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https://issues.apache.org/jira/browse/CASSANDRA-4175?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13258030#comment-13258030
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Jonathan Ellis edited comment on CASSANDRA-4175 at 4/20/12 5:34 AM:
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identityHashCode is basically the object's location in memory, so it's not
going to be the same on different nodes. (So it would work for approach 2, I
suppose, but I'd rather use a simple int counter.)
was (Author: jbellis):
Hashcode just isn't designed to be collision-resistant; it prioritizes
speed. Even with a better (from the standpoint of collisions) general-purpose
hash like Murmur, 32bits is just too small. The smallest cryptographic hash I
know of is md5, and ballooning to 128bits puts a serious crimp in the potential
savings here.
> Reduce memory (and disk) space requirements with a column name/id map
> ---------------------------------------------------------------------
>
> Key: CASSANDRA-4175
> URL: https://issues.apache.org/jira/browse/CASSANDRA-4175
> Project: Cassandra
> Issue Type: Improvement
> Reporter: Jonathan Ellis
> Fix For: 1.2
>
>
> We spend a lot of memory on column names, both transiently (during reads) and
> more permanently (in the row cache). Compression mitigates this on disk but
> not on the heap.
> The overhead is significant for typical small column values, e.g., ints.
> Even though we intern once we get to the memtable, this affects writes too
> via very high allocation rates in the young generation, hence more GC
> activity.
> Now that CQL3 provides us some guarantees that column names must be defined
> before they are inserted, we could create a map of (say) 32-bit int column
> id, to names, and use that internally right up until we return a resultset to
> the client.
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