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 Post subject: The "CPU bottleneck" explained
PostPosted: Wed Jul 30, 2014 9:50 pm 
Smithfield
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I think this video sums it up best.



tl;dw - The "CPU bottleneck" is a condition where the CPU is so busy with processing the game that it doesn't get much time to send render commands to the GPU. i.e., you're CPU is at 100% utilization when running the game.

That's the only criteria.


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 Post subject: Re: The "CPU bottleneck" explained
PostPosted: Wed Jul 30, 2014 11:44 pm 
Little Foot
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So it seems like it's actually hard to bottle neck a CPU. I;ve heard from some that my own i5-3350P isint likely to get bottle necked with any GPU out as of the 700 series without sli going. Which to me makes me figure the common recommendation of buying i5s over i7s is fairly good advice.


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 Post subject: Re: The "CPU bottleneck" explained
PostPosted: Thu Jul 31, 2014 5:56 am 
Smithfield
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While that's true, don't be afraid to recommend lower-end processors. A gaming rig should have a majority of its budget directed to the GPU.


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 Post subject: Re: The "CPU bottleneck" explained
PostPosted: Thu Jul 31, 2014 8:11 am 
Little Foot
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Of course I as I stated I run a slightly lower mid range i5 still a good cpu I think I was noting more how I likely will not have to upgrade my cpu until I replace the motherboard unless I buy a sli set up for my gpu. As any single card won't bottleneck me.


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 Post subject: Re: The "CPU bottleneck" explained
PostPosted: Sat Apr 04, 2015 5:31 am 
Thunderbird
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Good video, but I love the comment ~15:28 - I say that ALL the time! :)

As well as the comment ~18:08 and immediately following it ~18:15


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 Post subject: Re: The "CPU bottleneck" explained
PostPosted: Sat Apr 04, 2015 8:38 am 
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why stay within the same architecture of cpu, granted it changes some things but not others. would it not be more realistic to go back to something older than the I7, I5, I3 CPUs which are the newer ones on the market, would that not create a real bottleneck? I would guess there are more older CPUs still working than newer. I am still using a socket 775 system, what is my "bottleneck"? I still do not know what gpu he was using to test.


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 Post subject: Re: The "CPU bottleneck" explained
PostPosted: Sat Apr 04, 2015 10:18 am 
Smithfield
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Architecture doesn't matter because the problem is in regards to the performance of the part. If two parts get the same Whizzbangmarks, what difference does it make if they're different architectures?


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 Post subject: Re: The "CPU bottleneck" explained
PostPosted: Sat Apr 04, 2015 6:52 pm 
Thunderbird
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Quote:
would it not be more realistic to go back to something older than the I7, I5, I3 CPUs which are the newer ones on the market, would that not create a real bottleneck? I would guess there are more older CPUs still working than newer. I am still using a socket 775 system, what is my "bottleneck"?

Wait a minute...I thought he demonstrated bottlenecks existed in both i3 tests. A bottleneck is a bottleneck, why would he have to use an older CPU to prove it even more? I'm guessing he probably modified the i7 to mimic i5 and i3 conditions (mainly out of expediency). Now! I am curious about another question, but I'll explain it at the end.

Quote:
I still do not know what gpu he was using to test.

He mentioned he was using a GTX 680 - towards the beginning, he mocked people he anticipated would say he needed to use a 780Ti to do the test.

If I'm wrong, please correct me, but I see two causes for a CPU to bottleneck a process:

  • First, the game is CPU intensive and the CPU itself is having a hard time keeping up with the application, which in turn means the GPU is waiting for the CPU to send/receive commands...OR...
  • Second, the GPU is powerful and is overloading the CPU with requests/results and the CPU is having a hard time keeping up with the GPU. Again, the GPU sits idly by waiting for the CPU.

Are both examples legitimate causes for the CPU bottleneck? If the latter is true, then my question is: he shows the i3 is bottlenecking and slowing down his 680, but would the i3 CPU be a bottleneck if he was using, say, a non-Ti GTX 750?


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 Post subject: Re: The "CPU bottleneck" explained
PostPosted: Sat Apr 04, 2015 7:06 pm 
Smithfield
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The GPU doesn't make requests, the CPU tells the GPU what to display.

So I'm doing some investigation regarding this and I'm having trouble defining a point where the CPU is still bottlenecking. If the CPU is without a shadow of a doubt at 100%, it's bottlenecking. But I've found even at 80%, it's still bottlenecking. I don't know if this is just a scheduling quirk or if the sampling rate is too low.


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 Post subject: Re: The "CPU bottleneck" explained
PostPosted: Wed Apr 08, 2015 5:17 pm 
Thunderbird
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Ok. So my second point is incorrect - the GPU does not poll the CPU, the CPU sends requests to the GPU to do some work. Easy enough.

Still, a 680 GTX is going to process requests much faster than the non-Ti 750. I wonder if there would be as big of a performance hit. Using made-up numbers, let's say the non-Ti 750 spit out 45 fps with an i7/i5 setup up...would it produce the same 45 with an i3, or would it drop to say 42 fps (a much smaller hit)? Obviously, the i3 CPU wouldn't be able to send as many instructions to the GPU, but then again the GPU wouldn't be able to process as many commands to begin with.

Quote:
So I'm doing some investigation regarding this and I'm having trouble defining a point where the CPU is still bottlenecking. If the CPU is without a shadow of a doubt at 100%, it's bottlenecking. But I've found even at 80%, it's still bottlenecking. I don't know if this is just a scheduling quirk or if the sampling rate is too low.


This is kind of a cop out, but the easiest definition of bottlenecking is when a CPU can no longer process efficiently causing other system components to be underused. Of course, the harder part is determining when this occurs. I think it would be reasonable that any usage above 90% would cause some type of bottlenecking. Between 80 - 90%...? Hmmm, I would think there's enough breathing room that it wouldn't occur (or it would be very minimal), but I'm not a circuit-level expert and if you're seeing it at 80% that is interesting.


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 Post subject: Re: The "CPU bottleneck" explained
PostPosted: Wed Apr 08, 2015 11:47 pm 
Little Foot
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I'd say from my own understanding the bottleneck is not likely to cause as massive a hit as people think. By the time either architectures so old and inefficient so as to be unable to do these things effectively you shoulda long upgraded as is and the bottlenecks the least of your problems. If you still run a 9800GTX and a Pentium D thats what your gonna get. I am fairly sure if my I5-3350P were pitted with a Titan X thats water cooled and over clocked it's standard 3.1ghz clock speed and even it's turbo of 3.5ghz would be fine enough. Hell I bet a I3 running at over 3ghz would still not suffer horribly though it may not maximize the card. But seems to be any CPU and GPu made within the past say 3 years or maybe even 4 isn't gonna suddenly dip from 40fps to 20 cause of a bottle neck it's gonna do it cause the game your playing needs better hard ware. But thats my take if people feel i am wrong guess you'll let me know.


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 Post subject: Re: The "CPU bottleneck" explained
PostPosted: Thu Apr 09, 2015 8:03 am 
Smithfield
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As I said before, I'm doing some investigation into the matter using the same method as Jayz2Cents. However, for comparison's sake I'm running the setup through PassMark's PerformanceTest software to see roughly where each configuration lies (it's not perfect, but I don't have representative hardware). I've adjusted my hypothesis though, it's whenever the CPU load is constantly higher than the GPU load that presents a bottlenecking condition.

However, you cannot make any accurate guesses across upgrades and hardware architectures. All I'm trying to do is expose when the condition occurs such that anyone can run similar tests on their system to determine if any bottlenecking will likely occur.

That is to say, I'm trying to see if correlation implies causation.


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