goodhistogram: add windowed histogram with Prometheus integration#4
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angles-n-daemons merged 1 commit intoMay 4, 2026
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kyle-a-wong
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Apr 27, 2026
| } | ||
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| // tick performs the actual baseline rotation. Must be called with w.mu held. | ||
| func (w *Windowed) tick() { |
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nit: Can we call this tickLocked just to make it clear that it is expected that a lock was already acquired?
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sure, doing this now
| w.prevBaseline = empty | ||
| w.curBaseline = empty | ||
| w.nextTick = time.Now().Add(w.interval) | ||
| w.mu.Unlock() |
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any reason why we dont defer this after taking the lock?
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no reason, changing this now
kyle-a-wong
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Apr 27, 2026
| labelNames []string | ||
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| mu sync.RWMutex | ||
| histograms map[string]*labeledWindowed |
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nit: If a lock needs to be acquired to access it, i like to put it in an embeded struct, something like:
mu struct {
sync.RWMutex
histograms map[string]*labeledWindowed
}
that way, the histograms is accessed like w.mu.histograms, which makes it a little more clear that a lock needs to be acquired. Not gonna block on it, but just food for thought if you wanna make the change.
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yeah, i'll do that now
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| type Windowed struct { | ||
| h *Histogram // cumulative — the only histogram, never swapped or reset | ||
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| mu sync.Mutex |
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Sorry i missed this with the last review: can you do the same thing here that you did with windowedVec where the things that require locks are in the mutex struct?
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Good point, doing this now.
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| cur := w.h.Snapshot() | ||
| w.mu.Lock() | ||
| base := w.prevBaseline |
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One thing claude pointed out:
Subtle race in WindowedSnapshot (worth fixing)
w.maybeTick()
cur := w.h.Snapshot() // T1
w.mu.Lock()
base := w.prevBaseline // T2
w.mu.Unlock()
return cur.Sub(&base)
If another goroutine calls Tick() while h.Snapshot() is running at T1, the new curBaseline may capture per-bucket counts that are higher than the corresponding counts in our
partially-snapshotted cur (snapshots are not atomic across buckets). If a second Tick() then runs before we read prevBaseline at T2, that newer snapshot becomes our base, and
cur.Sub(&base) underflows uint64 per-bucket counts and TotalCount. With frequent Tick()s this is reachable, and the test TestWindowedConcurrentRecordAndTick doesn't exercise
WindowedSnapshot (it does check it, but only via the cumulative side, not the diff side).
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Ah nice, yeah that seems to make sense. We'll change that now
Cumulative histograms are the right primitive for Prometheus scraping, but operators and dashboards need rolling-window quantiles to see recent behavior. Windowed maintains a single cumulative histogram with two baseline snapshots rotated on a configurable interval. The windowed view is computed by subtracting the older baseline from the current cumulative state, covering 1-2x the window duration. Recording cost is identical to a plain Histogram (~20ns, lock-free); the mutex only protects baseline rotation. WindowedCollector and WindowedVec provide Prometheus integration for single and labeled windowed histograms, following the same adapter pattern as PrometheusCollector and HistogramVec. Co-Authored-By: roachdev-claude <roachdev-claude-bot@cockroachlabs.com>
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May 4, 2026
angles-n-daemons
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Summary
Stacks on #2.
Cumulative histograms are the right primitive for Prometheus scraping, but operators and dashboards need rolling-window quantiles to see recent behavior.
Windowedmaintains a single cumulative histogram with two baseline snapshots rotated on a configurable interval. The windowed view is computed by subtracting the older baseline from the current cumulative state, covering 1-2x the window duration. Recording cost is identical to a plainHistogram(~20ns, lock-free); the mutex only protects baseline rotation.WindowedCollectorandWindowedVecprovide Prometheus integration for single and labeled windowed histograms, following the same adapter pattern asPrometheusCollectorandHistogramVec.