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From "Todd Lipcon (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-5898) Consider double-checked locking for block cache lock
Date Wed, 09 May 2012 22:49:50 GMT

    [ https://issues.apache.org/jira/browse/HBASE-5898?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13271901#comment-13271901
] 

Todd Lipcon commented on HBASE-5898:
------------------------------------

Shouldn't be hard to get that CPU time back -- we can just add an array of cacheline-padded
AtomicLongs to the cache. Whenever we add something to the cache, we do {{changeCounters[key.hashCode()
% changeCounters.length].getAndIncrement()}}. Then change the code to:

{code}
AtomicLong changeCounter = changeCounters[key.hashCode() % changeCounters.length];
long firstTimeChangeCounter = changeCounter.get();
first time:
  try to look up in cache
  if found: return it
second time:
  take lock:
    if changeCounter.get() == firstTimeChangeCounter: it's not in cache
    otherwise: look up in cache again
{code}

We'd probably want to cache-pad the AtomicLongs too to avoid false sharing.
                
> Consider double-checked locking for block cache lock
> ----------------------------------------------------
>
>                 Key: HBASE-5898
>                 URL: https://issues.apache.org/jira/browse/HBASE-5898
>             Project: HBase
>          Issue Type: Improvement
>          Components: performance
>    Affects Versions: 0.94.1
>            Reporter: Todd Lipcon
>            Assignee: Todd Lipcon
>            Priority: Critical
>         Attachments: 5898-TestBlocksRead.txt, hbase-5898.txt
>
>
> Running a workload with a high query rate against a dataset that fits in cache, I saw
a lot of CPU being used in IdLock.getLockEntry, being called by HFileReaderV2.readBlock. Even
though it was all cache hits, it was wasting a lot of CPU doing lock management here. I wrote
a quick patch to switch to a double-checked locking and it improved throughput substantially
for this workload.

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