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From "Jonathan Ellis (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-5863) In process (uncompressed) page cache
Date Sat, 01 Nov 2014 01:24:34 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-5863?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14192869#comment-14192869
] 

Jonathan Ellis commented on CASSANDRA-5863:
-------------------------------------------

bq. The Tricky part is tracking the "hotness" of these chunks. It needs to track the number
of times the chunk was decompressed in the last X seconds. 

Backing up a bit -- why not just take a LRU approach?  When we uncompress a chunk, we cache
it.  Add metrics so users can monitor cache churn and disable if it's not useful.  (But since
our chunks are fairly large, and thus decompressing is relatively expensive, I think we could
tolerate relatively high churn.)

> In process (uncompressed) page cache
> ------------------------------------
>
>                 Key: CASSANDRA-5863
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-5863
>             Project: Cassandra
>          Issue Type: New Feature
>          Components: Core
>            Reporter: T Jake Luciani
>              Labels: performance
>             Fix For: 3.0
>
>
> Currently, for every read, the CRAR reads each compressed chunk into a byte[], sends
it to ICompressor, gets back another byte[] and verifies a checksum.  
> This process is where the majority of time is spent in a read request.  
> Before compression, we would have zero-copy of data and could respond directly from the
page-cache.
> It would be useful to have some kind of Chunk cache that could speed up this process
for hot data, possibly off heap.



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