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https://issues.apache.org/jira/browse/HBASE-13082?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14334491#comment-14334491
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Lars Hofhansl commented on HBASE-13082:
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Wait. A. Minute... The scenario that I describe above already happens with the
current code. StoreScanner.next(List<Cells>) would loop - with the look held -
until either we found a row worth of data or exhausted the entire store. If the
region has only one CF that could mean the entire region is scanned.
So my change would not make it much worse, but cause any other stores of the
region not be able to flush/compact during that time.
(I also see what the issue with the comparison is, if the ts of the cell falls
before the minStamp of the we seek to the next column... We'll do this over and
over again. But that's for a different jira).
So back to this.
* Advantage: Much better scan performance, that can be even measure in 25%
higher disk read rate. (will try with rotating disks tomorrow)
* Disadvantage: a slow scan that does not match any cell in *any* store (CF)
can prevent *other* stores in the region from flushing/compacting until the
slow scan finished.
Worth doing?
> 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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