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https://issues.apache.org/jira/browse/HBASE-13082?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15181541#comment-15181541
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Anoop Sam John commented on HBASE-13082:
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So this means when there are parallel scanners, the thread doing compaction
will get blocked. Means it can delay other compaction requests in Q. Means if
there are some parallel writes also that may delay flushes. So may block writes
(?) Is these possible Lars?
> 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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