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From "Josh Rosen (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (SPARK-3358) PySpark worker fork()ing performance regression in m3.* / PVM instances
Date Wed, 03 Sep 2014 20:07:52 GMT

    [ https://issues.apache.org/jira/browse/SPARK-3358?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14120354#comment-14120354
] 

Josh Rosen commented on SPARK-3358:
-----------------------------------

Agreed.  Long term, I think it would be better to address the causes behind why we need to
fork so many processes.

> PySpark worker fork()ing performance regression in m3.* / PVM instances
> -----------------------------------------------------------------------
>
>                 Key: SPARK-3358
>                 URL: https://issues.apache.org/jira/browse/SPARK-3358
>             Project: Spark
>          Issue Type: Bug
>          Components: PySpark
>    Affects Versions: 1.1.0
>         Environment: m3.* instances on EC2
>            Reporter: Josh Rosen
>
> SPARK-2764 (and some followup commits) simplified PySpark's worker process structure
by removing an intermediate pool of processes forked by daemon.py.  Previously, daemon.py
forked a fixed-size pool of processes that shared a socket and handled worker launch requests
from Java.  After my patch, this intermediate pool was removed and launch requests are handled
directly in daemon.py.
> Unfortunately, this seems to have increased PySpark task launch latency when running
on m3* class instances in EC2.  Most of this difference can be attributed to m3 instances'
more expensive fork() system calls.  I tried the following microbenchmark on m3.xlarge and
r3.xlarge instances:
> {code}
> import os
> for x in range(1000):
>   if os.fork() == 0:
>     exit()
> {code}
> On the r3.xlarge instance:
> {code}
> real	0m0.761s
> user	0m0.008s
> sys	0m0.144s
> {code}
> And on m3.xlarge:
> {code}
> real    0m1.699s
> user    0m0.012s
> sys     0m1.008s
> {code}
> I think this is due to HVM vs PVM EC2 instances using different virtualization technologies
with different fork costs.
> It may be the case that this performance difference only appears in certain microbenchmarks
and is masked by other performance improvements in PySpark, such as improvements to large
group-bys.  I'm in the process of re-running spark-perf benchmarks on m3 instances in order
to confirm whether this impacts more realistic jobs.



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