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6850 Crossfire CPU Bottleneck?

Last response: in Graphics & Displays
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a b U Graphics card
June 7, 2011 6:57:16 PM

After some research into why my 6850 Crossfire system isn't increasing performance, I'm beginning to think I have a CPU bottleneck, I currently have an AMD Athlon X4 635 running at 3.4GHZ. How can I confirm that my CPU is bottlenecking my graphics cards? And also which CPU shall I buy that won't bottleneck the cards.

If I buy a new CPU then it'll have to be AM3 because I recently got a new motherboard and cant afford to switch to Intel.
a b U Graphics card
a b à CPUs
June 7, 2011 7:12:27 PM

The easiest way is to grab an app called GPU-z from guru3d. On the sensors page, there is a GPU load sensor. You want to run something like furmark or MSI kombuster, in windowed mode, so you can have the graphics on screen and see the sensor reading. This brings the GPU use to 100% usually. Also, open up the task manager and observe cpu load. If CPU load is high 80+%, and the GPU is lower, than its a bottleneck.

What games do you play that make you think its underperforming?
a b U Graphics card
June 7, 2011 7:34:31 PM

Followed what you said, with furmark I'm getting 25% CPU Usage when the gpu is at 100%, however furmark is only using one core. That result was with one HD6850 installed.

The reason I think its underperforming in crossfire is that there is no performance increase on any games and theres usually a drop in performance.
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a c 125 U Graphics card
a b à CPUs
June 7, 2011 7:48:00 PM

I would first suggest getting MSI Afterburner - it does a similar thing to GPUZ but it allows you to record more info on the graphs, and they're bigger and easier to read. Also you don't need 2 instances of the program open in order to see both GPUs.

Anyway then run FurMark again. You're right, it only uses 1 thread. IMO you need to run it full screen at max resolution and 2xAA just to really load up the GPUs. Run it for about a minute then close and check Afterburner. You should see both GPUs at 99% usage throughout, and CPU probably at 100% as well for a single core. You can also take the opportunity to disable CF and rerun the test, and see how much the FPS changes. CF should be around 90% faster than single.

If the results are good there try games out again, they should be 80-99% usage depending on the game and what's happening. Also both GPUs should have pretty much identical usage to eachother.

Another way of testing is to download something like 3DMark11 which is a free benchmark tool.

And finally just to cover the basics, did you take these steps when you installed the second 6850:
-uninstall old drivers
-Driver Sweep old drivers
-shut down
-install both cards, with bridge attached
-install latest (11.5b currently) drivers
-install latest CAP (11.5 CAP 4 I think)
-enable Crossfire in CCC
-test away!
a b U Graphics card
June 7, 2011 7:58:37 PM

I will install the second card once I've finished my graphics coursework and test again. However I am getting 100% on one core with a single 6850, would this indicate that my CPU is bottlenecking and this would be the reason for my poor performance in Crossfire?
a c 125 U Graphics card
a b à CPUs
June 7, 2011 8:02:10 PM

Depends. I did some tests a while back with my crossfire 5850s. I first tested it in windowed mode so I could watch the Afterburner graphics. I had 100% CPU usage and around 200 FPS for single card. Switching to CF, I gained only about 10FPS. But then I went to fullscreen 1080p and my FPS went up by 90% over a single card. I'm not sure if CPU usage changed, but I think it was still 100%.
a c 216 U Graphics card
a b à CPUs
June 7, 2011 8:21:28 PM

wolfram23 said:
Depends. I did some tests a while back with my crossfire 5850s. I first tested it in windowed mode so I could watch the Afterburner graphics. I had 100% CPU usage and around 200 FPS for single card. Switching to CF, I gained only about 10FPS. But then I went to fullscreen 1080p and my FPS went up by 90% over a single card. I'm not sure if CPU usage changed, but I think it was still 100%.


The reason you guys are seeing no usage on the GPU's while watching in windowed mode is that Crossfire/SLI does not function at the desktop/windowed mode. You have to be in fullscreen mode to get use out of crossfire.

So do as Wolfram23 mentioned, and run in fullscreen and look at the recorded info.
a c 125 U Graphics card
a b à CPUs
June 8, 2011 7:29:22 AM

Honestly I'm not convinced you need to upgrade the CPU...

What are your full system specs (including monitor res)?

If you've diagnosed that crossfire is working and at least showing some gains, the next step is to test if there really is a bottleneck. The easiest thing to do would be run some sort of benchmark, like 3DMark11, Unigene Heaven, or a video game bench (several free ones available). Then try it again with the CPU overclocked 200-500mhz and see if the average and minimum FPS went up. If not, then it's not the issue. Could be the motherboard's bandwidth (if it's AM2+). If the HT or NB can get OCed then that might clear up some headroom... it really depends on the system.

That said, if you're convinced you want to upgrade the CPU a 965 is very good and can OC to 980 levels.
a b U Graphics card
June 8, 2011 11:07:40 AM

My monitor is 1920x1080
CPU - amd athlon x4 635
Mobo- Asrock M3a770de
Ram - 4gb ddr3 1333mhz
Gpus - 2x gigabyte 6850s
a c 125 U Graphics card
a b à CPUs
June 8, 2011 3:18:42 PM

Ok, so definitely a CPU upgrade is a good option for that setup.

What sort of gains are you currently seeing between CF and single card?
a b U Graphics card
June 8, 2011 3:26:03 PM

I've run furmark with one 6850 installed and when they're in Crossfire. There was a performance increase in FurMark.

Single 6850 - 23FPS
Crossfire 6850 - 43FPS

On both occasions the CPU usage on the CPU was 100% and the 6850(s) ran at 100% as well.
a b U Graphics card
June 8, 2011 4:04:20 PM

Just played Crysis 2 and Dirt 3, there is no longer a performance decrease with crossfire but there is no increase either, CPU ran at 78% with Crysis 2 and 85-90% with Dirt 3
a c 125 U Graphics card
a b à CPUs
June 8, 2011 5:23:52 PM

Interesting. Do you have the FPS results? I don't know if these have it but original Crysis, and Warhead, as well as Dirt 2 all had benchmarks for them.

But that is definitely high CPU usage so yeah, I guess upgrading to a PII X4 might just be what you need to do although I will say that I played the Crysis 2 demo and my CPU usage was very high as well like 60% or more with an i5 750 at 4ghz.
a b U Graphics card
June 8, 2011 5:32:03 PM

OP:

you really shouldn't be seeing much of a bottleneck at all, can you overclock your cpu any further, sounds like shitty drivers again, this is why I abandoned 6870 crossfire on my rig below, sell the cards and get a single powerful one is what I would do, unless you want to wait around for it to work right, in the end I was too fed up
a b U Graphics card
June 8, 2011 6:17:05 PM

I've got Crysis Warhead, I'll try it out
June 24, 2011 7:09:45 PM

It might be ur BIOS to be honest, I was playing Heroes of Newerth with my 6850 only getting 60 FPS when i flashed my BIOS i got 125 FPS..
June 24, 2011 7:21:37 PM

Nvm did the test myself, you running at average. and I was gunna ask whats the power consumption of running both 6850s at full load
June 24, 2011 9:06:34 PM

jjb8675309 said:
OP:

you really shouldn't be seeing much of a bottleneck at all, can you overclock your cpu any further, sounds like shitty drivers again, this is why I abandoned 6870 crossfire on my rig below, sell the cards and get a single powerful one is what I would do, unless you want to wait around for it to work right, in the end I was too fed up


to have something similiar in performance to 2 6870 what did you have to buy then? because checking reviews 2 6870s achieve great fps than just one card dont think could get, at least not for the price of 2 6870, not close anyway.
a b U Graphics card
June 24, 2011 9:38:22 PM

well I bought a single gtx 570 which is about a 20-25% performance decrease but without all the problem that seem to be inherent of crossfire 6870s, in the end it more than enough power for 1080p and I am much more happy with a powerful single card, other will argue, but for me this is what made sense
a b U Graphics card
June 24, 2011 9:40:49 PM

the thing is I had much higher minimum fps with the single 570 and with only a 60hz monitor that is what mattered to me the most, the micro stuttering made me want to throw my rig off a cliff
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