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Ensure 64bit alignment on 32bit platforms for histograms 

Signed-off-by: beorn7 <beorn@soundcloud.com>
This commit is contained in:
beorn7 2018-09-24 13:28:13 +02:00
parent 73edb9af66
commit 7973e4709d
1 changed files with 25 additions and 20 deletions
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45
prometheus/histogram.go
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@ -187,6 +187,7 @@ func newHistogram(desc *Desc, opts HistogramOpts, labelValues ...string) Histogr
desc: desc,
upperBounds: opts.Buckets,
labelPairs: makeLabelPairs(desc, labelValues),
counts: [2]*histogramCounts{&histogramCounts{}, &histogramCounts{}},
}
for i, upperBound := range h.upperBounds {
if i < len(h.upperBounds)-1 {
@ -223,6 +224,21 @@ type histogramCounts struct {
}
type histogram struct {
// countAndHotIdx 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.
//
// This has to be first in the struct for 64bit alignment. See
// http://golang.org/pkg/sync/atomic/#pkg-note-BUG
countAndHotIdx uint64
selfCollector
desc *Desc
writeMtx sync.Mutex // Only used in the Write method.
@ -230,23 +246,12 @@ type histogram struct {
upperBounds []float64
// Two counts, one is "hot" for lock-free observations, the other is
// "cold" for writing out a dto.Metric.
counts [2]histogramCounts
// "cold" for writing out a dto.Metric. It has to be an array of
// pointers to guarantee 64bit alignment of the histogramCounts, see
// http://golang.org/pkg/sync/atomic/#pkg-note-BUG.
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
}
@ -270,7 +275,7 @@ func (h *histogram) Observe(v float64) {
// 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]
hotCounts := h.counts[n%2]
if i < len(h.upperBounds) {
atomic.AddUint64(&hotCounts.buckets[i], 1)
@ -322,13 +327,13 @@ func (h *histogram) Write(out *dto.Metric) error {
if h.hotIdx == 0 {
count = atomic.AddUint64(&h.countAndHotIdx, 1) >> 1
h.hotIdx = 1
hotCounts = &h.counts[1]
coldCounts = &h.counts[0]
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]
hotCounts = h.counts[0]
coldCounts = h.counts[1]
}
// Now we have to wait for the now-declared-cold counts to actually cool