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Subject: Re: [OMPI devel] KNEM + user-space hybrid for sm BTL
From: Aurélien Bouteiller (bouteill_at_[hidden])
Date: 2013-07-18 11:52:38


Le 18 juil. 2013 à 11:12, "Iliev, Hristo" <iliev_at_[hidden]> a écrit :

> Hello,
>
> Could someone, who is more familiar with the architecture of the sm BTL, comment on the technical feasibility of the following: is it possible to easily extend the BTL (i.e. without having to rewrite it completely from scratch) so as to be able to perform transfers using both KNEM (or other kernel-assisted copying mechanism) for messages over a given size and the normal user-space mechanism for smaller messages with the switch-over point being a user-tunable parameter?
>
> From what I’ve seen, both implementations have something in common, e.g. both use FIFOs to communicate controlling information.
> The motivation behind this are our efforts to become greener by extracting the best possible out of the box performance on our systems without having to profile each and every user application that runs on them. We’ve already determined that activating KNEM really benefits some collective operations on big shared-memory systems, but the increased latency significantly slows down small message transfers, which also hits the pipelined implementations.
>

Hristo,

The knem BTL currently available in the trunk does just this :) You can use either Knem or Linux CMA to accelerate interprocess transfers. You can use the following mca parameters to turn on knem mode:

-mca btl_sm_use_knem 1

If my memory serves me well, anything under eager limit is sent by regular double copy:

-mca btl_sm_eager_limit 4096 (is the default, so anything below 1 page is copy-in, copy-out). If I remember correctly, anything below 16k decreased performance.

We also have a collective component leveraging on knem capabilities. If you want more info about the details,
you can look at the following paper we published at IPDPS last year. It covers what we found to be the best cutoff values for using (or not) knem in several collective.

Teng Ma, George Bosilca, Aurelien Bouteiller, Jack Dongarra, "HierKNEM: An Adaptive Framework for Kernel-Assisted and Topology-Aware Collective Communications on Many-core Clusters," Parallel and Distributed Processing Symposium, International, pp. 970-982, 2012 IEEE 26th International Parallel and Distributed Processing Symposium, 2012

http://www.computer.org/csdl/proceedings/ipdps/2012/4675/00/4675a970-abs.html

Enjoy,
Aurelien

> sm’s code doesn’t seem to be very complex but still I’ve decided to ask first before diving any deeper.
>
> Kind regards,
> Hristo
> --
> Hristo Iliev, PhD – High Performance Computing Team
> RWTH Aachen University, Center for Computing and Communication
> Rechen- und Kommunikationszentrum der RWTH Aachen
> Seffenter Weg 23, D 52074 Aachen (Germany)
>
>
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--
* Dr. Aurélien Bouteiller
* Researcher at Innovative Computing Laboratory
* University of Tennessee
* 1122 Volunteer Boulevard, suite 309b
* Knoxville, TN 37996
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