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At what point does the CPU bottleneck the GPU?

Last response: in Systems
February 3, 2011 2:19:31 AM

Hey all,

Building a rig with my son for homework gaming. ;)  We live about 20 miles from a Microcenter, so keep that in mind when pricing. Anyways, here's what we are thinking. Already have a monitor, a 22" 1680 x 1050 and no SLI or Crossfire.

Antec 750 Earthwatts $80 @ Newegg

Biostar A770E3 AM3 $39.99 @ MC

X3 450 $80

G.Skill 4GB 1600 PC 12800 CL9 $50 Newegg

Antec 300 $55 @ MC

Seagate 7200.12 500GB $38 Newegg

Hyper 212+ $30

So we're at about ~$375 right now. He has just under $600 to work with, but I'm willing to help if need be. So with about $225 for a GPU left, my main question is, at what point is does the GPU become irrelevant due to the limitations of the CPU? We were thinking GTX 460 1GB, but a Zotac 470 is $200 after rebate at MC. Not to mention the recent availability of the 5870 @ $200. What about the 560 Ti for $250? Will the X3 cut it?


February 4, 2011 5:13:59 PM

**crickets** Ok then just rate this build:

Antec 750 Earthwatts 80 plus -- $80 @ Newegg

Biostar A770E3 AM3 $39.99 @ MC [...] id=0349768

X3 450 $80

G.Skill 4GB 1600 PC 12800 CL9 $50 Newegg

Antec 300 $55 @ MC

Seagate 7200.12 500GB $38 Newegg

Hyper TX3 $18 @ MC

Sapphire 5870 - $200 AR

Total out the door -- $561

I decided to go with the 5870 after reading Tom's GTX 560 review. Can't beat the price...
Related resources
February 4, 2011 5:31:08 PM

About $50 more, but the upgrades are significant...

Athlon x4 640 + ASUS M4A87TD (USB 3.0 and SATA 6.0) $144
IMHO, skip the CPU cooler below for now and spend that money opting up for the Phenom x4 965.

CM Hyper 212+ $37

G.SKILL Ripjaws DDR3-1600 4GB (2x2GB) $50

HD 5870 $200

Hitachi 2TB SATA 6.0 $80

DVD RW $18

Antec 300 $53

Antec True Power 650W $94

Best solution

February 4, 2011 5:52:51 PM

Re: question in the title

There are many factors that determine this point. So I'll divide it into CPu and GPU

CPU half of the explanation-
Short answer. Depending upon resolution and type of game. IE how well threaded it is and whether it has physics and AI, or just AI. Also if it's AI intensive like RTS or not, like FPS.

Long explanation
Games are very poorly threaded. The part of gaming that is well
threaded, the graphics portions, is handled by the GPU, which already
have hundreds of cores.

In gaming, the most CPU intensive task is AI. AI, by definition is not
a parallel process. It is extremely difficult to thread AI. Most games
that are "multi threaded" actually keep AI on 1 thread and throw the
rest (minor far less intensive stuff) on the other.

Can you design a game to utilize 4 or more cores? Sure, you can throw
all the CPU non intensive calculations onto their own threads, but
until someone figures out a good way to thread nonparallel
computations, the performance increase will be minimal, as the hard
work is still restricted to 1 thread.

This issue has been stumping programmers for decades. There are ways
to do this in specific situations, but no general solution yet. A
general solution allowing infinite threading of nonparallel
calculations would be the programming equivalent of finding the cure
for cancer, noble prize stuff for sure.

Basically think of it this way. On a math exam you have a 3 part
question in which the answer to part each part depends on previous
answers. IE

A. Add up 3 and 5.
B. Use the answer from part A and divide by 2
C. Use the answer from part B and triple it.

what is the final answer?

This is the type of thinking AI requires. Threading this is the
equivalent of calculating the answer to A, B and C simultaneously.
It's not impossible like the mathematical equivalent is, but it's not

For this reason, more than 3 threads has very little benefit. So if a game is well threaded, it'll utilize CPU up to 3 threads.

On the other hand, if it's not well threaded you're better off with high speed single or dual core.

GPU half of the answer:
Short- depends on resolution, eye candy and drivers.

Long Answer
The higher the resolution, the more the GPU needs to calculate. Hence, the higher the resolution, the more the the bottleneck shifts towards GPU.

At the same time, enabling AF and AA also require further GPu calculations. So if you turn up the eye candy, GPU has to work harder and further pushes bottleneck to GPU.

Additional features such as tessellation also are GPU calculated. So these will also shift bottleneck to GPU.

At the same time, even if the hardware is capable, if drivers aren't then you got issues. If drivers are extremely poor (take TERA currently for ex) both ATI and Nvidia GPU's are getting unplayable FPS when theoretically they should be fine. So in this case, the bottleneck is with the drivers, not the CPU or GPU technically.

Bottleneck moves toward CPU if:
Lots of Physics
Lots of AI
Poor threading
Low resolution

Bottleneck moves to GPU if:
High Resolution
Eye candy enabled
Tessellation is used

Bottleneck moves to drivers if drivers just fail.

In general, since people want to game at the highest res and with as much eye candy as they can, you'll find GPu bottleneck.

The one exception to this is poor software coding. If a game isn't coded to use CPU /GPU properly all bets are off and it'll depend upon game where bottleneck goes.
February 5, 2011 7:08:22 PM

Best answer selected by goatsword.