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Subject: Re: [OMPI devel] Very poor performance with btl sm on twin nehalem servers with Mellanox ConnectX installed
From: Paul H. Hargrove (PHHargrove_at_[hidden])
Date: 2010-05-16 20:43:16

If I google "ompi sm btl performance" the top match is

I scanned the entire page from top to bottom and don't see any questions
of the form
    Why is SM performance slower than ...?

The words "NFS", "network", "file system" or "filesystem" appear nowhere
on the page. The closest I could find is
> 7. Where is the file that sm will mmap in?
> The file will be in the OMPI session directory, which is typically
> something like /tmp/openmpi-sessions-myusername_at_mynodename* . The file
> itself will have the name shared_mem_pool.mynodename. For example, the
> full path could be
> /tmp/openmpi-sessions-myusername_at_node0_0/1543/1/shared_mem_pool.node0.
> To place the session directory in a non-default location, use the MCA
> parameter orte_tmpdir_base.
which says nothing about where one should or should not place the
session directory.

Not having read the entire FAQ from start to end, I will not contradict
Ralph's claim that the "your SM performance might suck if you put the
session directory on a remote filesystem" FAQ entry does exist, but I
will assert that I did not find it in the SM section of the FAQ. I
tried google on "ompi session directory" and "ompi orte_tmpdir_base" and
still didn't find whatever entry Ralph is talking about. So, I think
the average user with no clue about the relationship between the SM BLT
and the session directory would need some help finding it. Therefore, I
still feel an FAQ entry in the SM category is warranted, even if it just
references whatever entry Ralph is referring to.


Ralph Castain wrote:
> We have had a FAQ on this for a long time...problem is, nobody reads it :-/
> Glad you found the problem!
> On May 14, 2010, at 3:15 PM, Paul H. Hargrove wrote:
>> Oskar Enoksson wrote:
>>> Christopher Samuel wrote:
>>>> Subject: Re: [OMPI devel] Very poor performance with btl sm on twin
>>>> nehalem servers with Mellanox ConnectX installed
>>>> To: devel_at_[hidden]
>>>> Message-ID:
>>>> <D45958078CD65C429557B4C5F492B6A60770E51F_at_[hidden]>
>>>> Content-Type: text/plain; charset="iso-8859-1"
>>>> On 13/05/10 20:56, Oskar Enoksson wrote:
>>>>> The problem is that I get very bad performance unless I
>>>>> explicitly exclude the "sm" btl and I can't figure out why.
>>>> Recently someone reported issues which were traced back to
>>>> the fact that the files that sm uses for mmap() were in a
>>>> /tmp which was NFS mounted; changing the location where their
>>>> files were kept to another directory with the orte_tmpdir_base
>>>> MCA parameter fixed that issue for them.
>>>> Could it be similar for yourself ?
>>>> cheers,
>>>> Chris
>>> That was exactly right, as you guessed these are diskless nodes that
>>> mounts the root filesystem over NFS.
>>> Setting orte_tmpdir_base to /dev/shm and btl_sm_num_fifos=9 and then
>>> running mpi_stress on eight cores measures speeds of 1650MB/s for both
>>> 1MB messages and 1600MB/s for 10kB messages.
>>> Thanks!
>>> /Oskar
>>> _______________________________________________
>>> devel mailing list
>>> devel_at_[hidden]
>> Sounds like a new FAQ entry is warranted.
>> -Paul
>> --
>> Paul H. Hargrove PHHargrove_at_[hidden]
>> Future Technologies Group
>> HPC Research Department Tel: +1-510-495-2352
>> Lawrence Berkeley National Laboratory Fax: +1-510-486-6900
>> _______________________________________________
>> devel mailing list
>> devel_at_[hidden]
> _______________________________________________
> devel mailing list
> devel_at_[hidden]

Paul H. Hargrove                          PHHargrove_at_[hidden]
Future Technologies Group                 Tel: +1-510-495-2352
HPC Research Department                   Fax: +1-510-486-6900
Lawrence Berkeley National Laboratory