zx1 PCI DMA

From: Matthew Chapman <matthewc_at_cse.unsw.edu.au>
Date: 2008-01-31 03:31:28
Hi folks,

I'm trying to track down a PCI performance problem - part of my
never-ending thesis troubles - and one thing I'm finding is that my HP
zx1-based Itaniums are taking surprisingly long to satisfy PCI DMA
reads.

On a 66Mhz PCI bus it seems to be taking about 60-75 bus cycles, i.e.
~1000ns, to initiate a read targetting a cache line that was previously
owned by a processor.  Even cache lines that have recently been accessed
by the PCI device, without being touched by a processor, seem to be
taking of the order of 50 bus cycles.

This is a big surprise to me, since I know that zx1 performs really well
CPU<->memory (order of 100ns).

Does anyone know what the achievable DMA latency should be, and what I
can tune on the zx1 chipset or PCI card?

Thanks,
Matt

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Received on Thu Jan 31 03:38:10 2008

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