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From "Sylvain Lebresne (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-4883) Optimize mostRecentTomstone vs maxTimestamp check in CollationController.collectAllData
Date Tue, 13 Nov 2012 10:58:11 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-4883?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13496109#comment-13496109
] 

Sylvain Lebresne commented on CASSANDRA-4883:
---------------------------------------------

bq. Any reason to not use ImmutableSet in DataTracker?

Not really. I used unmodifiableSet initially to avoid the copy, but since said copy is not
on a hot path and since ImmutableSet is probably slightly cheaper to iterate, committed with
ImmutableSet. Thanks
                
> Optimize mostRecentTomstone vs maxTimestamp check in CollationController.collectAllData
> ---------------------------------------------------------------------------------------
>
>                 Key: CASSANDRA-4883
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-4883
>             Project: Cassandra
>          Issue Type: Improvement
>            Reporter: Sylvain Lebresne
>            Assignee: Sylvain Lebresne
>            Priority: Minor
>             Fix For: 1.2.0 rc1
>
>         Attachments: 4883.txt
>
>
> CollationController.collectAllData eliminates a sstable if we've already read a row tombstone
more recent that its maxTimestamp. This is however done in 2 passes and can be inefficient
(or rather, it's not as efficient as it could). More precisely, say we have 10 sstables s0,
... s9, where s0 is the most recent and s9 the least one (and their maxTimestamp reflect that)
and s0 has a row tombstone that is more recent than all of s1-s9 maxTimestamps. Now in collectAllData(),
we first iterate over sstables in a "random" order (because DataTracker keeps sstable in a
more or less random order). Meaning that we may iterate in the order s9, s8, ... s0. In that
case, we will end up reading the row header from all the sstable (hitting disk each time).
Then, and only then, the 2nd pass of collectAllData will eliminate s1 to s9.
> However, if we were to iterate sstable in maxTimestamps order (as we do in collectTimeOrdered),
we would only need one pass but more importantly we would minimize the number of row header
we read to perform that sstable eliminination. In my example, we would only ever read the
row tombstone from s0 and eliminate all other sstable directly, simply based on their maxTimestamp.

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