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https://issues.apache.org/jira/browse/LUCENE-10448?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17504574#comment-17504574
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Vigya Sharma commented on LUCENE-10448:
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{quote}With many the statistics, all the detailBytes(mb) are always ~0.73mb, it
is a high probability that there are no large chunk bytes. the reason why
instant rate of up to 460 mb/s is that there is no wait. if we write 1mb to
disk, the writing rate can be 500mb/s+
{quote}
Without large chunk bytes, I don't understand why this would happen. Can you
share more details on how you are calculating {{detailRate}} ? If you are
looking at {{{}(bytes-written-between-two-pauses /
time-between-two-pauses){}}}, then that can easily be higher than the
configured rate, as the rate is only imposed during the pauses. Between them,
the writes are only really limited by the limits of your IO.
> MergeRateLimiter doesn't always limit instant rate.
> ---------------------------------------------------
>
> Key: LUCENE-10448
> URL: https://issues.apache.org/jira/browse/LUCENE-10448
> Project: Lucene - Core
> Issue Type: Bug
> Components: core/other
> Affects Versions: 8.11.1
> Reporter: kkewwei
> Priority: Major
> Time Spent: 20m
> Remaining Estimate: 0h
>
> We can see the code in *MergeRateLimiter*:
> {code:java}
> private long maybePause(long bytes, long curNS) throws
> MergePolicy.MergeAbortedException {
>
> double rate = mbPerSec;
> double secondsToPause = (bytes / 1024. / 1024.) / rate;
> long targetNS = lastNS + (long) (1000000000 * secondsToPause);
> long curPauseNS = targetNS - curNS;
> // We don't bother with thread pausing if the pause is smaller than 2
> msec.
> if (curPauseNS <= MIN_PAUSE_NS) {
> // Set to curNS, not targetNS, to enforce the instant rate, not
> // the "averaged over all history" rate:
> lastNS = curNS;
> return -1;
> }
> ......
> }
> {code}
> If a Segment is been merged, *maybePause* is called in 7:00, lastNS=7:00,
> then the *maybePause* is called in 7:05 again, so the value of
> *targetNS=lastNS + (long) (1000000000 * secondsToPause)* must be smaller than
> *curNS*, no matter how big the bytes is, we will return -1 and ignore to
> pause.
> I count the total times(callTimes) calling *maybePause* and ignored pause
> times(ignorePauseTimes) and detail ignored bytes(detailBytes):
> {code:java}
> [2022-03-02T15:16:51,972][DEBUG][o.e.i.e.I.EngineMergeScheduler] [node1]
> [index1][21] merge segment [_4h] done: took [26.8s], [123.6 MB], [61,219
> docs], [0s stopped], [24.4s throttled], [242.5 MB written], [11.2 MB/sec
> throttle], [callTimes=857], [ignorePauseTimes=25], [detailBytes(mb) =
> [0.28899956, 0.28140354, 0.28015518, 0.27990818, 0.2801447, 0.27991104,
> 0.27990723, 0.27990913, 0.2799101, 0.28010082, 0.2799921, 0.2799673,
> 0.28144264, 0.27991295, 0.27990818, 0.27993107, 0.2799387, 0.27998447,
> 0.28002167, 0.27992058, 0.27998066, 0.28098202, 0.28125, 0.28125, 0.28125]]
> {code}
> There are 857 times calling *maybePause*, including 25 times which is ignored
> to pause, we can see that the ignored detail bytes (such as 0.28125mb) are
> not small.
> As long as the interval between two *maybePause* calls is relatively long,
> the pause action that should be executed will not be executed.
>
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