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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  • PrinceGaz - Wednesday, August 04, 2004 - link

    The amount of System memory (above 512MB) is unlikely to have any impact on framerate in the timedemo as I doubt it would need to swap anything out after the first run (which is discarded anyway).

    I found my 128MB graphics-card (a Ti4200) gave an almost identical framerate at Low, Medium, and High quality settings in the timedemo even when gfx-card limited, provided Aniso was disabled in the driver for High quality mode (which would otherwise use 8x Aniso and impact on performance in other ways). So increasing the videocard memory from 128MB to 256MB will have no effect whatsoever on the timedemo, except maybe at Ultra quality which I didn't bother testing.
    Reply
  • Steve Guilliot - Wednesday, August 04, 2004 - link

    #27
    That's the OS load balancing between the two procs. Two D3 threads aren't running at once. That's why sum utilization of both procs won't go over 100%.
    Reply
  • Succorso - Wednesday, August 04, 2004 - link

    Is this review using XP or XP64 beta with the amd64 ? Are the benefits the same using a 32bitXP as opposed to the 64 bit XP ?

    Succorso
    Reply
  • SignalPST - Wednesday, August 04, 2004 - link

    its interesting how DOOM3 runs best in the Nvidia/AMD combo along with the amazing price/performance that they offer over their competitors

    the Athlon64 3000+ is on par with Intel's 3.4GHz EE, while the price difference is $840

    the GeForce 6800 GT is faster than ATI's X800XT PE, the price difference being $160

    so in this scenario, the Nvidia+AMD combo can save you $1000 and still outperform the ATI+Intel combo

    bottom line, for DOOM3 and future DOOM3 engine games, ATI+Intel=losers
    Reply
  • cKGunslinger - Wednesday, August 04, 2004 - link

    Yes, I would also like to see some numbers benchmarking 256/364/512/768/1024/etc MB memory configurations. When does the average system have *enough* ram to run WinXP and play a game? Reply
  • xtf - Wednesday, August 04, 2004 - link

    Would it be possible to add the cache (and other) specs of the K7s to certain charts?
    Because sometimes the 2700 and 2800 are slower than then the 2500 and it'd be interesting to know why.
    Reply
  • tdent1138 - Wednesday, August 04, 2004 - link

    Great article AT! I'm happy to know my 2.53Ghz @ 2.717Ghz P4 and 9800pro will happily run D3 at 8x6 in medium quality. I can now wait until HL2 at least to upgrade to whatever makes sense at the time (A64 something I imagine). Thanks again! Reply
  • tdent1138 - Wednesday, August 04, 2004 - link

    Reply
  • Philbill - Wednesday, August 04, 2004 - link

    Great article, Do you plan to give an update with the high end ATI cards?
    Phil
    Reply
  • dangereuxjeux - Wednesday, August 04, 2004 - link

    Somehow, I feel ashamed that the Sempron 3100+ crushes my ol' P4 2.4C.... please please please stop publishing articles like this that encourage me to spend any more of my money upgrading to a new AMD platform to go along with my 6800. Reply

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