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https://issues.apache.org/jira/browse/HBASE-2248?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12837074#action_12837074
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Dan Washusen commented on HBASE-2248:
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@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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