Crossfire cpu bottleneck?

[fizzy]

Hi,

I've been trying to get a bit more "oomph" out of my gaming rig. Its a twin ati x1900xt crossfire setup, using an asus board.

I ran 3dmark06, got a score of around 8600. Overclocked my gfx cards using the ati overclocking options - it stayed pretty much the same.

Reverted gfx overclock back and slowly overclocked the cpu to a stable max, and now getting around 9300. Again tried overclocking the gfx cards, but no real difference.

I can only think of two reasons - either the overclocking options in the ati drivers isn't actually having any effect, or its a cpu bottleneck. Anybody know at which point/speed does the cpu not become a bottleneck on a x1900 crossfire setup?

 

[melbourne720]

You could try underclocking the gfx cards until you start to get slowdown/reduced 3d mark score, that would tell you.

 

[fizzy]

Well, that would just prove that the cpu is not fast enough for the cards. I'm trying to find out what speed cpu needs to be to max out/keep up, rather than just realising I could have easily managed with two cheaper cards..

Ati drivers by default don't allow underclocking, so I guess I would need to use atitweak or similar instead...?

 

[melbourne720]

I see, my bad.

I think that the CPU is bottle-necking the cards (your work so far seems to indicate that) and this can be resolved by either a faster CPU, or more overclocking of your current one using a better cooler.

3d marks aside, in the 'real' world usage of games, can't you just turn up the AA and AF? When you start to get slowdown by doing this, then the handicap on the graphics card will mean that you will have eliminated (or at least substantially reduced) the bottleneck on the CPU.

Sorry I don't have any real numbers for you though, your setup is much better than mine!

 

[fizzy]

Hmm... upping the AA/AF sounds like a good idea. That would put far more work the gfx cards way, which would the benefit of looking nice too...

Getting pretty decent frame rates on oblivion at 1920x1600 in outside areas too - although the FPS app seems to make it crash a lot more often.

My CPU is an amd x2 4600, currently clocked from its default of 2.4 to 2.76. I ran 2.83 for a bit, but one of the cores wasn't prime95 stable (although I haven't messed with voltages etc yet). Gonna need a very, very beefy cpu if its the bottleneck...? Better start tweaking my memory too I think...

 

[Rroff]

incase you didn't know in oblivion - you can get the framerate counter up ingame without using fraps by pressing the ` key to get console, entering tdt and pressing enter, then pressing ` again to close the console...

An X2 4600+ @ 2.75gig would be about the minimum CPU speed to max your crossfire rig without the ATI cards being overclocked, once you start OCing the cf cards you are probably going to need a conroe clocked to 3gig to max them again.

I'd go for the extra eye candy aslong as you are getting playable fps, as the beauty of 2 cards in cf/SLI you can turn up FSAA, AF, overall IQ, without hurting your framerate.

 

[Durzel]

Anybody know at which point/speed does the cpu not become a bottleneck on a x1900 crossfire setup?

Well, I have an HIS X1900XT Crossfire setup too, and I get practically the same scores in 3DMark06 at 3.6Ghz with my Conroe E6700 (2.66Ghz) as I do when its overclocked further to 4.3Ghz. In fact, the only thing that changes noticeably is the CPU score (for obvious reasons).

I get nearly 12k @ 4.3Ghz and only a few hundred less at 3.6Ghz - so you are definitely CPU limited at the moment.

(In both cases the cards are modestly overclocked to about 680/860)

 

[Rroff]

I'd say it maxes out around 3.0-3.2gig on a conroe - I tested a similiar performance setup (but nVidia SLI not cf) at stock, 3gig and 3.6gig and saw a huge increase from stock to 3gig but hardly any increase from 3gig to 3.6gig.