[jira] [Updated] (MAPREDUCE-3235) Improve CPU cache behavior in map side sort

["Gopal V (JIRA)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/MAPREDUCE-3235?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Gopal V updated MAPREDUCE-3235:
-------------------------------

    Attachment: hashed-sort-MAPREDUCE-3235.patch

Update patch to trunk.

Move the byte[] into an int and place it into the HashComparator interface instead of into Comparables
                
> 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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