PCI Latency settings


    I have recently built my own PC and have been reading on the internet ways to optimize its performance in the motherboard settings etc.

    On stock settings the Counterstrike stress test gave me 182fps all on full, and after I changed the PCI latency from 32 to 64 I had an increase of 5fps to 187fps. There are a few higher numbers is it worth fiddling or have I reached the optimum setting.



    The PCI latency timer specifies the amount of time a PCI device holds the PCI bus for data transfer before it is released. When a PCI bus is gained ownership from a PCI device, other PCI devices are in a wait state. By setting too low a value the PCI device only holds ownership of the PCI bus for too short a period and requires very frequent ownership and release of the bus resulting in low performance. Setting too high a value will increase the performance of the PCI device but hinders the other PCI devices while they wait for ownership of the PCI bus. Remember that the PCI bus is shared across all PCI devices (PCI Express is different, by the way).

    I suggest you keep the settings at their default values but if you need optimum calculations then you need to understand latency multipliers (which is the number of PCI clock cycles per quarter microsecond), minimum grant clocks and maximum latency clocks. As an example, a PCI device with 64k buffers will have the buffer treated as a bidirectional 2x32k buffer which means 8 DWORDs of transfer and this equates to 8 clock cycles of the PCI bus. At 33 MHz, the 8 cycles equate to a quarter of a microsecond as I mentioned earlier. The MIN_GNT value for this is 1 so when data is being transferred the device needs to fill up one of the 32k buffers every 3.2 microseconds which equates to a MAX-LAT value of 12.

    If the above sounds too convoluted then I really suggest you use the default settings and not to change settings willy nilly. Yes you may see an improvement in your graphics but you will hinder the devices on the rest of the PCI bus.

    Original Poster

    Thanks for the reply. I have no other pci devices on the mobo as my sound is intergrated and my internet is usb wireless. I will keep experimenting till I find a happy medium.
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