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https://issues.apache.org/jira/browse/MAPREDUCE-3235?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13296127#comment-13296127
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Hal Mo commented on MAPREDUCE-3235:
-----------------------------------
@Todd
bq. How did the wall clock differ?
Almost the same, depends on if there are rack local maps. the more rack local
maps, the slower.
BTW, I know you are interested in JVM intrinsic binary array compare. Our team
might implement it, if we can see enough requirement. We work on JVM to support
hadoop. For example, our GCIH share side data between maps(share java object
directly between JVMs), saving 2000G physical memory on 200 nodes, and at the
same time improve GC performance/job performance, for one of our critical daily
job.
> 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: 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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