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From "Joshua McKenzie (JIRA)" <>
Subject [jira] [Commented] (CASSANDRA-6890) Remove mmap'ed i/o from read path
Date Wed, 26 Mar 2014 19:43:19 GMT


Joshua McKenzie commented on CASSANDRA-6890:

See CASSANDRA-4050 for further details - ran into some performance issues with conversion
to nio.2 and tracked it down to me combining a bypass on mmap'ed data on that patch.  Removed
that portion of things on that patch and will work further on getting to the bottom of that
here.  Currently a simple switch from a memory mapped to non-memory-mapped file in SegmentedFile's
getBuilder is showing a 15-40% drop in throughput depending on work-load characteristics.

Some more detailed yammer timers on small subsets of the code (a single FileChannel seek,
a single byte read operation) show that #'s are comparable up to somewhere between 50th and
75th percentile and then they balloon upwards from there.

I'll run jstat against a jvm in both scenarios and see what it shows.

> Remove mmap'ed i/o from read path
> ---------------------------------
>                 Key: CASSANDRA-6890
>                 URL:
>             Project: Cassandra
>          Issue Type: Improvement
>            Reporter: Joshua McKenzie
>            Assignee: Joshua McKenzie
>            Priority: Minor
>             Fix For: 3.0
>         Attachments: mmap_perf.txt
> Since we actively unmap unreferenced SSTR's and also copy data out of those readers on
the read path, the current memory mapped i/o is a lot of complexity for very little payoff.
 Clean out the mmapp'ed i/o on the read path.

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