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From "ryan rawson (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HBASE-2248) New MemStoreScanner copies memstore for each scan, makes short scans slow
Date Tue, 23 Feb 2010 07:57:28 GMT

    [ https://issues.apache.org/jira/browse/HBASE-2248?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12837127#action_12837127
] 

ryan rawson commented on HBASE-2248:
------------------------------------

I had a look at the implementation of clone, and it is really not appropriate for what we
are doing.

I would like to open up discussions to revert the original patch.  I would argue there has
been too many lurking issues, and the additional functionality, while useful, doesnt justify
crippling performance.

> New MemStoreScanner copies memstore for each scan, makes short scans slow
> -------------------------------------------------------------------------
>
>                 Key: HBASE-2248
>                 URL: https://issues.apache.org/jira/browse/HBASE-2248
>             Project: Hadoop HBase
>          Issue Type: Bug
>    Affects Versions: 0.20.3
>            Reporter: Dave Latham
>             Fix For: 0.20.4
>
>         Attachments: threads.txt
>
>
> HBASE-2037 introduced a new MemStoreScanner which triggers a ConcurrentSkipListMap.buildFromSorted
clone of the memstore and snapshot when starting a scan.
> After upgrading to 0.20.3, we noticed a big slowdown in our use of short scans.  Some
of our data repesent a time series.   The data is stored in time series order, MR jobs often
insert/update new data at the end of the series, and queries usually have to pick up some
or all of the series.  These are often scans of 0-100 rows at a time.  To load one page, we'll
observe about 20 such scans being triggered concurrently, and they take 2 seconds to complete.
 Doing a thread dump of a region server shows many threads in ConcurrentSkipListMap.biuldFromSorted
which traverses the entire map of key values to copy it.  

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