Github user mateiz commented on the pull request:
https://github.com/apache/spark/pull/1499#issuecomment-50297730
Are map tasks spilling by any chance? There is one issue in this right now,
which is that if your map task spills to disk, you need to spill multiple times
with the sort-based shuffle, whereas with the old code you would directly open
one file for each reduce task and write data to it. I think we can add the same
behavior here if the # of reduce tasks is small enough (the problem before was
that you'd open 10,000 files for 10,000 reduce tasks and allocate lots of space
for buffers, not to mention do lots of random I/O). I would do that in a
separate pull request though. For other things, like reduceByKey, this should
have similar performance to the hash-based shuffle.
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