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Subject: Re: [OMPI users] About MPI_TAG_UB
From: Jeff Squyres (jsquyres_at_[hidden])
Date: 2012-09-28 10:00:18

On Sep 28, 2012, at 9:50 AM, Sébastien Boisvert wrote:

> I did not know about shared queues.
> It does not run out of memory. ;-)

It runs out of *registered* memory, which could be far less than your actual RAM. Check this FAQ item in particular:

> But the latency is not very good.
> ** Test 1
> --mca btl_openib_max_send_size 4096 \
> --mca btl_openib_eager_limit 4096 \
> --mca btl_openib_rndv_eager_limit 4096 \
> --mca btl_openib_receive_queues S,4096,2048,1024,32 \
> I get 1.5 milliseconds.
> =>
> ** Test 2
> --mca btl_openib_receive_queues S,65536,256,128,32 \
> I get around 1.5 milliseconds too.
> =>

Are you saying 1.5us is bad? That's actually not bad at all. On the most modern hardware with a bunch of software tuning, you can probably get closer to 1us.

> With my virtual router I am sure I can get something around 270 microseconds.

OTOH, that's pretty bad. :-)

I'm not sure why it would be so bad -- are you hammering the virtual router with small incoming messages? You might need to do a little profiling to see where the bottlenecks are.

> Just out of curiosity, does Open-MPI utilize heavily negative values
> internally for user-provided MPI tags ?

I know offhand we use them for collectives. Something is tickling my brain that we use them for other things, too (CID allocation, perhaps?), but I don't remember offhand.

I'm just saying: YMMV. Buyer be warned. And all that. :-)

> If the negative tags are internal to Open-MPI, my code will not touch
> these private variables, right ?

It's not a variable that's the issue. If you do a receive for tag -3 and OMPI sends an internal control message with tag -3, you might receive it instead of OMPI's core. And that would be Bad.

Jeff Squyres
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