+1 on what Gilles said. :-)
Check out this part of the v1.10 README file:
https://github.com/open-mpi/ompi-release/blob/v1.10/README#L585-L625
Basically:
- PML is the back-end to functions like MPI_Send and MPI_Recv.
- The ob1 PML uses BTL plugins in a many-of-many relationship to potentially
utilize multiple networks.
- The cm PML uses matching-style network APIs in CM plugins to utilize a single
underlying network.
- The yalla PML was written by Mellanox as a replacement for cm and ob1, in
that it directly utilizes the MXM network library without going through any of
the abstractions in ob1 and cm. It was written at a time when cm was not well
optimized, and basically just added a latency penalty before dispatching to the
underlying MTL module. Since then, cm has been optimized such that its
abstraction penalty before invoking the underlying MTL module is negligible.
So the question really comes down to:
- if you have a network stack API that does MPI-style matching, you should
write an MTL.
- if not, you should write a BTL
Does that help?
> On Feb 4, 2016, at 2:29 AM, Gilles Gouaillardet <gil...@rist.or.jp> wrote:
>
> Durga,
>
> did you confuse PML and MTL ?
>
> basically, a BTL (Byte Transport Layer ?) is used with "primitive"
> interconnects that can only send bytes.
> (e.g. if you need to transmit a tagged message, it is up to you send/recv the
> tag and manually match the tag on the receiver side so you can put the
> message into the right place)
> on the other hand, MTL (Message Transport Layer ?) can be used with more
> advanced interconnects, that can "natively" send/recv (tagged) messages.
>
> for example, with infiniband, you can use the openib BTL, or the mxm MTL
> (note the openib BTL only requires the free ibverbs libraries
> and mxm MTL requires proprietary extensions provided by mellanox)
>
> a good starting point is the video Jeff posted at
> https://www.open-mpi.org/video/?category=internals
>
> Cheers,
>
> Gilles
>
> On 2/4/2016 2:20 PM, dpchoudh . wrote:
>> Hi developers
>>
>> I am trying to add support for a new (proprietary) RDMA capable fabric
>> to OpenMPI and have the following question:
>>
>> As I understand, some networks are implemented as a PML framework and
>> some are implemented as a BTL framework. It seems there is even
>> overlap as Myrinet seems to exist in both.
>>
>> My question is: what is the difference between these two frameworks?
>> When adding support for a new fabric, what factors one should consider
>> when choosing between one type of framework over the other?
>>
>> And, with apologies for asking a summary question: is there any kind
>> of documentation and/or book that explains all the internal details of
>> the implementation (which looks little like voodoo to a newcomer like
>> me)?
>>
>> Thanks for your help.
>>
>> Durga Choudhury
>>
>> Life is complex. It has real and imaginary parts.
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>>
>
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