How high can you go?

[towcutter]

I am curious about increasing the speed of the pci bus. I know if you go too high, you risk data corruption. My pci bus might be locked. I just wondered if anyone has increased the speed with good results. Would it make any difference in benchies on my rig? Also does the pci latency timer have any affect?

[Sonny]

Yes increasing the PCI BUS will result in increased performance for most systems but not all. The older VIA chipsets, KT133A/KT266A/KT333/KT400, benefited from this to varying degrees.



PCI-Latency Timer

Each PCI slot has a certain number of clock cycles for uninterrupted access to the system bus / CPU. Since each access also involves initial latencies (penalty cycles), the ratio between idle cycles and active cycles is better if the number of bus cycles (PCI-latency) is increased. The main argument for abandoning ISA slots has been that in a system with ISA devices, the PCI latency cannot be increased beyond 64 cycles. Doubling the latency from 64 to 128 cycles allegedly resulted in an overall system performance increase of 15% as reported by Boot Magazine (May 1998). We have never been able to replicate these findings, however we do see a performance increase by stepping up the latency from 32 to 64 cycles, especially with respect to HDD transfer rates.

Interestingly, our findings have also been implemented in George Breese's VIA IDE patch which increases the PCI latency to 96 cycles even on boards that hide the latency setting from the users.