pauldovi

Alright, some high level guys over at XS have discovered something interesting with the P965 / 975X chipsets. You may want to read through it all here, or just take my summary:

Your north bridge has an internal clock speed and latencies just like your CPU and memory. The FSB of your north bridge can be found by dividing your original CPU multiplier by your set CPU multiplier and then multiplying by your FSB.

So if you are running a E6600 (266 * 9) at 400Mhz x 8 your NB FSB is:

(9 / 8) x 400 = 450Mhz FSB (1800Mhz Total)

Just like your memory may be able to run at 4-4-4-12 at 1000Mhz but needs to run at 5-5-5-15 at 1200Mhz, your north bridge has a series of latencies which it must adjust in order to maintain stability at its FSB. These latencies seem to play a far more significant role in system performance than memory latencies.

Intel has predefined specific latencies at specific NB FSB speeds. They are referred to as straps. There is a strap for when the NB FSB is 1066Mhz and under, 1333Mhz FSB and under, 1600Mhz FSB and under, ect. When you go from the 1066Mhz FSB strap to the 1333Mhz FSB strap, the north bridge's internal latencies loosen to allow for greater stability.

ASUS has redefined the NB strap so that the 1333Mhz FSB strap does not come into effect until 401Mhz FSB (1604Mhz). Other perimeters of straps are somewhat unknown.

Tony, the guy who pretty much figured all this out has since been hired by OCZ (they were impressed). He is now a lobbyist trying to get ASUS and other major motherboard manufacturers to give the end user the control of when straps start.

There are 2 ways to beat the NB strap:

1. Boot to windows in the 1066Mhz strap and then use Clockgen to increase your CPU's FSB. You can then get to a much higher FSB while maintaining the 1066Mhz strap simply because the BIOS does not adjust the north bridge's latencies in real time.
2. Get a X6800 or QX6700 (or even a ES chip). To the north bridge, you are always at a default multiplier with a Extreme Edition processor. This allows you to set a much lower or higher multiplier without the NB FSB being effected.


Look at these Super Pi 32M results:




I will be adding to this as I learn more.

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Fatal05

Good find!

MADMAX22

did you read this

i read up on this a while back, its very interesting stuff, just through this in with what you have and make a faq or something like that for this site

edit: for this same reason is why people hit apparent fsb walls at 395 or so and have to jump over 400 to like 402 or so with certain mobos

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E6400@3856E4300@4158E6600@4510,Opti148@3102,opti170@2918,Xeon3220(B3)@3430(490fsb)
UD3P@8400600FSB

Joeking78

so 450 x 8 (which I currently run at) is no faster than 400 x9, perhaps slower?

And to notice an improvment 460 x 8 would be my best option?

pauldovi

What seems to be the best at the moment is stock multiplier and a 400Mhz FSB or greater than 460Mhz FSB (at stock multi).

You see, you can find the speed of the NB by taking your default multi and dividing it by whatever you have it set by and then multiply by your FSB.

So - 9/8 * 450 = 506.25Mhz (2025Mhz FSB for the NB)

It seems like the FSB strap will come into effect at a lower FSB when using a lower than default multiplier.

The 1333Mhz strap is suppose to start at 333Mhz FSB. However, ASUS manipulated the strap to allow to go to 400Mhz FSB before the 1333Mhz strap latencies kick in.

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lightsource

Other than super pi, does it have an effect on any realword situations?

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MADMAX22

thread


yeah if you read to the bottom of that thread it shows ( i think) that a higher fsb say 500 or so coupled with a decrease of 1 from the stock multi gives you the best results, this can give yousome really good stuff with the 6300/6400

i would test more but my mobo has locked multi's

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E6400@3856E4300@4158E6600@4510,Opti148@3102,opti170@2918,Xeon3220(B3)@3430(490fsb)
UD3P@8400600FSB

Ihatethedukes

Rep +1 Nice. Very Nice. I did notice a lack of superpi progress after about 3.6Ghz after looking at the 1M superpi bench thread.

I attributed it to the same principle that drag racers succumb to. A 500HP car runs in the 11ish range in the 1/4 mile but a 1000hp car is only in the 9ish range. Twice the power, only about 1/4 the speed increase.

Now I know it might just be the latencies of the strap.

pauldovi

Yes, this results in significant performance and stability changes in your system.

That is a good analogy, however it has nothing to do with this. You see, what you are referring to is the law of diminishing returns. The more you put into something the less you get out of it.

What effect that is happening in the P965 / 975X is the north bridge is increasing its latencies after specific FSB levels in order to remain stable. At stock settings (Intel defined) the north bridge will increase latencies at 1066Mhz, 1333Mhz, and 1600Mhz FSB levels within the northbridge. However, ASUS has reconfigured its motherboards so that you can go to the 400Mhz FSB level while maintaining the 1066Mhz FSB latencies within the north bridge. However, once you reach 401Mhz FSB, the 1333Mhz FSB strap kicks in and you get significant north bridge latencies.

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Ihatethedukes

I'm gonna test this idea when I get home on my DFI for you and see if it encompasses that board as well.