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From "Elliott Clark (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-8665) bad compaction priority behavior in queue can cause store to be blocked
Date Fri, 31 May 2013 05:01:20 GMT

    [ https://issues.apache.org/jira/browse/HBASE-8665?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13671155#comment-13671155
] 

Elliott Clark commented on HBASE-8665:
--------------------------------------

bq. If blocked store comes into a queue full of non-blocked stores why should we bump them?
And how much.
I was suggesting bumping the priority instead of queueing another. Bumping the compaction
request to what would currently be computed seems reasonable.

bq.3) Additionally if we pre-select in advance we simply end up with inefficient compactions,
for example in this case we could have compacted 6 small files in one go, but would have ended
up compacting 3 and 3 if it spent less time in the queue.

In this case pre-selecting doesn't seem to be a bad thing.  Prioritizing a fast compaction
over being a little more efficient seems like a trade off that most people would want.  So
since current implementation biases towards what I would expect users to want maybe we shouldn't
change that until we can put those smarts into the compaction selection (plumbing reason the
compaction was requested into selection).
                
> bad compaction priority behavior in queue can cause store to be blocked
> -----------------------------------------------------------------------
>
>                 Key: HBASE-8665
>                 URL: https://issues.apache.org/jira/browse/HBASE-8665
>             Project: HBase
>          Issue Type: Bug
>            Reporter: Sergey Shelukhin
>            Assignee: Sergey Shelukhin
>
> Note that this can be solved by bumping up the number of compaction threads but still
it seems like this priority "inversion" should be dealt with.
> There's a store with 1 big file and 3 flushes (1 2 3 4) sitting around and minding its
own business when it decides to compact. Compaction (2 3 4) is created and put in queue, it's
low priority, so it doesn't get out of the queue for some time - other stores are compacting.
Meanwhile more files are flushed and at (1 2 3 4 5 6 7) it decides to compact (5 6 7). This
compaction now has higher priority than the first one. After that if the load is high it enters
vicious cycle of compacting and compacting files as they arrive, with store being blocked
on and off, with the (2 3 4) compaction staying in queue for up to ~20 minutes (that I've
seen).
> I wonder why we do thing thing where we queue compaction and compact separately. Perhaps
we should take snapshot of all store priorities, then do select in order and execute the first
compaction we find. This will need starvation safeguard too but should probably be better.
> Btw, exploring compaction policy may be more prone to this, as it can select files from
the middle, not just beginning, which, given the treatment of already selected files that
was not changed from the old ratio-based one (all files with lower seqNums than the ones selected
are also ineligible for further selection), will make more files ineligible (e.g. imagine
with 10 blocking files, with 8 present (1-8), (6 7 8) being selected and getting stuck). Today
I see the case that would also apply to old policy, but yesterday I saw file distribution
something like this: 4,5g, 2,1g, 295,9m, 113,3m, 68,0m, 67,8m, 1,1g, 295,1m, 100,4m, unfortunately
w/o enough logs to figure out how it resulted.

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