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Allow to tune the scrape tolerance (#9283) 

* Allow to tune the scrape tolerance

In most of the classic monitoring use cases, a few milliseconds
difference can be omitted.

In Prometheus, a few millisecond difference can however make a big
difference.

Currently, Prometheus will ignore up to 2 ms difference in the
alignments.

It turns out that for users who can afford a 10ms difference, there is a
lot of resources and disk space to win, as shown in this graph, which
shows the bytes / samples over a production Prometheus server. You can
clearly see the switch from 2ms to 10ms tolerance.

This pull request enables the adjustment of the scrape timestamp
alignment tolerance.

Signed-off-by: Julien Pivotto <roidelapluie@inuits.eu>

* Fix golint

Signed-off-by: Julien Pivotto <roidelapluie@inuits.eu>
This commit is contained in:
Julien Pivotto 2021-09-08 13:57:33 +02:00 committed by GitHub
parent 2327236bb5
commit 48a101be1b
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2 changed files with 9 additions and 6 deletions
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5
cmd/prometheus/main.go
View File

@ -295,9 +295,12 @@ func main() {
a.Flag("rules.alert.resend-delay", "Minimum amount of time to wait before resending an alert to Alertmanager.").
Default("1m").SetValue(&cfg.resendDelay)
a.Flag("scrape.adjust-timestamps", "Adjust scrape timestamps by up to 2ms to align them to the intended schedule. See https://github.com/prometheus/prometheus/issues/7846 for more context. Experimental. This flag will be removed in a future release.").
a.Flag("scrape.adjust-timestamps", "Adjust scrape timestamps by up to `scrape.timestamp-tolerance` to align them to the intended schedule. See https://github.com/prometheus/prometheus/issues/7846 for more context. Experimental. This flag will be removed in a future release.").
Hidden().Default("true").BoolVar(&scrape.AlignScrapeTimestamps)
a.Flag("scrape.timestamp-tolerance", "Timestamp tolerance. See https://github.com/prometheus/prometheus/issues/7846 for more context. Experimental. This flag will be removed in a future release.").
Hidden().Default("2ms").DurationVar(&scrape.ScrapeTimestampTolerance)
a.Flag("alertmanager.notification-queue-capacity", "The capacity of the queue for pending Alertmanager notifications.").
Default("10000").IntVar(&cfg.notifier.QueueCapacity)

10
scrape/scrape.go
View File

@ -49,10 +49,10 @@ import (
"github.com/prometheus/prometheus/storage"
)
// Temporary tolerance for scrape appends timestamps alignment, to enable better
// compression at the TSDB level.
// ScrapeTimestampTolerance is the tolerance for scrape appends timestamps
// alignment, to enable better compression at the TSDB level.
// See https://github.com/prometheus/prometheus/issues/7846
const scrapeTimestampTolerance = 2 * time.Millisecond
var ScrapeTimestampTolerance = 2 * time.Millisecond
// AlignScrapeTimestamps enables the tolerance for scrape appends timestamps described above.
var AlignScrapeTimestamps = true
@ -1159,14 +1159,14 @@ mainLoop:
// Calling Round ensures the time used is the wall clock, as otherwise .Sub
// and .Add on time.Time behave differently (see time package docs).
scrapeTime := time.Now().Round(0)
if AlignScrapeTimestamps && sl.interval > 100*scrapeTimestampTolerance {
if AlignScrapeTimestamps && sl.interval > 100*ScrapeTimestampTolerance {
// For some reason, a tick might have been skipped, in which case we
// would call alignedScrapeTime.Add(interval) multiple times.
for scrapeTime.Sub(alignedScrapeTime) >= sl.interval {
alignedScrapeTime = alignedScrapeTime.Add(sl.interval)
}
// Align the scrape time if we are in the tolerance boundaries.
if scrapeTime.Sub(alignedScrapeTime) <= scrapeTimestampTolerance {
if scrapeTime.Sub(alignedScrapeTime) <= ScrapeTimestampTolerance {
scrapeTime = alignedScrapeTime
}
}