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tsdb: add gauge metric for max head chunks pending m-mapping per series

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Add prometheus_tsdb_head_chunks_max_pending_mmap, a gauge updated on
each mmapHeadChunks pass to the maximum headChunkCount observed among
series with headChunkCount >= 2 (i.e. those that are candidates for
m-mapping). The metric is scoped to mmap-ready series rather than all
series, so it stays correct after the per-stripe mmapReady optimisation
in #18541.

Signed-off-by: György Krajcsovits <gyorgy.krajcsovits@grafana.com>
This commit is contained in:
György Krajcsovits 2026-04-24 11:39:19 +02:00
parent 551b5b1c56
commit ba27c12834
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GPG Key ID: 47A8F9CE80FD7C7F
2 changed files with 17 additions and 2 deletions
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16
tsdb/head.go
View File

@ -420,7 +420,8 @@ type headMetrics struct {
mmapChunkCorruptionTotal prometheus.Counter
snapshotReplayErrorTotal prometheus.Counter // Will be either 0 or 1.
oooHistogram prometheus.Histogram
mmapChunksTotal prometheus.Counter
mmapChunksTotal prometheus.Counter
headChunksMaxPendingMmap prometheus.Gauge
walReplayUnknownRefsTotal *prometheus.CounterVec
wblReplayUnknownRefsTotal *prometheus.CounterVec
}
@ -560,6 +561,10 @@ func newHeadMetrics(h *Head, r prometheus.Registerer) *headMetrics {
Name: "prometheus_tsdb_mmap_chunks_total",
Help: "Total number of chunks that were memory-mapped.",
}),
headChunksMaxPendingMmap: prometheus.NewGauge(prometheus.GaugeOpts{
Name: "prometheus_tsdb_head_chunks_max_pending_mmap",
Help: "Maximum number of head chunks pending m-mapping observed in any individual series during the last m-map pass.",
}),
walReplayUnknownRefsTotal: prometheus.NewCounterVec(prometheus.CounterOpts{
Name: "prometheus_tsdb_wal_replay_unknown_refs_total",
Help: "Total number of unknown series references encountered during WAL replay.",
@ -598,6 +603,7 @@ func newHeadMetrics(h *Head, r prometheus.Registerer) *headMetrics {
m.checkpointCreationTotal,
m.oooHistogram,
m.mmapChunksTotal,
m.headChunksMaxPendingMmap,
m.mmapChunkCorruptionTotal,
m.snapshotReplayErrorTotal,
// Metrics bound to functions and not needed in tests
@ -1945,12 +1951,17 @@ func (h *Head) getOrCreateWithOptionalID(id chunks.HeadSeriesRef, hash uint64, l
// since holding the lock during an append could delay the next scrape or cause query timeouts.
func (h *Head) mmapHeadChunks() {
var count int
var maxPendingMmapChunks uint32
for i := range h.series.size {
h.series.locks[i].RLock()
for _, series := range h.series.series[i] {
if series.headChunkCount.Load() < 2 { // < 2 means 0 or 1 head chunks, nothing to mmap.
hcc := series.headChunkCount.Load()
if hcc < 2 { // < 2 means 0 or 1 head chunks, nothing to mmap.
continue
}
if hcc > maxPendingMmapChunks {
maxPendingMmapChunks = hcc
}
series.Lock()
count += series.mmapChunks(h.chunkDiskMapper)
@ -1958,6 +1969,7 @@ func (h *Head) mmapHeadChunks() {
}
h.series.locks[i].RUnlock()
}
h.metrics.headChunksMaxPendingMmap.Set(float64(maxPendingMmapChunks))
h.metrics.mmapChunksTotal.Add(float64(count))
}

3
tsdb/head_test.go
View File

@ -8231,12 +8231,14 @@ func TestHead_mmapHeadChunks(t *testing.T) {
require.Equal(t, uint32(1), getCount(lblsB), "headChunkCount should be 1 after mmap")
require.Equal(t, uint32(1), getCount(lblsC), "headChunkCount should be 1 after mmap")
require.Equal(t, 0, countReady(), "ready set should be empty after mmapHeadChunks")
require.Equal(t, float64(3), prom_testutil.ToFloat64(h.metrics.headChunksMaxPendingMmap), "max pending mmap should be 3 (both series had 3 head chunks)")
// A second call should be a no-op.
beforeMetric := prom_testutil.ToFloat64(h.metrics.mmapChunksTotal)
h.mmapHeadChunks()
afterMetric := prom_testutil.ToFloat64(h.metrics.mmapChunksTotal)
require.Equal(t, beforeMetric, afterMetric, "second call should mmap 0 chunks")
require.Equal(t, float64(0), prom_testutil.ToFloat64(h.metrics.headChunksMaxPendingMmap), "max pending mmap should be 0 when no series needs mmapping")
// Only newly ready series should be processed.
app = h.Appender(t.Context())
@ -8257,6 +8259,7 @@ func TestHead_mmapHeadChunks(t *testing.T) {
afterMetric = prom_testutil.ToFloat64(h.metrics.mmapChunksTotal)
require.Greater(t, afterMetric, beforeMetric, "third call should mmap chunks from series B")
require.Equal(t, uint32(1), getCount(lblsB), "series B headChunkCount should be 1 after mmap")
require.Equal(t, float64(3), prom_testutil.ToFloat64(h.metrics.headChunksMaxPendingMmap), "max pending mmap should be 3 (only series B had 3 head chunks)")
}
func TestHead_mmapHeadChunks_oooDoesNotInflateCount(t *testing.T) {