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From "Enis Soztutar (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-16698) Performance issue: handlers stuck waiting for CountDownLatch inside WALKey#getWriteEntry under high writing workload
Date Fri, 14 Oct 2016 22:39:20 GMT

    [ https://issues.apache.org/jira/browse/HBASE-16698?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15576766#comment-15576766
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Enis Soztutar commented on HBASE-16698:
---------------------------------------

I also have a hard time understanding the patch. We are still serializing the seq assining,
but this time via a lock. This lock is used from handlers to append to the ring buffer as
well, which basically means that we are going back to the old model (0.98) and have no benefits
coming from disruptor. Is the perf gains from context switches in case there is less contention
for the mvcc lock? 
We have to serialize the edits via ring buffer and assign seq ids in the same order anyway.
Is the RBEH doing too much work for appending? maybe we need two consumers for the ring buffer,
one for assigning seq ids, and the other for doing the actual append? 


> Performance issue: handlers stuck waiting for CountDownLatch inside WALKey#getWriteEntry
under high writing workload
> --------------------------------------------------------------------------------------------------------------------
>
>                 Key: HBASE-16698
>                 URL: https://issues.apache.org/jira/browse/HBASE-16698
>             Project: HBase
>          Issue Type: Improvement
>          Components: Performance
>    Affects Versions: 1.2.3
>            Reporter: Yu Li
>            Assignee: Yu Li
>             Fix For: 2.0.0
>
>         Attachments: HBASE-16698.branch-1.patch, HBASE-16698.branch-1.v2.patch, HBASE-16698.patch,
HBASE-16698.v2.patch, hadoop0495.et2.jstack
>
>
> As titled, on our production environment we observed 98 out of 128 handlers get stuck
waiting for the CountDownLatch {{seqNumAssignedLatch}} inside {{WALKey#getWriteEntry}} under
a high writing workload.
> After digging into the problem, we found that the problem is mainly caused by advancing
mvcc in the append logic. Below is some detailed analysis:
> Under current branch-1 code logic, all batch puts will call {{WALKey#getWriteEntry}}
after appending edit to WAL, and {{seqNumAssignedLatch}} is only released when the relative
append call is handled by RingBufferEventHandler (see {{FSWALEntry#stampRegionSequenceId}}).
Because currently we're using a single event handler for the ringbuffer, the append calls
are handled one by one (actually lot's of our current logic depending on this sequential dealing
logic), and this becomes a bottleneck under high writing workload.
> The worst part is that by default we only use one WAL per RS, so appends on all regions
are dealt with in sequential, which causes contention among different regions...
> To fix this, we could also take use of the "sequential appends" mechanism, that we could
grab the WriteEntry before publishing append onto ringbuffer and use it as sequence id, only
that we need to add a lock to make "grab WriteEntry" and "append edit" a transaction. This
will still cause contention inside a region but could avoid contention between different regions.
This solution is already verified in our online environment and proved to be effective.
> Notice that for master (2.0) branch since we already change the write pipeline to sync
before writing memstore (HBASE-15158), this issue only exists for the ASYNC_WAL writes scenario.



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