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Subject: Re: [OMPI users] Limit to number of processes on one node?
From: Prentice Bisbal (prentice_at_[hidden])
Date: 2010-03-04 09:27:14

Ralph Castain wrote:
> On Mar 3, 2010, at 12:16 PM, Prentice Bisbal wrote:
>> Eugene Loh wrote:
>>> Prentice Bisbal wrote:
>>>> Eugene Loh wrote:
>>>>> Prentice Bisbal wrote:
>>>>>> Is there a limit on how many MPI processes can run on a single host?
>>> Depending on which OMPI release you're using, I think you need something
>>> like 4*np up to 7*np (plus a few) descriptors. So, with 256, you need
>>> 1000+ descriptors. You're quite possibly up against your limit, though
>>> I don't know for sure that that's the problem here.
>>> You say you're running 1.2.8. That's "a while ago", so would you
>>> consider updating as a first step? Among other things, newer OMPIs will
>>> generate a much clearer error message if the descriptor limit is the
>>> problem.
>> While 1.2.8 might be "a while ago", upgrading software just because it's
>> "old" is not a valid argument.
>> I can install the lastest version of OpenMPI, but it will take a little
>> while.
> Maybe not because it is "old", but Eugene is correct. The old versions of OMPI required more file descriptors than the newer versions.
> That said, you'll still need a minimum of 4x the number of procs on the node even with the latest release. I suggest talking to your sys admin about getting the limit increased. It sounds like it has been set unrealistically low.
I *am* the system admin! ;)

The file descriptor limit is the default for RHEL, 1024, so I would not
characterize it as "unrealistically low". I assume someone with much
more knowledge of OS design and administration than me came up with this
default, so I'm hesitant to change it without good reason. If there was
good reason, I'd have no problem changing it. I have read that setting
it to more than 8192 can lead to system instability.

This is admittedly unusual situation - in normal use, no one would ever
want to run that many processes on a single system - so I don't see any
justification for modifying that setting.

Yesterday I spoke to the researcher who originally asked me this limit -
he just wanted to know what the limit was, and doesn't actually plan to
do any "real" work with that many processes on a single node, rendering
this whole discussion academic.

I did install OpenMPI 1.4.1 yesterday, but I haven't had a chance to
test it yet. I'll post the results of testing here.

>>>>>> I have a user trying to test his code on the command-line on a single
>>>>>> host before running it on our cluster like so:
>>>>>> mpirun -np X foo
>>>>>> When he tries to run it on large number of process (X = 256, 512), the
>>>>>> program fails, and I can reproduce this with a simple "Hello, World"
>>>>>> program:
>>>>>> $ mpirun -np 256 mpihello
>>>>>> mpirun noticed that job rank 0 with PID 0 on node
>>>>>> exited on signal 15 (Terminated).
>>>>>> 252 additional processes aborted (not shown)
>>>>>> I've done some testing and found that X <155 for this program to work.
>>>>>> Is this a bug, part of the standard, or design/implementation decision?
>>>>> One possible issue is the limit on the number of descriptors. The error
>>>>> message should be pretty helpful and descriptive, but perhaps you're
>>>>> using an older version of OMPI. If this is your problem, one workaround
>>>>> is something like this:
>>>>> unlimit descriptors
>>>>> mpirun -np 256 mpihello
>>>> Looks like I'm not allowed to set that as a regular user:
>>>> $ ulimit -n 2048
>>>> -bash: ulimit: open files: cannot modify limit: Operation not permitted
>>>> Since I am the admin, I could change that elsewhere, but I'd rather not
>>>> do that system-wide unless absolutely necessary.
>>>>> though I guess the syntax depends on what shell you're running. Another
>>>>> is to set the MCA parameter opal_set_max_sys_limits to 1.
>>>> That didn't work either:
>>>> $ mpirun -mca opal_set_max_sys_limits 1 -np 256 mpihello
>>>> mpirun noticed that job rank 0 with PID 0 on node
>>>> exited on signal 15 (Terminated).
>>>> 252 additional processes aborted (not shown)

Prentice Bisbal
Linux Software Support Specialist/System Administrator
School of Natural Sciences
Institute for Advanced Study
Princeton, NJ