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From "Eric Evans (JIRA)" <j...@apache.org>
Subject [jira] Commented: (CASSANDRA-51) Memory footprint for memtable
Date Wed, 15 Apr 2009 16:44:14 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-51?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12699271#action_12699271
] 

Eric Evans commented on CASSANDRA-51:
-------------------------------------

There is now instrumentation for memtable data size and column count, and a counter for the
number of memtable switches (see CASSANDRA-75). Combined with the attached patch for the default/sample
config that makes the applicable thresholds more obvious to everyone, tuning a cluster to
avoid exhausting the JVM heap should be straightforward.

My vote would be to leave EBM as it is for now, and close this issue.


>  Memory footprint for memtable
> ------------------------------
>
>                 Key: CASSANDRA-51
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-51
>             Project: Cassandra
>          Issue Type: Improvement
>         Environment: all
>            Reporter: Sandeep Tata
>            Assignee: Eric Evans
>             Fix For: 0.3
>
>         Attachments: 0001-more-conservative-defaults-for-memtable-constraints.patch
>
>
> The implementation of EfficientBidiMap(EBM) today stores the column in two place, a map
and a sorted set. Both data structures store exactly the same values.
> I assume we're storing this twice so that the map can give us O(1) reads while the sortedset
is important for efficient flush. Is this tradeoff important ? Do we want to store the data
twice to get O(1) reads over O(log(n)) reads from sortedset? Is the sortedset implementation
broken? Perhaps we should consider a configuration option that turns off the map -- write
performance will be slightly improved, read performance will be somewhat worse, and the memory
footprint will probably be about half. Certainly sounds like a good alternative tradeoff.

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