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From "Joshua McKenzie (JIRA)" <>
Subject [jira] [Commented] (CASSANDRA-9658) Re-enable memory-mapped index file reads on Windows
Date Wed, 01 Jul 2015 19:37:04 GMT


Joshua McKenzie commented on CASSANDRA-9658:

Thanks for the quick turnaround!

bq. The log statement is repeated 3 times in code you can just put it at the end of the if/else
I think that was originally in there to differentiate "auto" from other modes in the log which
requires either logging the way we do now or building a log statement after depending on conf.disk_access_mode.
Both seem equally distasteful to me so I figured leave well enough alone.

bq. Use the FBUtiliies.getProtectedField method vs doing it yourself?
Didn't know we had that - updated.

bq. Should you log every file at info? should it be debug?
Yes. Mistake / left over from testing. Fixed.

bq. Bad import org.hsqldb.Database;
IDE got over-eager and I missed it on final pass. Removed.

bq. Is there anything we need todo if the delete fails? like kill -9? or stack overflow on
your recursive delete? Should we have a .deleteme file added that can be checked on start?
Or at least a log message if a Major exception is thrown that users should expect to clean
up themselves
I thought about that in passing but originally landed on the side of "if a node hard-dies
and an operator sees there's large disk usage on there, they should turn up snapshots pretty
quick and can correlate it with the log message". In retrospect the log message should be
more clear about it being "best effort" on JVM shutdown w/certain caveats (crashing, stack
overflow, OOM) if we go that route.

Given the work taking place in CASSANDRA-7066, I think I'll peruse Stefania's work on there
and follow her lead w/an implementation to delete old snapshots on startup. It's really the
most robust solution and it may be a long road for us to get buffered and memory-mapped into
parity. Pulling this out of patch available for now while I get that implemented.

CI currently running:

> Re-enable memory-mapped index file reads on Windows
> ---------------------------------------------------
>                 Key: CASSANDRA-9658
>                 URL:
>             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|].
> 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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