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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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