[Beowulf] Re: Re: Home beowulf - NIC latencies

Patrick Geoffray patrick at myri.com
Wed Feb 16 07:04:31 EST 2005


Ashley Pittman wrote:

> Interesting enough the STEN is a very good example of what is being
> discussed here, doing a remote write (Or MPI send) using the STEN is
> lower latency than using a DMA but uses more CPU cycles (as the STEN
> needs the data to be "pushed" from the main CPU whereas a (R)DMA only
> needs the DMA descriptor to be "pushed" and the NIC then "pulls" the
> actual data).

It seems to be common practice to use PIO for small messages on the send 
side. MX/Myrinet does that too (whereas GM/Myrinet does not), SCI does 
it, Greg's IB on HT does it. I don't know who is not burning some cycles 
to get lower latency for small messages these days.

Patrick
-- 

Patrick Geoffray
Myricom, Inc.
http://www.myri.com
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