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 - To unsubscribe from this list: send the line "unsubscribe linux-ia64" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
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