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From: Jeff Squyres (jsquyres_at_[hidden])
Date: 2007-07-12 12:39:49


There's a few benefits:

- Remember that you post a big pool of buffers instead of num_peers
individual sets of receive buffers. Hence, if you post M buffers for
each of N peers, each peer -- due to flow control -- can only have M
outstanding sends at a time. So if you have apps sending lots of
small messages, you can get better utilization of buffer space
because a single peer has more than M buffers to receive into.

- You can also post less than M*N buffers by playing the statistics
of your app -- if you know that you won't have more than M*N messages
outstanding at any given time, you can post fewer receive buffers.

- At the same time, there's a problem with flow control (meaning that
there is none): how can a sender know when they have overflowed the
receiver (other than an RNR)? So it's not necessarily as safe.

- So if you want to simply eliminate the flow control, choose M high
enough (or just a total number of receive buffers to post to the SRQ)
that you won't ever run out of resources and you should see some
speedup from lack of flow control. This obviously mainly helps apps
with lots of small messages; it may not help in many other cases.

On Jul 12, 2007, at 12:29 PM, Don Kerr wrote:

> Through mca parameters one can select the use of shared receive queues
> in the openib btl, other than having fewer queues I am wondering what
> are the benefits of using this option. Can anyone eleborate on using
> them vs the default?
>
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-- 
Jeff Squyres
Cisco Systems