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From "graham sanderson (JIRA)" <j...@apache.org>
Subject [jira] [Comment Edited] (CASSANDRA-6890) Standardize on a single read path
Date Wed, 22 Oct 2014 00:55:36 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-6890?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14179429#comment-14179429
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graham sanderson edited comment on CASSANDRA-6890 at 10/22/14 12:55 AM:
------------------------------------------------------------------------

I haven't followed this entire thread - I just happened across it - so this may be moot at
this point, but I thought I'd throw it out there.

bq. mmap is better for the number of reasons but mostly because we have to do less syscalls
and less copies (especially cross mode boundary). So I would, as well, vote to leave it and
remove buffered I/O instead.

We just observed this in the wild on similar high end machines to that on which we run C*
(40 cores, lots of high perf disks, RAM etc). With a large number of reader threads (in this
cased doing random pread64 I/O against a single fd) we were seeing a *significant* cliff where
performance dropped off alarmingly quickly (very high user and kernel CPU, with little to
no actual disk I/O). In this case the data files fit in the page cache which obviously makes
the problem more visible. Anyway.


was (Author: graham sanderson):
I haven't followed this entire thread - I just happened across it - so this may be moot at
this point, but I thought I'd throw it out there.

&bq. mmap is better for the number of reasons but mostly because we have to do less syscalls
and less copies (especially cross mode boundary). So I would, as well, vote to leave it and
remove buffered I/O instead.

We just observed this in the wild on similar high end machines to that on which we run C*
(40 cores, lots of high perf disks, RAM etc). With a large number of reader threads (in this
cased doing random pread64 I/O against a single fd) we were seeing a *significant* cliff where
performance dropped off alarmingly quickly (very high user and kernel CPU, with little to
no actual disk I/O). In this case the data files fit in the page cache which obviously makes
the problem more visible. Anyway.

> Standardize on a single read path
> ---------------------------------
>
>                 Key: CASSANDRA-6890
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-6890
>             Project: Cassandra
>          Issue Type: Improvement
>          Components: Core
>            Reporter: Joshua McKenzie
>            Assignee: Joshua McKenzie
>              Labels: performance
>             Fix For: 3.0
>
>         Attachments: 6890_v1.txt, mmap_gc.jpg, mmap_jstat.txt, mmap_perf.txt, nommap_gc.jpg,
nommap_jstat.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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