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Subject: Re: [OMPI devel] openib fragment alignment
From: Jeff Squyres (jsquyres) (jsquyres_at_[hidden])
Date: 2013-02-20 16:56:43


I waffled on this issue a bit (and talked w/ Nathan about it in IM), but with my RM hat on, I'm giving a final ruling: no.

This is too "last second", and it's for an incredibly small set of platforms and configuration options.

I see that the risk is pretty small for this commit, but history is littered with "but that should have worked!". I'd rather be conservative and have a good 1.6.4 release. Since this has been committed on the trunk already, we can see what happens (likely: it'll cause no problems), and someday move it over to 1.6.5 if anyone cares.

- Grouchy old RM

On Feb 20, 2013, at 4:51 PM, Nathan Hjelm <hjelmn_at_[hidden]> wrote:

> I talked to Pasha about the change. He suggests fragments are 2-byte aligned to save space. I suspect that on 64-bit platforms the fragment size is already a multiple of 8 bytes so this change will likely only affect 32-bit systems (which is where the bus error is occurring).
>
> -Nathan
>
> On Wed, Feb 20, 2013 at 09:39:09PM +0000, Joshua Ladd wrote:
>> I would hold off, if possible, until I can investigate the issue. I don't, off-hand, know why the 2-byte alignment, although I would suspect it's for performance reasons.
>>
>>
>> Josh
>>
>>
>> -----Original Message-----
>> From: devel-bounces_at_[hidden] [mailto:devel-bounces_at_[hidden]] On Behalf Of Jeff Squyres (jsquyres)
>> Sent: Wednesday, February 20, 2013 4:35 PM
>> To: <devel_at_[hidden]>
>> Subject: [OMPI devel] openib fragment alignment
>> Importance: High
>>
>> Arrgh. I think you're going to make me eat my words (http://www.open-mpi.org/community/lists/devel/2013/02/12143.php).
>>
>> I just recently lost my access to InfiniBand test gear, so I can't test this myself. Hypothetically, it should be fine. But throwing in an untested change literally right before a release without IB vendor say-so really, really gives me pause...
>>
>> Mellanox?
>>
>>
>> On Feb 20, 2013, at 4:27 PM, Open MPI <bugs_at_[hidden]> wrote:
>>
>>> #3519: Move r28083 to v1.6 branch
>>> -----------------------------------+--------------------------
>>> Reporter: hjelmn | Owner: hjelmn
>>> Type: changeset move request | Status: new
>>> Priority: major | Milestone: Open MPI 1.6
>>> Version: trunk |
>>> -----------------------------------+--------------------------
>>> (In [28083]) btl/openib: don't align fragments on 2 byte boundaries
>>> (changed to 8)
>>>
>>> cmr:v1.6,v1.7
>>>
>>> --
>>> Ticket URL: <https://svn.open-mpi.org/trac/ompi/ticket/3519>
>>> Open MPI <http://www.open-mpi.org/>
>>>
>>> _______________________________________________
>>> bugs mailing list
>>> bugs_at_[hidden]
>>> http://www.open-mpi.org/mailman/listinfo.cgi/bugs
>>
>>
>> --
>> Jeff Squyres
>> jsquyres_at_[hidden]
>> For corporate legal information go to: http://www.cisco.com/web/about/doing_business/legal/cri/
>>
>>
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-- 
Jeff Squyres
jsquyres_at_[hidden]
For corporate legal information go to: http://www.cisco.com/web/about/doing_business/legal/cri/