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From "Todd Lipcon (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (MAPREDUCE-3235) Improve CPU cache behavior in map side sort
Date Wed, 07 Nov 2012 22:22:14 GMT

    [ https://issues.apache.org/jira/browse/MAPREDUCE-3235?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13492758#comment-13492758
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Todd Lipcon commented on MAPREDUCE-3235:
----------------------------------------

Hi Gopal. I agree the patch isn't suitable for commit right now as it could break stuff --
was just using it to try to understand the cache effects.

How big was the dataset that you're sorting? Somewhere around 90MB it sounds like? Were you
able to compare cpu seconds elapsed? Even if the wall clock doesn't improve much, saving cpu-seconds
means that you can run more tasks in parallel, etc.
                
> Improve CPU cache behavior in map side sort
> -------------------------------------------
>
>                 Key: MAPREDUCE-3235
>                 URL: https://issues.apache.org/jira/browse/MAPREDUCE-3235
>             Project: Hadoop Map/Reduce
>          Issue Type: Improvement
>          Components: performance, task
>    Affects Versions: 0.23.0
>            Reporter: Todd Lipcon
>            Assignee: Todd Lipcon
>         Attachments: hashed-sort-MAPREDUCE-3235.patch, map_sort_perf.diff, mr-3235-poc.txt
>
>
> When running oprofile on a terasort workload, I noticed that a large amount of CPU usage
was going to MapTask$MapOutputBuffer.compare. Upon disassembling this and looking at cycle
counters, most of the cycles were going to memory loads dereferencing into the array of key-value
data -- implying expensive cache misses. This can be avoided as follows:
> - rather than simply swapping indexes into the kv array, swap the entire meta entries
in the meta array. Swapping 16 bytes is only negligibly slower than swapping 4 bytes. This
requires adding the value-length into the meta array, since we used to rely on the previous-in-the-array
meta entry to determine this. So we replace INDEX with VALUELEN and avoid one layer of indirection.
> - introduce an interface which allows key types to provide a 4-byte comparison proxy.
For string keys, this can simply be the first 4 bytes of the string. The idea is that, if
stringCompare(key1.proxy(), key2.proxy()) != 0, then compare(key1, key2) should have the same
result. If the proxies are equal, the normal comparison method is used. We then include the
4-byte proxy as part of the metadata entry, so that for many cases the indirection into the
data buffer can be avoided.
> On a terasort benchmark, these optimizations plus an optimization to WritableComparator.compareBytes
dropped the aggregate mapside CPU millis by 40%, and the compare() routine mostly dropped
off the oprofile results.

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