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https://issues.apache.org/jira/browse/HBASE-13082?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14349791#comment-14349791
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Lars Hofhansl commented on HBASE-13082:
---------------------------------------
Did some more tests with Phoenix.
In a case with 5 columns the gain is moderate but measurable (5-10%). For a
table with only 1 column this shaves of 30-35%.
4m rows in both cases, all aggregates, i.e. all the work is at the server. This
shows that this translates to wins even when the "filtering" is done up at the
coprocessor level.
The only reason why I am still hesitant on this one is that we'll now be forced
to keep the RegionScanner lock in the future. I do not actually see a reason to
get rid of it, but still...
> Coarsen StoreScanner locks to RegionScanner
> -------------------------------------------
>
> Key: HBASE-13082
> URL: https://issues.apache.org/jira/browse/HBASE-13082
> Project: HBase
> Issue Type: Bug
> Reporter: Lars Hofhansl
> Attachments: 13082-test.txt, 13082.txt, 13082.txt, 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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