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From "Aleksey Yeschenko (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-9658) Re-enable memory-mapped index file reads on Windows
Date Fri, 26 Jun 2015 23:27:07 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-9658?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14603768#comment-14603768
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Aleksey Yeschenko commented on CASSANDRA-9658:
----------------------------------------------

I, would still strongly prefer us to get to the core of the issue and fix it, without reenabling
the mmaped path. We should absolutely standardize on a single read path for everything. If
we can pin-point the issue quickly, then even delaying 2.2 (if we are otherwise going to make
this change for 2.2.0).

If we cannot do it quickly, then I guess temporarily switching mmap read bath back is unavoidable
here, which makes me sad a little.

> Re-enable memory-mapped index file reads on Windows
> ---------------------------------------------------
>
>                 Key: CASSANDRA-9658
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-9658
>             Project: Cassandra
>          Issue Type: Improvement
>            Reporter: Joshua McKenzie
>            Assignee: Joshua McKenzie
>              Labels: Windows, performance
>             Fix For: 2.2.x
>
>
> It appears that the impact of buffered vs. memory-mapped index file reads has changed
dramatically since last I tested. [Here's some results on various platforms we pulled together
yesterday w/2.2-HEAD|https://docs.google.com/spreadsheets/d/1JaO2x7NsK4SSg_ZBqlfH0AwspGgIgFZ9wZ12fC4VZb0/edit#gid=0].
> TL;DR: On linux we see a 40% hit in performance from 108k ops/sec on reads to 64.8k ops/sec.
While surprising in itself, the really unexpected result (to me) is on Windows - with standard
access we're getting 16.8k ops/second on our bare-metal perf boxes vs. 184.7k ops/sec with
memory-mapped index files, an over 10-fold increase in throughput. While testing w/standard
access, CPU's on the stress machine and C* node are both sitting < 4%, network doesn't
appear bottlenecked, resource monitor doesn't show anything interesting, and performance counters
in the kernel show very little. Changes in thread count simply serve to increase median latency
w/out impacting any other visible metric that we're measuring, so I'm at a loss as to why
the disparity is so huge on the platform.
> The combination of my changes to get the 2.1 branch to behave on Windows along with [~benedict]
and [~Stefania]'s changes in lifecycle and cleanup patterns on 2.2 should hopefully have us
in a state where transitioning back to using memory-mapped I/O on Windows will only cause
trouble on snapshot deletion. Fairly simple runs of stress w/compaction aren't popping up
any obvious errors on file access or renaming - I'm going to do some much heavier testing
(ccm multi-node clusters, long stress w/repair and compaction, etc) and see if there's any
outstanding issues that need to be stamped out to call mmap'ed index files on Windows safe.
The one thing we'll never be able to support is deletion of snapshots while a node is running
and sstables are mapped, but for a > 10x throughput increase I think users would be willing
to make that sacrifice.
> The combination of the powercfg profile change, the kernel timer resolution, and memory-mapped
index files are giving some pretty interesting performance numbers on EC2.



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