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From "Benedict (JIRA)" <j...@apache.org>
Subject [jira] [Comment Edited] (CASSANDRA-8630) Faster sequential IO (on compaction, streaming, etc)
Date Wed, 19 Aug 2015 19:27:47 GMT

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Benedict edited comment on CASSANDRA-8630 at 8/19/15 7:26 PM:
--------------------------------------------------------------

bq. In what scenario would we not want to map the file with as few 2 gigabyte buffers as possible?

During early opening we currently remap our buffers every interval-, meaning for a 2Gb buffer
by default we will map it 20 times (plus once every 2Gb)-. This is not horrible, but I would
prefer if - at least during reopening - we only mapped once, and each time we reopened/extended
the size of the file, we just mapped the bit that wasn't previously mapped. Once we cross
a 2Gb boundary (or we are opening the final copy of the file) we should certainly remap into
contiguous 2Gb chunks.

edit: currently we actually map it much more than this; we map each 2Gb range every 50Mb,
so for a 100Gb file we might map several thousand times. So whatever we do will be a dramatic
improvement, but I generally am on a mission to sanitise the code base, and while we're here
we might as well do it right.


was (Author: benedict):
bq. In what scenario would we not want to map the file with as few 2 gigabyte buffers as possible?

During early opening we currently remap our buffers every interval, meaning for a 2Gb buffer
by default we will map it 20 times (plus once every 2Gb). This is not horrible, but I would
prefer if - at least during reopening - we only mapped once, and each time we reopened/extended
the size of the file, we just mapped the bit that wasn't previously mapped. Once we cross
a 2Gb boundary (or we are opening the final copy of the file) we should certainly remap into
contiguous 2Gb chunks.


> Faster sequential IO (on compaction, streaming, etc)
> ----------------------------------------------------
>
>                 Key: CASSANDRA-8630
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-8630
>             Project: Cassandra
>          Issue Type: Improvement
>          Components: Core, Tools
>            Reporter: Oleg Anastasyev
>            Assignee: Stefania
>              Labels: compaction, performance
>             Fix For: 3.x
>
>         Attachments: 8630-FasterSequencialReadsAndWrites.txt, cpu_load.png, flight_recorder_001_files.tar.gz,
flight_recorder_002_files.tar.gz, mmaped_uncomp_hotspot.png
>
>
> When node is doing a lot of sequencial IO (streaming, compacting, etc) a lot of CPU is
lost in calls to RAF's int read() and DataOutputStream's write(int).
> This is because default implementations of readShort,readLong, etc as well as their matching
write* are implemented with numerous calls of byte by byte read and write. 
> This makes a lot of syscalls as well.
> A quick microbench shows than just reimplementation of these methods in either way gives
8x speed increase.
> A patch attached implements RandomAccessReader.read<Type> and SequencialWriter.write<Type>
methods in more efficient way.
> I also eliminated some extra byte copies in CompositeType.split and ColumnNameHelper.maxComponents,
which were on my profiler's hotspot method list during tests.
> A stress tests on my laptop show that this patch makes compaction 25-30% faster  on uncompressed
sstables and 15% faster for compressed ones.
> A deployment to production shows much less CPU load for compaction. 
> (I attached a cpu load graph from one of our production, orange is niced CPU load - i.e.
compaction; yellow is user - i.e. not compaction related tasks)



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