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Subject: Re: [OMPI devel] Shared Memory RDMA and eager messages
From: Torje Henriksen (torjeh_at_[hidden])
Date: 2008-05-15 07:28:06


Thanks Jeff.

Always fast answers from this mailing list. Very nice indeed. I'll
look at the PML OB1 module next then.

Best regards,

Torje

On May 15, 2008, at 1:15 PM, Jeff Squyres wrote:

> On May 15, 2008, at 5:55 AM, Torje Henriksen wrote:
>
>> 1. In the BTL SM component (both in "ompi_info --param btl sm" and
>> the
>> source), there is mention of RDMA. Is RDMA used in ths component? Is
>> any special hardware needed for using it?
>
> No and no. The SM BTL uses a copyout/copyin mechanism (copy out to
> shared memory, copy in to the target process) -- it does not directly
> copy from one process to another.
>
> The OB1 PML uses "RDMA-like" semantics; I'm guessing that's what it's
> referring to.
>
>> 2. As for eager vs non-eager messages. Is the only difference between
>> these the amount of memory allocated for a fragment? Is there any
>> rendevouz protocol used for large messages or anything like that?
>
> The rendesvouz protocol is enforced by the upper level PML (OB1) -- it
> is basically the lower-layer-independent "protocol engine". OB1
> decides the exact protocol to use (short, long, pipelined, etc.), and
> the lower later BTLs decide how to get the bits there.
>
>> The module based architecture is great, but sometimes I find it hard
>> to figure out what is happening :)
>
> Indeed. This is definitely a tradeoff when you only see small pieces
> of the puzzle at a time. :-\
>
> --
> Jeff Squyres
> Cisco Systems
>
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