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Subject: Re: [OMPI devel] [OMPI svn-full] svn:open-mpi r22762
From: Terry Dontje (Terry.Dontje_at_[hidden])
Date: 2010-03-03 16:06:36


Terry Dontje wrote:
> Iain Bason wrote:
>> On Mar 3, 2010, at 3:04 PM, Jeff Squyres wrote:
>>
>>
>>> Mmmm... good point. I was thinking specifically of the
>>> if_in|exclude behavior in the openib BTL. That uses strcmp, not
>>> strncmp. Here's a complete list:
>>>
>>> ompi_info --param all all --parsable | grep include | grep :value:
>>> mca:opal:base:param:opal_event_include:value:pollmca:btl:ofud:param:btl_ofud_if_include:value:
>>>
>>> mca:btl:openib:param:btl_openib_if_include:value:
>>> mca:btl:openib:param:btl_openib_ipaddr_include:value:mca:btl:openib:param:btl_openib_cpc_include:value:
>>>
>>> mca:btl:sctp:param:btl_sctp_if_include:value:
>>> mca:btl:tcp:param:btl_tcp_if_include:value:
>>> mca:btl:base:param:btl_base_include:value:
>>> mca:oob:tcp:param:oob_tcp_if_include:value:
>>>
>>> Do we know what these others do? I only checked openib_if_*clude --
>>> it's strcmp.
>>>
>>
>> I haven't looked at those, but it's easy to grep for strncmp...
>>
>> It looks as though sctp is the only other BTL that uses strncmp.
>>
>> Of course, if we decide to change the default so that it no longer
>> includes "lo" then maybe using strncmp doesn't matter. The problem
>> has been that the name of the interface is different on different
>> platforms.
>>
>> (I should note that the default also excludes "sppp". I don't know
>> anything about that interface.)
>>
> I may be wrong for the usage here but the old Sun Starcats had a tcp
> interface named sppp to its diagnostic processor that we needed to skip.
> Not sure if this is the same reason done here, I couldn't find where
> sppp was referenced so I could find the history of the line in opengrok.
>
> --td
Close, r19988 added sppp to the excluded interfaces for the Sun M9000
server. I believe for the same reason I gave above.

--td
>>
>>>>> Additionally, if loopback is now handled properly via change #2,
>>>>> shouldn't the default value for the btl_tcp_if_exclude parameter
>>>>> now be empty?
>>>>>
>>>> That's a good question. Enabling the "lo" interface results in
>>>> intra-node messages being striped across that interface in addition
>>>> to the others on a system. I don't know what impact that would
>>>> have, if any.
>>>>
>>> sm and self should still be prioritized above it, right? If so, we
>>> should be ok.
>>>
>>
>> Yes, that's true. It would only affect those who restrict intra-node
>> communication to TCP.
>>
>>
>>> However, I think you're right that the addition of striping across
>>> lo* in addition to the other interfaces might have an unknown effect.
>>>
>>> Here's a random question -- if a user does not use the sm btl, would
>>> sending messages through lo for on-node communication be potentially
>>> better than sending it through a real device, given that that real
>>> device may be far away (in the NUMA sense of "far")? I.e., are OS's
>>> typically smart enough to know that loopback traffic may be able to
>>> stay local to the NUMA node, vs. sending it out to a device and
>>> back? Or are OS's smart enough to know that if the both ends of a
>>> TCP socket are on the same node -- regardless of what IP interface
>>> they use -- and if both processes are on the same NUMA locality,
>>> that the data can stay local and not have to make a round trip to
>>> the device?
>>>
>>> (I admit that this is a fairly corner case -- doing on-node
>>> communication but *not* using the sm btl...)
>>>
>>
>> Good question. For the loopback interface there is no physical
>> device, so there should be no NUMA effect. For an interface with a
>> physical device there may be some reason that a packet would actually
>> have to go out to the device. If there is no such reason, I would
>> expect Unix to be smart enough not to do it, given how much
>> intra-node TCP traffic one commonly sees on Unix. I couldn't hazard
>> a guess about Windows.
>>
>> Iain
>>
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>>
>
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