PCI-64: how to find

Patrick Geoffray patrick at myri.com
Mon Feb 25 12:22:20 PST 2002

Velocet wrote:
> On Mon, Feb 25, 2002 at 02:11:03PM -0500, Patrick Geoffray's all...
>>Velocet wrote:
>>>6.5us for Myrinet vs 32/33 pci being 8? Doesnt seem like the win that
>>>it usually seems to be spec'd at. Are these #s accurate? ('out of the blue'
>>>warning notwithstanding ;)
>>Euh, yes, I think they are: 6.5 us with a 64/66 PCI, 8 us with a 32/33 
>>PCI (Lanai9 at 200 MHz). The main difference here is the PCI clock, the 
>>latency with a 64/33 MHz is between 7.5 and 8 us.
> with what size packets? 

0-sized packets.

> Wouldnt the latency reduce for a several hundred
> byte packet just because the bus is that much wider? and twice the effect
> for 66/64 pci? 

A 64 bits PCI is not always 2 times faster than a 32 bits one. It's true 
for medium and large transactions, but not for tiny packets where the 
number of cycles used for the arbitration and to reach the PCI device is 
the main parameter. In short, 64 bits is good to do burst, but 66 Mhz is 
what you want to just cross the PCI.

> Is the cost of 64 gigabit cards vs 32 worth it for things
> sensitive to latency?

I would say there should not be any difference in cost between 64 bits 
and 32 bits cards these days. If we are talking about Gigabit Ethernet, 
there would be no latency difference for tiny packets, it's so small 
compared to the IP stack latency.


|   Patrick Geoffray, Ph.D.      patrick at myri.com
|   Myricom, Inc.                http://www.myri.com
|   Cell:  865-389-8852          685 Emory Valley Rd (B)
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