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Date: Tue, 27 Sep 2016 04:03:20 +0000 (UTC)
From: "Heng Chen (JIRA)" <jira@apache.org>
To: issues@hbase.apache.org
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Subject: [jira] [Commented] (HBASE-16698) Performance issue: handlers stuck
 waiting for CountDownLatch inside WALKey#getWriteEntry under high writing
 workload
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archived-at: Tue, 27 Sep 2016 04:03:22 -0000


    [ https://issues.apache.org/jira/browse/HBASE-16698?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15524987#comment-15524987 ] 

Heng Chen commented on HBASE-16698:
-----------------------------------

How much the performance will be downgrade when ops are just for one region.  [~carp84] do you have some performance results?  In our production cluster (Not big cluster),  many tables have just few regions but QPS is high,  have a litter worried about it after we set it to be default.

> 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: 1.1.6, 1.2.3
>            Reporter: Yu Li
>            Assignee: Yu Li
>         Attachments: 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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