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https://issues.apache.org/jira/browse/HBASE-2248?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12837519#action_12837519
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Dave Latham commented on HBASE-2248:
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@Dan: Took a read over the patch, though it seemed to be based in a different
dir and didn't want to apply nicely. From what I can see the ScanTest still
does getScanner in testSetup before the timer is begun. This may be fine, if
the point of this test is to measure scan performance per-row and not
setup/teardown time. It just explains why the ScanTest doesn't exhibit this
issue. It does look like other tests, such as the RandomSeekScanTest and the
new RandomScanWithRangeTest do test setup/teardown time as part of each
"testRow" and should exhibit this issue, if run.
> 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: hbase-2248.gc, Screen shot 2010-02-23 at 10.33.38
> AM.png, 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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