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Subject: Re: [OMPI users] strange IMB runs
From: Eugene Loh (Eugene.Loh_at_[hidden])
Date: 2009-08-13 01:51:33

I was away on vacation for two weeks and therefore missed most of this
thread, but I'm quite interested.

Michael Di Domenico wrote:

>I'm not sure I understand what's actually happened here. I'm running
>IMB on an HP superdome, just comparing the PingPong benchmark
>HP-MPI v2.3
>Max ~ 700-800MB/sec
>OpenMPI v1.3
>-mca btl self,sm - Max ~ 125-150MB/sec
>-mca btl self,tcp - Max ~ 500-550MB/sec
>Is this behavior expected? Are there any tunables to get the OpenMPI
>sockets up near HP-MPI?

First, I want to understand the configuration. It's just a single
node. No interconnect (InfiniBand or Ethernet or anything). Right?

If so, without knowing too much about the Superdome, I assume the only
puzzle here is why the Open MPI sm bandwidth is so low. The other stuff
(like HP results or OMPI tcp results) are fine so far as I know.

Specifically, I tried some on-node bandwidth tests on another system
comparing sm and tcp, and tcp is about 1.4x slower than sm. I think
this is consistent with expectations and makes the OMPI tcp performance
roughly consistent with the HP MPI performance.

So, again, the single oddity here appears to be the very slow OMPI sm

George Bosilca wrote:
>The leave pinned will not help in this context.

Michael Di Domenico wrote:
>mpi_leave_pinned didn't help still at ~145MB/sec

Right. The "leave pinned" variable should be irrelevent, both for TCP
(which isn't the issue here) and for sm (which is disturbingly low).

Michael Di Domenico wrote:

>On Thu, Jul 30, 2009 at 10:08 AM, George Bosilca<bosilca_at_[hidden]>
>>The Open MPI version is something you compiled or it came installed
with the
>>OS? If you compiled it can you please provide us the configure line?
>OpenMPI was compiled from source v1.3 with only a --prefix line, no
>other options.

I think a configure line with only --prefix is okay, but for performance
you probably need compiler optimization flags set one way or the other.
One way is with environment variables. E.g., for csh shell and GCC
compilers, maybe something like:

setenv CFLAGS "-O -m64 -g"
setenv CXXFLAGS "-O -m64 -g"
setenv FFLAGS "-O -m64 -g"
setenv FCFLAGS "-O -m64 -g"

or whatever.

That said, I just tried building OMPI with and without optimization, and
the on-node bandwidth seems basically unaffected. I suppose that is
perhaps no surprise since the data movement will basically just be
driven by memcpy calls anyhow.

Michael Di Domenico wrote:
>Here's an interesting data point. I installed the RHEL rpm version of
>OpenMPI 1.2.7-6 for ia64
>mpirun -np 2 -mca btl self,sm -mca mpi_paffinity_alone 1 -mca
>mpi_leave_pinned 1 $PWD/IMB-MPI1 pingpong
>With v1.3 and -mca btl self,sm i get ~150MB/sec
>With v1.3 and -mca btl self,tcp i get ~550MB/sec
>With v1.2.7-6 and -mca btl self,sm i get ~225MB/sec
>With v1.2.7-6 and -mca btl self,tcp i get ~650MB/sec

Michael Di Domenico wrote:
>So pushing this along a little more
>running with openmpi-1.3 svn rev 20295
>mpirun -np 2
> -mca btl sm,self
> -mca mpi_paffinity_alone 1
> -mca mpi_leave_pinned 1
> -mca btl_sm_eager_limit 8192
>$PWD/IMB-MPI1 pingpong
>Yields ~390MB/sec
>So we're getting there, but still only about half speed

One of the differences among MPI implementations is the default
placement of processes within the node. E.g., should processes by
default be collocated on cores of the same socket or on cores of
different sockets? I don't know if that issue is applicable here (that
is, HP MPI vs Open MPI or on Superdome architecture), but it's
potentially an issue to look at. With HP MPI, mpirun has a -cpu_bind
switch for controlling placement. With Open MPI, mpirun controls
placement with -rankfile.

E.g., what happens if you try

% cat rf1
rank 0=XX slot=0
rank 1=XX slot=1
% cat rf2
rank 0=XX slot=0
rank 1=XX slot=2
% cat rf3
rank 0=XX slot=0
rank 1=XX slot=3
% mpirun -np 2 --mca btl self,sm --host XX,XX -rf rf1 $PWD/IMB-MPI1 pingpong
% mpirun -np 2 --mca btl self,sm --host XX,XX -rf rf2 $PWD/IMB-MPI1 pingpong
% mpirun -np 2 --mca btl self,sm --host XX,XX -rf rf3 $PWD/IMB-MPI1 pingpong

where XX is the name of your node and you march through all the cores on
your Superdome node?

Also, I'm puzzled why you should see better results by changing
btl_sm_eager_limit. That shouldn't change long-message bandwidth, but
only the message size at which one transitions from short to long
messages. If anything, tweaking btl_sm_max_send_size would be the
variable to try.

Final note: Notice the space in "Open MPI". Some legal thing.