Re: PCI_LATENCY_TIMER
Doug Ledford (dledford@dialnet.net)
Mon, 28 Sep 1998 19:48:13 -0500
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
> 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. ;-)
Your interpretation of the PCI spec and what Adaptec thinks these values
mean in terms of the PCI bus are two different things. One of you is wrong
about what the MAX_LAT value is all about.
--
Doug Ledford <dledford@dialnet.net>
Opinions expressed are my own, but
they should be everybody's.
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