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    SubjectRe: PCI_LATENCY_TIMER


    I did some mistakes in my previous calculations that need some fixes:
    Sorry for the bandwitch.

    On Sun, 27 Sep 1998, Gerard Roudier wrote:

    >
    > 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
    2

    > MAX_LAT = 8 --> 8 x 0.25 = 4 micro-seconds
    2

    > 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
    2

    > 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.

    Changed to:
    In order to achieve the SCSI device desired maximum bus latency, the PCI
    system software could assign a LATENCY TIMER of 33 to PCI devices, which
    is probaly a too short value at least for the SCSI device. But, the
    MAX_LAT requirement of the SCSI device looks like a bogus value and
    assuming the right value could be at least twice the MIN_GNT seems a lot
    more realistic.

    > 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.

    Changed to:
    The MAX_LAT value provided by the SCSI device is pain in the ass for the
    PCI software.

    > 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.

    (Same remark for 2 us)

    >
    > > 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. ;-)

    No change needed, there. ;-)

    >
    >
    > Regards,
    > Gerard.
    >
    >


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