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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:
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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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