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GTX 460 768 MB bandwidth bottleneck found?

post #1 of 4
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hey guys
just bought my GTX 460 on new year got a thread dedicated to that here.
http://www.overclock.net/nvidia/9061...ures-dial.html
but this thread is about something else.
i've read a lot of people said that overclocking the video RAM doesn't do much to help performance. so i'm trying to test out that concept today.

Core clock is at 900 MHz and shader is at 1800 MHz at all time during benching. voltage is at 1.087

Crysis 1.21
1440x900
2x AA
16x AF (driver forced)
all Very High settings except a few tweaks to the LOD and texture in .cfg file to make the distant mountains look sharp.

924 MHz (stock OC)
Min 21.12
Avg 39.27
Max 43.90

1150 MHz (final OC)
Min 30.70
Avg 42.43
Max 47.46

I've tried to bench at every 50 MHz increase since i heard that if you push GDDR5 too high it will degrade performance. each time i bring it higher i see additional fps.

Bandwidth increased from 88.7 GB/s at 924 MHz to 110.4 GB/s at 1150 MHz
a 28% overclock if you count the stock clock is 900 results in about 8% improvement in avg fps; larger improvement in min and max.

I'm not sure if this shows a bandwidth bottleneck or not?
i can't believe i'm already bandwidth limited at this low res and low aa.
someone with higher end card please confirm this.
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post #2 of 4
Keep in mind that on a gfx card, the memory is not some ancillary component, sitting kind of 'off to the side' like your system RAM is. It is a direct part of the graphical 'pipeline' in the sense that the image you are viewing was, at one time, built/stored on your memory chips.

Thus, on a video card, memory bandwidth is not generally the kind of thing where you either 'have enough', or 'don't have enough'. IOW, a 'bandwidth bottleneck' is not necessarily the proper concept. Rather, it is almost always the case that having more is better (and never worse), just like having more core speed is almost always better (and never worse).

I mean there are some really rare exceptions with certain cards, which are generally referred to as 'not well balanced', wherein there is such a plethora of bandwidth relative the power of the core (the 4890 is actually probably the most notable example) that raising the bandwidth does almost nothing ... but this is a rare circumstance. And even then ... it will do SOMETHING to OC the memory.

Getting an 8% improvement in perf from a roughly 25% memory OC is a pretty significant level of perf gain, but it's not indicative of a 'bottleneck', per se.

I mean, it is, in the strictest technical sense ... but it's also totally normal ... cards are set up this way on purpose. It's certainly not anymore of a 'bottleneck' than your core is, because a 28% OC on the core is going to give you like 20-24% perf improvement, right? But .. do you think of your core as 'bottlenecking'?

Again, only a very 'unbalanced' part, with way too much bandwidth vs processing power, is not going to show at least a measurable gain with OC'ing the memory.
Edited by brettjv - 1/6/11 at 10:14pm
    
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post #3 of 4
I also found that with the 768 overclocking the memory did a great deal for performance whereas overclocking the memory on my 480 seems like an exercise in futility (not to mention the fact that is doesn't seem to have very good memory chips )
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post #4 of 4
Quote:
Originally Posted by TickleMeElmo View Post
I also found that with the 768 overclocking the memory did a great deal for performance whereas overclocking the memory on my 480 seems like an exercise in futility (not to mention the fact that is doesn't seem to have very good memory chips )
That makes sense ... the GF104 is quite a powerful chip, with it's 336 CUDA cores. And a 192-bit bus is not especially wide for a chip w/that much gfx horsepower.

The 480 has only 1.4X the cores of the 460, but it has 2X the available bandwidth (at equal memory clocks).

So it makes perfect sense that the particular config of the 460 768MB card would lend itself to a more significant % gains from memory OC'ing than a 480.

A lot of what you see in terms of perf gains from OC'ing the vram comes down to the particular test, too, and what settings you run it at. Crysis favors a lot of bandwidth (it also likes a lot of ROP power), and using AA in general tends to shift the balance towards bandwidth as a limiting perf factor. You'll nearly always see higher scaling from memory OC's when using AA vs. when you're not using it.

Last point ... for fairly obvious reasons ... the faster you clock your core, the better scaling you'll also see from a memory OC.

In the case of the OP ... if you were to lower your core clock to 500 and run the same tests with the memory OC's, you'd find that you don't get anywhere near the same 8% FPS gain from your 25% memory OC. Part of the reason that gain is so significant ... is because your core is screaming at 900MHz, which increases the likelihood of the bandwidth being a limiting factor. Make sense?
Edited by brettjv - 1/6/11 at 10:14pm
    
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