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From "Dan Washusen (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 11:36:27 GMT

    [ https://issues.apache.org/jira/browse/HBASE-2248?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12837202#action_12837202
] 

Dan Washusen commented on HBASE-2248:
-------------------------------------

Very good point...  

Even if the clone took the scan start and stop rows into account, there is still the possibility
that only one or neither of them has been provided provided... 

> 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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