[Beowulf] Home beowulf - NIC latencies

James Cownie jcownie at etnus.com
Mon Feb 7 08:26:29 PST 2005

> Patrick Geoffray <patrick at myri.com> wrote:
> duncan.roweth at quadrics.com wrote:
> > This example reports the average time for 1000
> > blocking get calls. Patrick's description of the
> > mechanism is essentially correct, apart from the
> > detail that we have a fast path for short operations
> > that avoids the need to set up a DMA.
> How can you do one-sided operations without a DMA on the target side ?!?
> The only way that I can think of is to map the host virtual memory
> into the NIC memory space and let all memory writes generates PIO
> writes to actually modify the NIC memory. Surely, you must be talking
> about another DMA.

I think you're talking at cross-purposes.

Patrick is right that in the target machine there is a DMA operation
initiated by the NIC.

However Duncan is saying that Quadrics don't send a DMA request packet
over their network, but have a more optimised less general request that
they can issue without having to build a full DMA descriptor in the host
machine and transfer it to the target. 

Therefore in Quadrics' terms no DMA operation is sent over the net,
whereas from Patricks' viewpoint a DMA operation _does_ occur.

-- Jim
James Cownie	<jcownie at etnus.com>
Etnus, LLC.     +44 117 9071438

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