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Subject: Re: [OMPI users] Optimal mapping/binding when threads are used?
From: Saliya Ekanayake (esaliya_at_[hidden])
Date: 2014-04-11 01:22:25


Thank you Ralph for the details and it's a good point you mentioned on
mapping by node vs socket. We have another program that uses a chain of
send receives, which will benefit from having consecutive ranks nearby.

I've a question on bind to none being equal to bind to all. I understand
the two concepts mean the same thing, but I remember seeing poor
performance when bind to none is explicitly given. I need to check the
options I used and will let you know.

Yes, this test was mainly to understand how different patterns perform and
it seems P=1 is not suitable for this hardware configuration and may be in
general as you've mentioned.

Thank you,
Saliya

On Fri, Apr 11, 2014 at 12:30 AM, Ralph Castain <rhc_at_[hidden]> wrote:

> Interesting data. Couple of quick points that might help:
>
> option B is equivalent to --map-by node --bind-to none. When you bind to
> every core on the node, we don't bind you at all since "bind to all" is
> exactly equivalent to "bind to none". So it will definitely run slower as
> the threads run across the different NUMA regions on the node.
>
> You might also want to try --map-by socket, with no binding directive.
> This would map one process to each socket, binding it to the socket - which
> is similar to what your option A actually accomplished. The only difference
> is that the procs that share a node will differ in rank by 1, whereas
> option A would have those procs differ in rank by N. Depending on your
> communication pattern, this could make a big difference.
>
> Map-by socket is typically the fastest performance for threaded apps. You
> generally don't want P=1 unless you have a *lot* of threads in the process
> as it removes any use of shared memory, and so messaging will run slower -
> and you want the ranks that share a node to be the ones that most
> frequently communicate to each other, if you can identify them.
>
> HTH
> Ralph
>
> On Apr 10, 2014, at 5:59 PM, Saliya Ekanayake <esaliya_at_[hidden]> wrote:
>
> Hi,
>
> I am evaluating the performance of a clustering program written in Java
> with MPI+threads and would like to get some insight in solving a peculiar
> case. I've attached a performance graph to explain this.
>
> In essence the tests were carried out as TxPxN, where T is threads per
> process, P is processes per node, and N is number of nodes. I noticed an
> inefficiency with Tx*1*xN cases in general (tall bars in graph).
>
> To elaborate a bit further,
> 1. each node has 2 sockets with 4 cores each (totaling 8 cores)
> 2. used OpenMPI 1.7.5rc5 (later tested with 1.8 and observed the same)
> 3. with options
> A.) --map-by node:PE=4 and --bind-to core
> B.) --map-by node:PE=8 and --bind-to-core
> C.) --map-by socket and --bind-to none
>
> Timing of A,B,C came out as A < B < C, so used results from option A for Tx
> *1*xN in the graph.
>
> Could you please give some suggestion that may help to speed up these Tx
> *1*xN cases? Also, I expected B to perform better than A as threads could
> utilize all 8 cores, but it wasn't the case.
>
> Thank you,
> Saliya
>
>
> <image.png>
>
> --
> Saliya Ekanayake esaliya_at_[hidden]
> Cell 812-391-4914 Home 812-961-6383
> http://saliya.org
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-- 
Saliya Ekanayake esaliya_at_[hidden]
Cell 812-391-4914 Home 812-961-6383
http://saliya.org