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https://issues.apache.org/jira/browse/HBASE-15716?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15261638#comment-15261638
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Hiroshi Ikeda commented on HBASE-15716:
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{quote}
Can you say more please? I do not follow how it goes 'back' since we do
readPoint = currentReadPoint;
at the bottom of the loop.
{quote}
Calculating the smallest readpoint can be called by another thread, even before
the loop is complete. The thread can see that the smallest readpoint goes back,
theoretically.
{quote}
Yes, if we miss the close, a reference to the Scanner will be held here.
{quote}
I mean, it is enough to just make a reference count to zero, and drop later the
entry when calculating the smallest readpoint. Moreover, implementation of a
concurrent map is generally complex and heavy than a queue.
{quote}
Let me look at an implementation using your suggested types....
{quote}
To tell the truth, that suggestion might be a little sophisticated and
difficult (or overdoing :P)
> HRegion#RegionScannerImpl scannerReadPoints synchronization costs
> -----------------------------------------------------------------
>
> Key: HBASE-15716
> URL: https://issues.apache.org/jira/browse/HBASE-15716
> Project: HBase
> Issue Type: Bug
> Components: Performance
> Reporter: stack
> Assignee: stack
> Attachments: 15716.prune.synchronizations.patch,
> 15716.prune.synchronizations.v3.patch, 15716.prune.synchronizations.v4.patch,
> 15716.prune.synchronizations.v4.patch, Screen Shot 2016-04-26 at 2.05.45
> PM.png, Screen Shot 2016-04-26 at 2.06.14 PM.png, Screen Shot 2016-04-26 at
> 2.07.06 PM.png, Screen Shot 2016-04-26 at 2.25.26 PM.png, Screen Shot
> 2016-04-26 at 6.02.29 PM.png, Screen Shot 2016-04-27 at 9.49.35 AM.png,
> current-branch-1.vs.NoSynchronization.vs.Patch.png, hits.png,
> remove_cslm.patch
>
>
> Here is a [~lhofhansl] special.
> When we construct the region scanner, we get our read point and then store it
> with the scanner instance in a Region scoped CSLM. This is done under a
> synchronize on the CSLM.
> This synchronize on a region-scoped Map creating region scanners is the
> outstanding point of lock contention according to flight recorder (My work
> load is workload c, random reads).
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