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https://issues.apache.org/jira/browse/HBASE-13082?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14332476#comment-14332476
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Lars Hofhansl commented on HBASE-13082:
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So at least we have established that locking in the StoreScanner is bad :)
I now remember issues we have seen with timerange range scans, where in unlucky
circumstances it takes almost 20 minutes to finish scanning a single region
(and that time all spent inside a *single* RegionScanner.next() call, as in
this case no Cells matched the timerange)
So that would be 20 minutes(!) during which we would not be able to commit a
flush or finish a compaction.
So now, I do not think that is acceptable. The RegionScanner lock is too
coarse. We need something in between. Hmmm....
> 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.txt
>
>
> 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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