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From "Junegunn Choi (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-15213) Fix increment performance regression caused by HBASE-8763 on branch-1.0
Date Sun, 06 Mar 2016 15:31:40 GMT

    [ https://issues.apache.org/jira/browse/HBASE-15213?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15182200#comment-15182200
] 

Junegunn Choi commented on HBASE-15213:
---------------------------------------

[~carp84]

bq. This possibly will make T3 return earlier than T2 which is incorrect? 

It's not incorrect. It does not break consistency guarantee of HBase. If you think concurrent
clients should receive the responses to the concurrent transactions in the exact order they
are processed on the server, it's practically not possible and HBase does not guarantee that.

bq. I guess this is exactly why we remove only one element at a time from the queue in 0.98/master
codes

No, on both branches multiple entries are removed at once if possible. That's what the while
loops are for. Each handler thread first mark the entry at hand complete, try to remove multiple,
consecutive entries at the front of the queue (while loop), then go into the wait loop.

> Fix increment performance regression caused by HBASE-8763 on branch-1.0
> -----------------------------------------------------------------------
>
>                 Key: HBASE-15213
>                 URL: https://issues.apache.org/jira/browse/HBASE-15213
>             Project: HBase
>          Issue Type: Sub-task
>          Components: Performance
>            Reporter: Junegunn Choi
>            Assignee: Junegunn Choi
>             Fix For: 1.1.4, 1.0.4
>
>         Attachments: 15157v3.branch-1.1.patch, HBASE-15213-increment.png, HBASE-15213.branch-1.0.patch,
HBASE-15213.v1.branch-1.0.patch
>
>
> This is an attempt to fix the increment performance regression caused by HBASE-8763 on
branch-1.0.
> I'm aware that hbase.increment.fast.but.narrow.consistency was added to branch-1.0 (HBASE-15031)
to address the issue and a separate work is ongoing on master branch, but anyway, this is
my take on the problem.
> I read through HBASE-14460 and HBASE-8763 but it wasn't clear to me what caused the slowdown
but I could indeed reproduce the performance regression.
> Test setup:
> - Server: 4-core Xeon 2.4GHz Linux server running mini cluster (100 handlers, JDK 1.7)
> - Client: Another box of the same spec
> - Increments on random 10k records on a single-region table, recreated every time
> Increment throughput (TPS):
> || Num threads || Before HBASE-8763 (d6cc2fb) || branch-1.0 || branch-1.0 (narrow-consistency)
||
> || 1            | 2661                         | 2486        | 2359  |
> || 2            | 5048                         | 5064        | 4867  |
> || 4            | 7503                         | 8071        | 8690  |
> || 8            | 10471                        | 10886       | 13980 |
> || 16           | 15515                        | 9418        | 18601 |
> || 32           | 17699                        | 5421        | 20540 |
> || 64           | 20601                        | 4038        | 25591 |
> || 96           | 19177                        | 3891        | 26017 |
> We can clearly observe that the throughtput degrades as we increase the number of concurrent
requests, which led me to believe that there's severe context switching overhead and I could
indirectly confirm that suspicion with cs entry in vmstat output. branch-1.0 shows a much
higher number of context switches even with much lower throughput.
> Here are the observations:
> - WriteEntry in the writeQueue can only be removed by the very handler that put it, only
when it is at the front of the queue and marked complete.
> - Since a WriteEntry is marked complete after the wait-loop, only one entry can be removed
at a time.
> - This stringent condition causes O(N^2) context switches where n is the number of concurrent
handlers processing requests.
> So what I tried here is to mark WriteEntry complete before we go into wait-loop. With
the change, multiple WriteEntries can be shifted at a time without context switches. I changed
writeQueue to LinkedHashSet since fast containment check is needed as WriteEntry can be removed
by any handler.
> The numbers look good, it's virtually identical to pre-HBASE-8763 era.
> || Num threads || branch-1.0 with fix ||
> || 1            | 2459                 |
> || 2            | 4976                 |
> || 4            | 8033                 |
> || 8            | 12292                |
> || 16           | 15234                |
> || 32           | 16601                |
> || 64           | 19994                |
> || 96           | 20052                |
> So what do you think about it? Please let me know if I'm missing anything.



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