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From "Junegunn Choi (JIRA)" <j...@apache.org>
Subject [jira] [Created] (HBASE-15213) Fix increment performance regression caused by HBASE-8763 on branch-1.0
Date Thu, 04 Feb 2016 09:24:39 GMT
Junegunn Choi created HBASE-15213:
-------------------------------------

             Summary: 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: Bug
          Components: Performance
            Reporter: Junegunn Choi


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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