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From "ramkrishna.s.vasudevan (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-13082) Coarsen StoreScanner locks to RegionScanner
Date Sat, 05 Mar 2016 06:43:40 GMT

    [ https://issues.apache.org/jira/browse/HBASE-13082?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15181578#comment-15181578
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ramkrishna.s.vasudevan commented on HBASE-13082:
------------------------------------------------

[~lhofhansl]
In the current trunk and 1.3 actually we are not doing any updateReaders after compactions.
Only after flushes we are doing. So how does this patch work with flushes if you will wait
on the latch that is created per StoreScanner. Because for flush we are forced to update the
scanner because we are cleaning up the memstore snapshot. But yes we do have the reference
to the older memstore snapshot but holding it up for a longer time is not good is what was
discussed previously in this JIRA impl. So even if compactions get blocked if there are large
scans then it will affect the compaction requests that comes newly?

> Coarsen StoreScanner locks to RegionScanner
> -------------------------------------------
>
>                 Key: HBASE-13082
>                 URL: https://issues.apache.org/jira/browse/HBASE-13082
>             Project: HBase
>          Issue Type: Bug
>          Components: Performance, Scanners
>            Reporter: Lars Hofhansl
>            Assignee: ramkrishna.s.vasudevan
>             Fix For: 2.0.0
>
>         Attachments: 13082-test.txt, 13082-v2.txt, 13082-v3.txt, 13082-v4.txt, 13082.txt,
13082.txt, CountDownLatch-0.98.txt, HBASE-13082.pdf, HBASE-13082_1.pdf, HBASE-13082_12.patch,
HBASE-13082_13.patch, HBASE-13082_14.patch, HBASE-13082_15.patch, HBASE-13082_16.patch, HBASE-13082_17.patch,
HBASE-13082_18.patch, HBASE-13082_19.patch, HBASE-13082_1_WIP.patch, HBASE-13082_2.pdf, HBASE-13082_2_WIP.patch,
HBASE-13082_3.patch, HBASE-13082_4.patch, HBASE-13082_9.patch, HBASE-13082_9.patch, HBASE-13082_withoutpatch.jpg,
HBASE-13082_withpatch.jpg, LockVsSynchronized.java, gc.png, gc.png, gc.png, hits.png, next.png,
next.png
>
>
> Continuing where HBASE-10015 left of.
> We can avoid locking (and memory fencing) inside StoreScanner by deferring to the lock
already held by the RegionScanner.
> In tests this shows quite a scan improvement and reduced CPU (the fences make the cores
wait for memory fetches).
> There are some drawbacks too:
> * All calls to RegionScanner need to be remain synchronized
> * Implementors of coprocessors need to be diligent in following the locking contract.
For example Phoenix does not lock RegionScanner.nextRaw() and required in the documentation
(not picking on Phoenix, this one is my fault as I told them it's OK)
> * possible starving of flushes and compaction with heavy read load. RegionScanner operations
would keep getting the locks and the flushes/compactions would not be able finalize the set
of files.
> I'll have a patch soon.



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