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    SubjectRe: PCI_LATENCY_TIMER

    On Sun, 27 Sep 1998, Mike Black wrote:

    > I found the discussions concerning this to be quite interesting but not very
    > illuminating.

    What kind of illumination were you expecting?

    What about the following?

    > Here's my question: I've got a network server that primarily uses SCSI,
    > IDE, and a 100BaseT Network Card. All other PCI devices (e.g VGA) are not
    > used much. How do I optimize this setup for the SCSI/Network IDE/Network
    > throughput with the understanding that generally only SCSI or IDE access is
    > happening at one time??
    >
    > I see that my SCSI, IDE and Netcard report:
    > SCSI Latency: 8 min, 8 max, 64 set, cache line size 08
    > IDE Latency: 64 set
    > Network Latency: 8 min, 28 max, 64 set

    Let me translate this output into PCI language:

    SCSI:
    MIN_GNT = 8 --> 8 x 0.25 = 4 micro-seconds
    MAX_LAT = 8 --> 8 x 0.25 = 4 micro-seconds
    LATENCY_TIMER = 64 --> 64x0.030 = 1.92 micro-seconds

    IDE:
    LATENCY_TIMER = 64 --> 64x0.030 = 1.92 micro-seconds

    Network:
    MIN_GNT = 8 --> 8 x 0.25 = 4 micro-seconds
    MAX_LAT = 28 --> 28x 0.25 = 7 micro-seconds
    LATENCY_TIMER = 64 --> 64x0.030 = 1.92 micro-seconds

    If we only take into account these 3 devices, the predictable PCI BUS
    latency is 2*1.92 = 3.84 micro-seconds that fits the MAX_LAT requirement
    of the SCSI device that is the lowest value for MAX_LAT.

    My comments:

    1 - The system software that chose a latency timer of 64 for all
    devices has not been able to fit the MAX_GNT value due to the SCSI
    controller providing it very probably _wrong_ informations, but the
    MAX_LAT requirement of all devices has been achieved.

    2 - A device that desires to be granted 4 us for a BUS transaction and
    that want the maximum BUS latency to be at most that 4 us is kind of
    shit-maker for PCI BIOSes and PCI drivers that want to make things
    fine, unless it is required to be the unique device on a PCI BUS.

    > 00:0b.0 SCSI storage controller: Adaptec AIC-7880U

    What a great illumination I have had 4 years ago to go with Symbios
    controllers rather than Adaptec ones. ;-)


    Regards,
    Gerard.


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