AVS Forum banner
  • Our native mobile app has a new name: Fora Communities. Learn more.

PCI Latency's Improve Jitters!

1163 Views 13 Replies 8 Participants Last post by  Kram Sacul
Ok so I spent most of my day at work reading about PCI Latency's and how they can improve jittery or sluggish applications.

Apparently adjusting your Latency's can have a big impact on those occasional audio jitters or fram rate drops. Most new video cards are set at around 248 (although your bios reads 32) and adjusting through your bios does nothing.

I read all this mainly at http://www.geocities.com/phileosopho...cilatency.html

Where he discuss's the topic!

Definately worth a read for those HTPC users or even PC users that get agravated with occasional jitters etc!

Also if anyone can actually explain PCI Latency's please do. I understand changing it helps but I really don't understand what it controls?

Not open for further replies.
1 - 14 of 14 Posts
Cool item - thanks Oz! :)
very interesting article.

did anyone try to tweak their pci latency's and did it make stutters go away ?
I'm actually looking for more info on what the PCI Latency controls before I do! Can't really find optimal numbers if you don't know what it's changing!

But as soon as I know I'll be trying and will post back within a day or two!

3 cups of coffee improves my jitters. ;) I wonder if it would affect an SCSI system, which runs more "efficiently" than ATA, I'd also say yes. This informative link might help - http://www.reric.net/linux/pci_latency.html

"PCI latency timers are a mechanism for PCI bus-mastering devices to share the PCI bus fairly. "Fair" in this case means that devices won't use such a large portion of the available PCI bus bandwidth that other devices aren't able to get needed work done.

How this works is that each PCI device that can operate in bus-master mode is required to implement a timer, called the Latency Timer, that limits the time that device can hold the PCI bus. The timer starts when the device gains bus ownership, and counts down at the rate of the PCI clock. When the counter reaches zero, the device is required to release the bus. If no other devices are waiting for bus ownership, it may simply grab the bus again and transfer more data.

If the latency timer is set too low, PCI devices will interrupt their transfers unnecessarily often, hurting performance. If it's set too high, devices that require frequent bus access may overflow their buffers, losing data.

So in theory there's a compromise that can be reached somewhere in between, where all devices can get good performance plus a reasonable guarantee that they will get bus access on a timely basis."
See less See more
Ahh! Perfect Explanation!

Well judging by that I can safely say that the defaults for most newer systems certainly arent to equal! 249 For graphics cards compared to 32 for your Onboard NIC? No wonder I have slow performance and packets dropping!

Here's another nice link (although he's running Linux, still informative for Windows users) - he talks about IRQs and PCI, the PCI latency timer and PCI latency approaches in "Linux hardware stability guide, Part 2" - http://www-106.ibm.com/developerwork...ary/l-hw2.html
This is from the EnTech (PowerStrip) website's FAQ...

"My graphics card and PCI audio card don't seem to get along. Can the PowerStrip help?

Maybe. Graphics cards can sometimes be too aggressive in holding the bus, causing problems for other bus-master PCI devices, most notably PCI sound cards. Although most PCI BIOSes have an option where you can specify a default PCI latency, many cards ignore or override this setting. The PowerStrip has an undocumented feature that allows you to specify the latency for each device on the PCI bus, individually. There aren't any hard and fast rules on appropriate values (a multiple of 8), but as a general guideline higher values yield better performance at the expense of other devices, while lower values are more appropriate to systems with multiple PCI cards. If you want to experiment with this feature, you'll need to manually edit your pstrip.ini file as there are no visual controls available. An example follows:




In this example, 001812D2 is the PCI device (0018) and vendor (12D2) ID for a Riva128, which normally has a very high latency of 248, and which the PS reduces to 80. If you don't know the PCI device and vendor ID for a particular card, you can find out by checking the PS's hardware details screen, which is opened by clicking the [...] button on the Windows page of the PS Graphics System Information dialog box."

Hope some of ya'll find this exerpt helpful...
See less See more
Thanks for the info!

The link to the article by the Gentoo CEO is quite helpful also!

Since we obviouslt can't change the PCI Latency in the bios does anyone know of another way besides PStrip?

I just don't like using PStrip because I can use that 10mb it uses as a background process elsewhere! Im not sure if you need to keep the background proccess running after changing the Latency's or not!

Maybe I'll hit up Pstrip and find out!

See less See more
But if the graphics card is AGP, is the PCI latency relevant and if it is does it make any difference to what the setting should be?
as far as I can gather yes! How the PCI bus and AGP bus work and intergrate is beyond me but from what I've read adjusting your video card's PCI Latency still helps!

I remember back when I used a SB 128 under linux... in order to get jitter free playback, you had to increase the PCI latency for that card (some linux FAQ I read suggested to double it). Before the change, audio was practically unlistenable... as much jitter as music... afterwords it was great, no jitter at all. Of course that was all due to poor drives for the SB 128 at the time under linux.. windows never had that problem.

No real point to this post other than a 1st hand example of PCI latency making a difference...
OZ-E did you try adjusting the PCI latency ?

did it work ?
Adjusting the pci latency thru the bios on my system eliminated tremendous audio distortions while playing dvds. I was very happy with the results. :)
1 - 14 of 14 Posts
Not open for further replies.