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From Jacob R Rideout <apa...@jacobrideout.net>
Subject Re: Speculative execution
Date Thu, 03 Mar 2011 23:29:35 GMT
On Thu, Mar 3, 2011 at 2:04 PM, Keith Wiley <kwiley@keithwiley.com> wrote:
> On Mar 3, 2011, at 2:51 AM, Steve Loughran wrote:
>> yes, but the problem is determining which one will fail. Ideally you should find
the route cause, which is often some race condition or hardware fault. If it's the same server
ever time, turn it off.
>> You can play with the specex parameters, maybe change when they get kicked off. The
assumption in the code is that the slowness is caused by H/W problems (especially HDD issues)
and it tries to avoid duplicate work. If every Map was duplicated, you'd be doubling the effective
cost of each query, and annoying everyone else in the cluster. Plus increased disk and network
IO might slow things down.
>> Look at the options, have a play and see. If it doesn't have the feature, you can
always try coding it in -if the scheduler API lets it do it, you wont' be breaking anyone
else's code.
>> -steve
> Thanks.  I'll take it under consideration.  In my case, it would be really beneficial
to duplicate the work.  That task in question is a single task on a single node (numerous
mappers feed data into a single reducer), so duplicating the reducer represents very will
duplicated effort while mitigating a potential bottleneck in the job's performance since the
job simply is not done until the single reducer finishes.  I would really like to be able
to do what I am suggesting, to duplicate the reducer and kill the clones after the winner
> Anyway, thanks.

What is your reason for needing a single reducer? I'd first try to see
how I could parallelize that work first if possible.


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