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From "Andrew Purtell (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HBASE-2248) Provide new non-copy mechanism to assure atomic reads in get and scan
Date Mon, 12 Apr 2010 15:28:44 GMT

    [ https://issues.apache.org/jira/browse/HBASE-2248?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12856018#action_12856018
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Andrew Purtell commented on HBASE-2248:
---------------------------------------

>From Ryan via email:
{quote}
There is a busy wait loop which attempts to ensure a write completes only when it is visible
to others. With the log append as part of the "transaction" this is breaking down. The solution
is to either forgo the busy wait loop (probably not a great idea) or restructure the code
to do hlog appends first then memstore updates.

I'll talk to stack tomorrow and we can figure which route is better... Although I'd guess
option #2
{quote}

> Provide new non-copy mechanism to assure atomic reads in get and scan
> ---------------------------------------------------------------------
>
>                 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
>            Priority: Blocker
>             Fix For: 0.20.4
>
>         Attachments: HBASE-2248-demonstrate-previous-impl-bugs.patch, HBASE-2248-GetsAsScans3.patch,
HBASE-2248-rr-alpha3.txt, HBASE-2248-rr-pre-durability2.txt, HBASE-2248-rr-pre-durability3.txt,
hbase-2248.gc, HBASE-2248.patch, hbase-2248.txt, profile.png, put_call_graph.png, readownwrites-lost.2.patch,
readownwrites-lost.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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