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From "stack (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-15716) HRegion#RegionScannerImpl scannerReadPoints synchronization constrains random read
Date Fri, 01 Jul 2016 04:53:11 GMT

    [ https://issues.apache.org/jira/browse/HBASE-15716?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15358380#comment-15358380
] 

stack commented on HBASE-15716:
-------------------------------

I think it is fine. I went to hack on your class because I'd thought you'd left off consideration
of isolationlevel...but I think the way you do it is clean. The other method is for ourside
consumers and should probably go away anyways (it seems to be used by Scanners only...).

Let me make up a patch that has your nice class [~ikeda] and the other changes too... 

I spent a bit of a while testing this patch. Looks good. JFR no longer reports this as a point
of contention. If I thread dump, safe point no longer has the region scanner init showing.
Comparing perf runs, i see we are doing the same throughput, perhaps a very slight bit more.

Looking before and after using honest profiler https://github.com/RichardWarburton/honest-profiler,
it says that org.apache.hadoop.hbase.regionserver.HRegion$RegionScannerImpl appears now MORE
often in stack traces when samping but that as an endpoint in a stack trace, there is less
incidence; percentages are small -- less than 1% -- which would seem to indicate that focus
on this as a point of contention is probably overblown because it appears in safepoint thread
dumps (i.e. my manual thread dumping -- see "Why (Most) Sampling Java Profilers Are Fucking
Terrible" http://psy-lob-saw.blogspot.co.za/2016/02/why-most-sampling-java-profilers-are.html
-- though this post says JFR does not rely on safe point....).



> 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.implementation.using.ScannerReadPoints.branch-1.patch, 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, HBASE-15716.branch-1.001.patch, HBASE-15716.branch-1.002.patch,
HBASE-15716.branch-1.003.patch, HBASE-15716.branch-1.004.patch, ScannerReadPoints.java, 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, TestScannerReadPoints.java, before_after.png,
current-branch-1.vs.NoSynchronization.vs.Patch.png, hits.png, remove.locks.patch, 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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