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From "Lars Hofhansl (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-13082) Coarsen StoreScanner locks to RegionScanner
Date Mon, 23 Feb 2015 00:25:12 GMT

    [ https://issues.apache.org/jira/browse/HBASE-13082?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14332447#comment-14332447
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Lars Hofhansl commented on HBASE-13082:
---------------------------------------

Same with FAST_DIFF encoding (768mb on disk). All data in cache:

With patch: 13.2s
Without patch: 16.4s

Data not in cache (read from SSD, OS buffer cache also cleaned, short circuit reads SCR enabled):
With patch: 14.2s (reading about 51mb/s from disk)
Without patch: 17.5s (reading about 42mb/s from disk)

So assuming data locality so that we can do SCR this definitely improves things.

In all cases we save about 3.3s. So we save about 66ns per row.

Is it worth the hack? Not sure.


> Coarsen StoreScanner locks to RegionScanner
> -------------------------------------------
>
>                 Key: HBASE-13082
>                 URL: https://issues.apache.org/jira/browse/HBASE-13082
>             Project: HBase
>          Issue Type: Bug
>            Reporter: Lars Hofhansl
>         Attachments: 13082.txt
>
>
> Continuing where HBASE-10015 left of.
> We can avoid locking (and memory fencing) inside StoreScanner by deferring to the lock
already held by the RegionScanner.
> In tests this shows quite a scan improvement and reduced CPU (the fences make the cores
wait for memory fetches).
> There are some drawbacks too:
> * All calls to RegionScanner need to be remain synchronized
> * Implementors of coprocessors need to be diligent in following the locking contract.
For example Phoenix does not lock RegionScanner.nextRaw() and required in the documentation
(not picking on Phoenix, this one is my fault as I told them it's OK)
> * possible starving of flushes and compaction with heavy read load. RegionScanner operations
would keep getting the locks and the flushes/compactions would not be able finalize the set
of files.
> I'll have a patch soon.



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