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Date: Fri, 7 Aug 2015 09:15:47 +0000 (UTC)
From: "Stefania (JIRA)" <jira@apache.org>
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Subject: [jira] [Commented] (CASSANDRA-8630) Faster sequential IO (on
 compaction, streaming, etc)
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    [ https://issues.apache.org/jira/browse/CASSANDRA-8630?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14661541#comment-14661541 ] 

Stefania commented on CASSANDRA-8630:
-------------------------------------

About byte ordering, it seems OHC insists on native byte ordering, which is little-endian on linux x86_64. Not a big problem, we can force the ordering to big-endian in the serializers.

However, I think this means we always pay the price of swapping bytes when using direct byte buffers. Here is the implementation of {{getInt()}} in DirectByteBuffer.java:

{code}
private int getInt(long a) {
        if (unaligned) {
            int x = unsafe.getInt(a);
            return (nativeByteOrder ? x : Bits.swap(x));
        }
        return Bits.getInt(a, bigEndian);
    }
{code}

Forcing byte ordering to big-endian doesn't mean {{nativeByteOrder}] becomes true:

{code}
public final ByteBuffer order(ByteOrder bo) {
        bigEndian = (bo == ByteOrder.BIG_ENDIAN);
        nativeByteOrder =
            (bigEndian == (Bits.byteOrder() == ByteOrder.BIG_ENDIAN));
        return this;
    }
{code}

where {{Bits.byteOrder()}} return the platform endianess. 

So wouldn't we be better off forcing native byte ordering rather than big-endian?


> 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
>
>
> 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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