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https://issues.apache.org/jira/browse/HBASE-15716?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15266056#comment-15266056
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Hiroshi Ikeda commented on HBASE-15716:
---------------------------------------
{code}
+ // If the isolation level is READ_UNCOMMITTED, should we just not bother
registering?
+ // This was suggested by Hiroshi Ikeda. I was thinking that the
READ_UNCOMMITTED applied to
+ // the frontier of updates, not the rear where Cells might be dropped
because too many
+ // versions or beyond the TTL.
synchronized(scannerReadPoints) {
this.readPt = getReadpoint(isolationLevel);
scannerReadPoints.put(this, this.readPt);
{code}
I'm not sure about READ_UNCOMMITTED in detail, but I feel we should take the
current mvcc.memstoreReadPoint() regardless of isolationLevel, otherwise cells
can be freely dropped before or while scanning when using READ_UNCOMMITTED.
> HRegion#RegionScannerImpl scannerReadPoints synchronization constrains random
> read
> ----------------------------------------------------------------------------------
>
> 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, 15716.wip.more_to_be_done.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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