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

Ashley Pittman ashley at quadrics.com
Wed Feb 16 06:26:55 EST 2005


On Wed, 2005-02-16 at 00:05 -0800, Greg Lindahl wrote:
> Quadrics STEN (forgive me for classing this as
> dumb, I happen to think dumb is a compliment...) get this right.

In this context the STEN in used on the transmit side of the network as
a way of doing effectively PIO writes directly into the network.  On the
receive side the NIC is anything but dumb and does the MPI tag matching.
It's almost entirely bypasses the CPU leaving it free to do *whatever
the application desires*.

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).

Ashley,
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