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From Matei Zaharia <ma...@eecs.berkeley.edu>
Subject Re: Fairscheduler won't run more than 3 map tasks (from 48 available)
Date Thu, 18 Aug 2011 21:40:47 GMT
Okay, great!

Unfortunately the task launching is still slow even if you have JVM reuse set to -1 because
of heartbeats (the slave node only updates its state with the master every ~5 seconds).


On Aug 18, 2011, at 5:25 PM, Mck wrote:

>> How long are your tasks, and which version of Hadoop are you using? 
> Hadoop-0.20.1 (eventually we're looking to upgrade to Brisk).
> Tasks take 5-30 seconds.
>> In older versions (0.20.*), the fair
>> scheduler doesn't launch multiple tasks per heartbeat, so it performs poorly when
your tasks are small
>> (less than 5-10 seconds). You may be able to improve it a bit by setting
>> mapred.fairscheduler.assignmultiple to true in your mapred-site.xml. However, even
this will assign
>> too few tasks per heartbeat. I recommend using either Hadoop 0.21, where this issue
is fixed by default, or
>> Cloudera's Hadoop distribution, which is the only 0.20-based version that has backported
the relevant
>> fair scheduler improvements from 0.21.
> mapred.fairscheduler.assignmultiple gave no apparent benefit. Maps running is still too
low (4 or 5 now).
> [OT] Does Brisk-1.0-beta2 include this fix you mention?
>> If you have short tasks though, you should beware that Hadoop as a whole will be
inefficient because it will
>> spend most of its time launching JVMs and waiting on heartbeats to send back status
> I'm running with mapred.job.reuse.jvm.num.tasks=-1 so to re-use all JVMs.
>> You should try
>> to tune your task size (number of input records per task) so that each tasks takes
at least 30-60 seconds, or
>> you won't be running at the maximum efficiency possible for your cluster.
> The default here is 
> cassandra.input.split.size=65536
> Raising it to 262144 (default x 4) indeed fixes the problem :-)
> (Now i just need to check this doesn't send memory through the roof...)
> Thanks for your valuable and quick help Matei.
> ~mck
> -- 
> "Physics is to math what sex is to masturbation." Richard Feynman 
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