In a continuation of Doom 3 Week, we're bringing you the next installment of our coverage, this time focusing on CPU performance. If you haven't already, be sure to read our guide to graphics performance under Doom 3 before proceeding with this guide.

When does Doom 3 Need a Fast CPU?

We know by now that the GPU requirements of Doom 3 are quite high; the days of ultra high resolutions bringing us triple digit frame rates on mid range cards are gone with Doom 3, even cards like the Radeon 9800 Pro are best played at resolutions as low as 800x600. But is a fast GPU all you need to get the most out of Doom 3?

Remember that while your GPU will handle all of the rendering of the scenes in Doom 3, it is the CPU that handles all of the physics, artificial intelligence and 3D setup for sending vertex data to your GPU. So in order to get the most out of a fast GPU, you will also need to pair it up with a fast CPU - but how fast? The basic rule of thumb is this: the faster your GPU is, the faster your CPU will have to be to keep up with it.

Let's take the GeForce 6800 Ultra for example; as you've already seen, the GeForce 6 series is the fastest set of GPUs for running Doom 3, making it an ideal reference point for our discussion.

Below we have a graph of frame rate vs. resolution taken on a Pentium 4 Extreme Edition running at 3.4GHz with a GeForce 6800 Ultra running at 400MHz core/1.1GHz mem. The curve on the graph is what you'll want to pay attention to. If the graph were perfectly flat, as in there was no drop from 640x480 up to 1600x1200, our test system would be completely CPU limited (or bound by something other than the GPU). On the flip side, if the graph showed a clearly negative slope then we would have a much more GPU limited scenario, where the burden of rendering more pixels was not masked by an overly slow CPU.

In this particular case we see that at resolutions below 1280x1024 the GeForce 6800 Ultra is primarily CPU limited, making all of the lower resolutions perform identical to one another. It isn't until we hit 1280x1024 and 1600x1200 that there is a significant performance drop off. So it's clear that if you have a GeForce 6800 Ultra, pairing the chip up with a fast CPU is quite important. But what about on a slower card like a Radeon 9800 Pro?

Here we have a completely different graph, where not even at 800x600 is the card CPU bound. With a Radeon 9800 Pro, having a fast CPU doesn't help as much since you are mostly GPU limited, especially at higher resolutions. This doesn't mean that you can pair up a Radeon 9800 Pro with a 1.4GHz Celeron and be fine, but it does mean that a Pentium 4 Extreme Edition is going to be overkill for your 9800 Pro.

When investigating CPU performance under Doom 3 it's clear that we'll want to use a GeForce 6800 Ultra to put as much stress on the CPU as possible, but we've also looked at how slower cards like the Radeon 9800 Pro react to CPU speed improvements as you'll see on the next page.

How does CPU Speed Impact Graphics Performance?
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  • michael2k - Wednesday, August 04, 2004 - link

    Heh, what about frames/$ graphs? Reply
  • Da3dalus - Wednesday, August 04, 2004 - link

    Lovely graphs, *pats my Athlon 64 3200+ while waiting for Doom 3*

    Another week for Doom 3 to hit our european shelves, damnit I hate waiting for something you americans already have :-/
    Reply
  • elfy6x - Wednesday, August 04, 2004 - link

    I have a dual Opteron 246 setup, with 1Gb of ram, and a Radeon 9700 Pro. I'm not really a gamer, but I gave Doom 3 a shot, and it utilizes both of my CPU's when I play. I have nothing else running when I play the game. It doesn't tax both CPU's to 100% but one CPU fluctuates between 40-50% while the other one bounces around 10-20%. So something is processing two threads. Just my observation. :) Reply
  • PotatoMAN - Wednesday, August 04, 2004 - link

    I second the idea of benching memory and video memory at AT for doom week. I have a 3200+ A64 and I am wondering if I am starving it more with a 9800 pro (128) or with my 512mb of RAM. Thanks AT for being awesome! Reply
  • Gooberslot - Wednesday, August 04, 2004 - link

    I bet a 1-1.2ghz P3 or Athlon would cut it for a minimum. Just go back and look at the old benchmarks of the P4 1.5ghz on here. Only in Quake3 was the P4 on top. In UT the P4 actually tied with the P3 1ghz. Pathetic. Stating minimum requirements based on those old Williamete P4's is very misleading. Perhaps the real minimum cpu requirements should have been 1.2ghz P3 or Athlon, 1.5ghz P4, or 2.4ghz Celeron. :) Reply
  • matman326 - Wednesday, August 04, 2004 - link

    Man I always knew that an athlon 64 system was powerfull but a 180 dollar proc. beating Intels Extremly Expensive 1000 dollar proc is just mind blowing. So much for the Netburst design kicking butt. Reply
  • AtaStrumf - Wednesday, August 04, 2004 - link

    If you could find some time, I'd like to see a comparison between 128 and 256 MB R9800Pro, and 512 RAM and 1024 MB RAM. Basicly how much of a difference do video and system RAM make. Reply
  • at80eighty - Wednesday, August 04, 2004 - link

    dammit! looks like im gonna celebrate Christmas with a loan..grrrr... : )
    Reply
  • WooDaddy - Wednesday, August 04, 2004 - link

    Two words:

    AMD ROX!!!


    I've been an AMD user since the 386 days and even had a NexGen processor (pre-pentium, K5). Never went Intel, never will (maybe)...

    Thatsright,
    Naw.. The charts don't lie. But to be fair, let's wait for the Intel-funded people (aka Tom's (blow)hardware) to put up their benchmarks. 10 bucks says they'll make all kinds of excuses to why Intel procs aren't fast enough.
    Reply
  • Regs - Wednesday, August 04, 2004 - link

    Wow. Good info. Dual channel offers nothing. And the socket 939 2.2 Ghz CPU offers nothing over my 3000 A64 which costs 200 dollars less. Given that if you play on higher resolutions. Reply

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