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Subject: Re: [OMPI devel] collective problems
From: Richard Graham (rlgraham_at_[hidden])
Date: 2007-11-08 09:43:15


On 11/8/07 4:03 AM, "Gleb Natapov" <glebn_at_[hidden]> wrote:

> On Wed, Nov 07, 2007 at 11:25:43PM -0500, Patrick Geoffray wrote:
>> Richard Graham wrote:
>>> The real problem, as you and others have pointed out is the lack of
>>> predictable time slices for the progress engine to do its work, when relying
>>> on the ULP to make calls into the library...
>>
>> The real, real problem is that the BTL should handle progression at
>> their level, specially when the buffering is due to BTL-level flow
>> control. When I write something into a socket, TCP will take care of
>> sending it eventually, for example.
> In the case of TCP, kernel is kind enough to progress message for you,
> but only if there was enough space in a kernel internal buffers. If there
> was no place there, TCP BTL will also buffer messages in userspace and
> will, eventually, have the same problem.
>
> To progress such outstanding messages additional thread is needed in
> userspace. Is this what MX does?

Yes - this is the bottom line, with the current problem the high cost of
scheduling such threads at some sort of reasonable frequency.

Rich

>
> --
> Gleb.
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