[jira] [Commented] (CASSANDRA-4316) Compaction Throttle too bursty with large rows

["Jonathan Ellis (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/CASSANDRA-4316?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13549228#comment-13549228 ] 

Jonathan Ellis commented on CASSANDRA-4316:
-------------------------------------------

Looks like mayThrottle (both implementations) is missing conversion from bytes -> MB.  (Actually looking at the RateLimiter creation, it looks like the name {{dataSizeInMB}} is misleading since it is actually still bytes.)

Does StandaloneScrubber ratelimit?  It probably shouldn't.

Scanner in cleanup compaction needs a withRateLimit.

Is looping over scanner.getCurrentPosition for each row compacted going to eat CPU?  Maybe every N rows would be better, with N = 1MB / average row size.  Quite possibly it's not actually a problem and I'm prematurely complexifying.

Nits:
- "// throttle if needed" comment is redundant
- "maybeThrottle" is a better method name than "mayThrottle"
- May be able to simplify getCompactionRateLimiter by creating a default limiter even if we are unthrottled (since if we are unthrottled we ignore it anyway), so you don't need to worry about == null checks

Rest LGTM.  Should we open a new ticket to move FST to RateLimiter and get rid of Throttle entirely?

                
> Compaction Throttle too bursty with large rows
> ----------------------------------------------
>
>                 Key: CASSANDRA-4316
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-4316
>             Project: Cassandra
>          Issue Type: Improvement
>          Components: Core
>    Affects Versions: 0.8.0
>            Reporter: Wayne Lewis
>            Assignee: Yuki Morishita
>             Fix For: 1.2.1
>
>         Attachments: 4316-1.2.txt
>
>
> In org.apache.cassandra.db.compaction.CompactionIterable the check for compaction throttling occurs once every 1000 rows. In our workload this is much too large as we have many large rows (16 - 100 MB).
> With a 100 MB row, about 100 GB is read (and possibly written) before the compaction throttle sleeps. This causes bursts of essentially unthrottled compaction IO followed by a long sleep which yields inconsistence performance and high error rates during the bursts.
> We applied a workaround to check throttle every row which solved our performance and error issues:
> line 116 in org.apache.cassandra.db.compaction.CompactionIterable:
>                 if ((row++ % 1000) == 0)
> replaced with
>                 if ((row++ % 1) == 0)
> I think the better solution is to calculate how often throttle should be checked based on the throttle rate to apply sleeps more consistently. E.g. if 16MB/sec is the limit then check for sleep after every 16MB is read so sleeps are spaced out about every second.

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