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From "stack (JIRA)" <j...@apache.org>
Subject [jira] Updated: (HBASE-2248) New MemStoreScanner copies memstore for each scan, makes short scans slow
Date Fri, 26 Feb 2010 12:56:27 GMT

     [ https://issues.apache.org/jira/browse/HBASE-2248?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

stack updated HBASE-2248:
-------------------------

    Attachment: HBASE-2248.patch

Here is an attempt.  Tests pass.  Posting for review.  Need to do load tests yet.

"- Added a (transient) int updateId to KeyValue
- Memstore populates it on Adds and Deletes 
- When a MemstoreScanner is created it grabs the current id (actually increments  it to make
sure no KV has that same id) and ignores records from kvset having an id greater than the
one grabbed. Snapshots are scanned in full since they're not updated during the scanner's
lifetime hence there's no risk of partial updates being visible.  There may be an issue with
delete's becoming partly visible in this scheme, I'll check that later."


> 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-demonstrate-previous-impl-bugs.patch, hbase-2248.gc,
HBASE-2248.patch, 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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