PCI Latency - What is It and How does it affect performance?

[PrinceXizor]

There's an interesting forum thread in GH linking to an article I found a few days ago about PCI latency.

I know PCI latency "patches" and such are around mostly due to Creative vs. VIA problems.

But, I'm more interested in general ideas.

What is PCI Latency?
Why do VideoCards have High Numbers?
Why does PCI Latency affect performance? (i.e. what is its purpose and what does tweaking it accomplish).

Thanks!

P-X

 

[Peter]

The latency value, stored in each PCI and AGP device by system BIOS, denotes how long this device may own the bus before having to release bus ownership for a new arbitration.
All the devices waiting to win the bus ownership at that time will experience this allowance as a latency, hence the name.

AGP graphics cards typically are configured to high values, since their only contestant for AGP bus ownership is the system chipset itself - which will need to own the bus when the CPU or a media streaming agent (TV cards, DVD decoders ...) has transfers pending.

On a multi agent bus, high values increase single device throughput, low values increase bus time slice fairness between multiple agents.

 

[Peter]

So, Creative vs. VIA wasn't about all this at all - it was an electrical glitch problem on the SoundBlaster Live! card that upset the VIA chipset. There's a (performance reducing) workaround for that contained in VIA's drivers and most makers' BIOSes.

On a more related topic, in older BIOSes for VIA chipset boards, the programming of the chipset in regard to PCI performance tuning was rather poor. VIA has a "PCI performance patch" driver to remedy this - which of course you shouldn't apply when you have an original SB!Live in there. (SB!Live 5.1 series is OK.)

 

[Finnkc]

yep.

Using a KT400 chip with a Creative 5.1 card and works great.

 

[PrinceXizor]

Thanks Peter. I don't know if you read the article I'm talking about, some guy who had intermittent "stutters" when opening apps or streaming audio or whatever.

He lowered his video cards latency via powerstrip and something else hdd? will have to check and edit update later and left his soundcard the same and the problem he was having went away. Just wanted some technical info on PCI Latency.

So the longer it has to wait the more it processes when it DOES get on? is that how it works? I guess maybe I need some background on data transfer across a BUS

P-X

 

[zephyrprime]

So what units does pci latency have? Is it cycles or is it microseconds or is it milliseconds?

 

[Peter]

The value you see in the PCI device's register is in PCI clocks, divided by 8. So a value of 64 grants the device a maximum bus ownership time of 64x8 cycles.