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https://issues.apache.org/jira/browse/HADOOP-5830?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12709456#action_12709456
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Hong Tang commented on HADOOP-5830:
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This is probably complementary to Hadoop-2560 (btw, is the new
CombineFileInputFormat capable of picking blocks from different files and lump
them into one input split?). I consider this approach is better because it
could achieve better load balancing, and lower overhead of map failure or
speculative execution.
> Reuse output collectors across maps running on the same jvm
> -----------------------------------------------------------
>
> Key: HADOOP-5830
> URL: https://issues.apache.org/jira/browse/HADOOP-5830
> Project: Hadoop Core
> Issue Type: Improvement
> Components: mapred
> Reporter: Arun C Murthy
>
> We have evidence that cutting the shuffle-crossbar between maps and reduces
> (m * r) leads to perfomant applications since:
> # It cuts down the number of connections necessary to shuffle and hence
> reduces load on the serving-side (TaskTracker) and improves latency
> (terasort, HADOOP-1338, HADOOP-5223)
> # Reduces seeks required for the TaskTracker to serve the map-outputs
> So far we've had to manually tune applications to cut down the shuffle-
> crossbar by having fatter maps with custom input formats etc. For e.g. we saw
> a significant improvement while running the petasort when we went from
> ~800,000 maps to 80,00 maps (1.5G to 15G per map) i.e. from 48+ hours to 16
> hours,
> The downsides are:
> # The burden falls on the application-writer to tune this with custom
> input-formats etc.
> # The naive method of using a higher min.split.size leads to considerable
> non-local i/o on the maps.
> Given these, the proposal is to keep the 'output collector' open across jvm
> reuse for maps, there-by enabling 'combiners' across map-tasks. This would
> have the happy-effect of fixing both the above. The downsides are that it
> will add latency to jobs (since map-outputs cannot be shuffled till a few
> maps on the same jvm are done, then followed by a final sort/merge/combine)
> and the failure cases get a bit more complicated.
> Thoughts? Lets discuss...
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