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https://issues.apache.org/jira/browse/HBASE-2248?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12837326#action_12837326
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Dave Latham commented on HBASE-2248:
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Thanks, Dan, and others for looking into this issue. The table where we were
seeing these slow scans was definitely a tall, narrow table. Each row has one
cell, the column family and qualifier are each one byte. The row varies, but
is typically 8-20 bytes, and the value is usually 4 bytes or less. Most common
is probably row - 12 bytes, col fam - 1 byte, qualifier 1 byte, value - 3
bytes, giving 17 bytes plus overhead.
As I was trying to understand the discrepancy between the PE results you
mentioned and what I've observed, I looked in to PerformanceEvaluation. It
looks like the timer only starts after the scanner is constructed which means
that the MemStore clone isn't being timed as part of the test, so that would
probably explain why the test seems fast. Just reasoning, it seems hard to
believe that ConcurrentSkipListMap.buildFromSorted could complete a million
iterations that fast.
> 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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