# PCI latency?



## mR Yellow (Mar 16, 2005)

I noticed that one can change the PCI lantechy  with ATI Tool.

My current latency before changing it was 255. 
What is the best latency setting?  

Ive set mine to 64 in the mean time


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## FatMaserati (Mar 16, 2005)

PCI latency will only change things if you are using a PCI graphics card, I'm fairly sure (though I may be wrong).

Another question: Is PCI Latency the equivilant of the AGP Aperture size?


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## mR Yellow (Mar 16, 2005)

FatMaserati said:
			
		

> PCI latency will only change things if you are using a PCI graphics card, I'm fairly sure (though I may be wrong).
> 
> Another question: Is PCI Latency the equivilant of the AGP Aperture size?



I think u wrong  
I heard it makes a diff...less stutter


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## FatMaserati (Mar 16, 2005)

Yeah, fair enough. I just read some stuff on it myself. General consensus is that lower = better.


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## INSTG8R (Mar 16, 2005)

yeah basically ATI and Nvidia figure they need to hog all the PCI clocks, why I dont know but I have mine set to 64 as well


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## FatMaserati (Mar 16, 2005)

In my motherboard settings, my PCI latency is set to 32 (can't get lower), so I just set it to 32 in ATITool as well. I thought there was no point setting it lower if the motherboard seemingly doesn't support it.

Also, my AGP aperture is set to 256.


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## INSTG8R (Mar 16, 2005)

Well my AGP controller runs at 64 so best match


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## kenling (Mar 16, 2005)

uhh... i think pci latency means the priority of the device. for example if your pci latency is 32 and your agp card's latency is 128, the agp card will get four times more attention than any pci device u might have installed. this may cause problems cause the pci device can't get what it needs hence it sometimes helps stutter problems if u set your agp latency to something lower so it won't hord all the resources. hope u can understand my very laymans explanation.


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## mR Yellow (Mar 16, 2005)

ATI cards by default are set to 255   
Setting it lower, to say 64, will remove stutter and u should have smoother gameplay. 
It will also remove sound stuttering /popping.


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## INSTG8R (Mar 16, 2005)

Yep thats a pretty good explanation of how it works.
On a sidenote I have inquired to the Catalyst Makers as to why they do this


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## mR Yellow (Mar 16, 2005)

INSTG8R said:
			
		

> Yep thats a pretty good explanation of how it works.
> On a sidenote I have inquired to the Catalyst Makers as to why they do this



Now we will just see if he will reply...


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## INSTG8R (Mar 16, 2005)

Well he is fairly active in at this particular forum so Im sure he will read it.


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## mR Yellow (Mar 16, 2005)

INSTG8R said:
			
		

> Well he is fairly active in at this particular forum so Im sure he will read it.



Has CM posted here before?


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## INSTG8R (Mar 16, 2005)

No not at this Forum. Terry Makedon is a regular at Driverheaven


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## Prime2515102 (Mar 23, 2005)

PCI Latency is how long any device will "hold" the PCI bus for a given transaction. For example, if the graphics card needs to send or recieve something from the bus, and it's set to say, 64, then the bus will be dedicated to the graphics card for a minimum of 64 clocks. So, if what its doing only takes say, 32 clocks, other devices (such as a sound card for example) will have to wait 64 clocks anyway before it can use the bus. Vice versa too - if the sound card executes a transaction and it's set to 64 clocks and it does something that take less, then the other devices will have to wait. It's how long an IRQ has to wait pretty much.

If I'm not mistaken, there's is a different "PCI" latency for the AGP bus (that defaults to 255 whereas the normal PCI latency is usually set to 32 clocks by default but can be changed in the BIOS of the motherboard). 

Then again I might not know what I'm talking about... lol

Prime


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## Stephen M. Smith (Mar 24, 2005)

Hmmm, my understanding of PCI Latency is just the opposite -- it is how long any given device is allowed to have exclusive use of the PCI bus.


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## Prime2515102 (Mar 24, 2005)

Right, but while something has exclusive use, the rest of the devices on the bus have to wait until the number of clocks specified in the latency setting has passed whether they (the clocks) are being used by the device or not.

Edit: So the point is, if set too low, it can decrease performance because of the overhead in going from one IRQ to the next, and if set to high, it can cause problems for other devices (such as sound cards, as mentioned above) because they can't get enough clock cycles.

By the way, I didn't some poking around and there are 2 AGP latency settings: 1 for the AGP bus and 1 for the AGP device (there's also the regular PCI latency setting). Mine was set to 255 clocks for the bus and 64 for the video card (I use Omega drivers wich disables the driver latency setting) so I set it to 64 for the bus. I haven't had a chance to test for improvment yet, but I'm going to see if it helps at all with the stuttering problem in Half-Life 2.

Edit 2: You can check your settings using PCI Latency Tool (carefull with that lol).

Prime


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## INSTG8R (Mar 24, 2005)

yes I also have the PCI Latency Tool and if you check your card without drivers you will see that it is at 32 UNTIL you put in drivers. my AGP bus is at 64(not sure why but it is) and of course with drivers in(ATI) the card is set to 255. Omega sets the card to 64(makes more sense to me too)


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