[LUCENE-10265] IO write throttle rate will beyond the Ceiling(1024MB/s) in the merge

• Type: Bug
• Status: Open
• Priority: Major
• Resolution: Unresolved
• Affects Version/s: 8.6.2
• Component/s: core/other
• Lucene Fields: New
• Assignee: Unassigned
• Reporter: kkewwei
• Created: 26/Nov/21 08:10
• Updated: 28/Aug/22 16:31

Description

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:

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;
      }
     ......
}

The modification process is not a atomic operation:

1. `targetMBPerSec` is changed by the first merge thread from 1024 to 1024*1.2
2. other merge thread will read the new value(1024*1.2).
3. 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.

[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

If we shoud do the following:
1. changing it by the atomic operation.
2. adding the `volatile` attribute to `targetMBPerSec`.