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https://issues.apache.org/jira/browse/LUCENE-10265?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17449967#comment-17449967
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kkewwei commented on LUCENE-10265:
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Yes, you are right, I read it wrong.
If we should lower the value, or it can be change by the outside. e will use io
to build index, search, merge, after all, Slow disks(hdd) are so common, it can
only support Hundreds of MB/s write, if the merge speed is too high, it will
affect index and search.
> IO write throttle rate will beyond the Ceiling(1024MB/s) in the merge
> ---------------------------------------------------------------------
>
> Key: LUCENE-10265
> URL: https://issues.apache.org/jira/browse/LUCENE-10265
> Project: Lucene - Core
> Issue Type: Bug
> Components: core/other
> Affects Versions: 8.6.2
> Reporter: kkewwei
> Priority: Major
>
> It's known that merge io write throttle rate is under the control of
> `targetMBPerSec` In ConcurrentMergeSchedule, it should't beyond the
> Ceiling(1024MB/s).
> `targetMBPerSec` is shared by many merge threads, it will be changed by the
> way:
> {code:java}
> if (newBacklog) {
> // This new merge adds to the backlog: increase IO throttle by 20%
> targetMBPerSec *= 1.20;
> if (targetMBPerSec > MAX_MERGE_MB_PER_SEC) {
> targetMBPerSec = MAX_MERGE_MB_PER_SEC;
> }
> ......
> } else {
> // We are not falling behind: decrease IO throttle by 10%
> targetMBPerSec /= 1.10;
> if (targetMBPerSec < MIN_MERGE_MB_PER_SEC) {
> targetMBPerSec = MIN_MERGE_MB_PER_SEC;
> }
> ......
> }
> {code}
> The modification process is not a atomic operation:
> # `targetMBPerSec` is changed by the first merge thread from 1024 to 1024*1.2
> # other merge thread will read the new value(1024*1.2).
> # the first merge thread limit the value to be 1024.
> The bad case will happen.
> In product, we do find that IO write throttle rate is beyond the
> Ceiling(1024MB/s) in the merge.
> {code:java}
> [2021-11-26T15:27:19,861][TRACE][o.e.i.e.E.MS ] [data1]
> [test1][25] elasticsearch[data1][refresh][T#5] MS: io throttle: current merge
> backlog; leave IO rate at 3589.1 MB/sec
> [2021-11-26T15:27:20,304][TRACE][o.e.i.e.E.MS ] [data1]
> [test1][13] elasticsearch[data1][write][T#3] MS: io throttle: current merge
> backlog; leave IO rate at 192.4 MB/sec
> [2021-11-26T15:27:25,330][TRACE][o.e.i.e.E.MS ] [data1]
> [test1][22] elasticsearch[data1][[test1][22]: Lucene Merge Thread #1026] MS:
> io throttle: current merge backlog; leave IO rate at 96.3 MB/sec
> [2021-11-26T15:27:25,995][TRACE][o.e.i.e.E.MS ] [data1]
> [test1][16] elasticsearch[data1][[test1][16]: Lucene Merge Thread #1063] MS:
> io throttle: current merge backlog; leave IO rate at 419.2 MB/sec
> [2021-11-26T15:27:38,335][TRACE][o.e.i.e.E.MS ] [data1]
> [test1][19] elasticsearch[data1][write][T#2] MS: io throttle: current merge
> backlog; leave IO rate at 3051.5 MB/sec
> {code}
> If we shoud do the following:
> 1. changing it by the atomic operation.
> 2. adding the `volatile` attribute to `targetMBPerSec`.
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