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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 04:13:28 GMT

    [ https://issues.apache.org/jira/browse/HBASE-2248?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12837074#action_12837074
] 

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

@JD: that would explain it...

With --nomapred (10 client threads in a single VM) each scan took 120-140ms...  

Also, the randomSeekScan test each scan seems VERY slow.  Each scan takes about 15 seconds...?
 The scanRange100 uses a startRow and stopRow to get 100 rows back (well 70 rows).  The randomSeekScan
using a "scan.setFilter(new WhileMatchFilter(new PageFilter(120)));".  What's up with that?

Oh, also, those tests are on the latest 0.20 branch (not on the 0.20.3 release)... 

> 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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