histogram.go index 0cccbcca16..cd7e29a8c8 100644 --- a/src/runtime/histogram.go +++ b/src/runtime/histogram.go @@ -47,7 +47,7 @@ const ( // │ └---- 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 + // Following this pattern, super-bucket 44 will have the bit 47 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. @@ -139,36 +139,30 @@ func float64NegInf() float64 { func timeHistogramMetricsBuckets() []float64 { b := make([]float64, timeHistTotalBuckets+1) b[0] = float64NegInf() - for i := 0; i < timeHistNumSuperBuckets; i++ { - superBucketMin := uint64(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 - // 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 super 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) - } + // Super-bucket 0 has no bits above timeHistSubBucketBits + // set, so just iterate over each bucket and assign the + // incrementing bucket. + for i := 0; i < timeHistNumSubBuckets; i++ { + bucketNanos := uint64(i) + b[i+1] = float64(bucketNanos) / 1e9 + } + // Generate the rest of the super-buckets. It's easier to reason + // about if we cut out the 0'th bucket, so subtract one since + // we just handled that bucket. + for i := 0; i < timeHistNumSuperBuckets-1; i++ { 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. + // Set the super-bucket bit. + bucketNanos := uint64(1) << (i + timeHistSubBucketBits) + // Set the sub-bucket bits. + bucketNanos |= uint64(j) << i + // The index for this bucket is going to be the (i+1)'th super bucket + // (note that we're starting from zero, but handled the first super-bucket + // earlier, so we need to compensate), and the j'th sub bucket. + // Add 1 because we left space for -Inf. + bucketIndex := (i+1)*timeHistNumSubBuckets + j + 1 + // Convert nanoseconds to seconds via a division. // These values will all be exactly representable by a float64. - b[i*timeHistNumSubBuckets+j+1] = float64(subBucketMin) / 1e9 + b[bucketIndex] = float64(bucketNanos) / 1e9 } } b[len(b)-1] = float64Inf() diff --git a/src/runtime/histogram_test.go b/src/runtime/histogram_test.go index dbc64fa559..b12b65a41e 100644 --- a/src/runtime/histogram_test.go +++ b/src/runtime/histogram_test.go @@ -68,3 +68,43 @@ func TestTimeHistogram(t *testing.T) { dummyTimeHistogram = TimeHistogram{} } + +func TestTimeHistogramMetricsBuckets(t *testing.T) { + buckets := TimeHistogramMetricsBuckets() + + nonInfBucketsLen := TimeHistNumSubBuckets * TimeHistNumSuperBuckets + expBucketsLen := nonInfBucketsLen + 2 // Count -Inf and +Inf. + if len(buckets) != expBucketsLen { + t.Fatalf("unexpected length of buckets: got %d, want %d", len(buckets), expBucketsLen) + } + // Check the first non-Inf 2*TimeHistNumSubBuckets buckets in order, skipping the + // first bucket which should be -Inf (checked later). + // + // Because of the way this scheme works, the bottom TimeHistNumSubBuckets + // buckets are fully populated, and then the next TimeHistNumSubBuckets + // have the TimeHistSubBucketBits'th bit set, while the bottom are once + // again fully populated. + for i := 1; i <= 2*TimeHistNumSubBuckets+1; i++ { + if got, want := buckets[i], float64(i-1)/1e9; got != want { + t.Errorf("expected bucket %d to have value %e, got %e", i, want, got) + } + } + // Check some values. + idxToBucket := map[int]float64{ + 0: math.Inf(-1), + 33: float64(0x10<<1) / 1e9, + 34: float64(0x11<<1) / 1e9, + 49: float64(0x10<<2) / 1e9, + 58: float64(0x19<<2) / 1e9, + 65: float64(0x10<<3) / 1e9, + 513: float64(0x10<<31) / 1e9, + 519: float64(0x16<<31) / 1e9, + expBucketsLen - 2: float64(0x1f<<43) / 1e9, + expBucketsLen - 1: math.Inf(1), + } + for idx, bucket := range idxToBucket { + if got, want := buckets[idx], bucket; got != want { + t.Errorf("expected bucket %d to have value %e, got %e", idx, want, got) + } + } +}