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Old 2013-04-18, 21:39   #1
Zerowalker
 
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Default Large FFT tweaking

If i set custom Settings from Large FFT to, 8 to 256, and my CPU has 256kb L2.

Will it be able to run solely on the CPU then, as normally it will go over my L2 Cache, and then i guess it runs with RAM instead, like blend.

Though i am not to knowledgeable with this.


EDIT:

On simpler words.

What settings should i use to let it fill my L2 Cache which is 256kb (1024 total - quad core)

Last fiddled with by Zerowalker on 2013-04-18 at 22:15
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Old 2013-04-18, 23:33   #2
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Quote:
Originally Posted by Zerowalker View Post
On simpler words.

What settings should i use to let it fill my L2 Cache which is 256kb (1024 total - quad core)
I would leave the settings to Prime95 to choose if you are doing an assignment from PrimeNet. Most of the first time L-L tests being handed out use an FFT of 3072k or larger. The double checks are running in the range of 1536k and larger.
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Old 2013-04-18, 23:38   #3
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Quote:
Originally Posted by Uncwilly View Post
I would leave the settings to Prime95 to choose if you are doing an assignment from PrimeNet. Most of the first time L-L tests being handed out use an FFT of 3072k or larger. The double checks are running in the range of 1536k and larger.
I believe the OP is talking about stress testing.
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Old 2013-04-19, 01:47   #4
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Quote:
Originally Posted by Zerowalker View Post

What settings should i use to let it fill my L2 Cache which is 256kb (1024 total - quad core)
Settings of 8 to 16 (maybe 24) will operate entirely out of the L2 cache. Above that you will be hitting main memory.
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Old 2013-04-19, 01:56   #5
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I calculated it to 32, 32x8 = 256, will that go over the L2 anyway;S?
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Old 2013-04-19, 02:09   #6
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Quote:
Originally Posted by Zerowalker View Post
I calculated it to 32, 32x8 = 256, will that go over the L2 anyway;S?
http://mersenneforum.org/showpost.ph...70&postcount=3 I think may help
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Old 2013-04-19, 12:46   #7
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I checked that, but didn´t wuite understand:

"there is also some sin/cos data required by the FFT as well as some other constants that are way above the scope of this conversation"

I guess that means that it isn´t precisely 8x(x), as other stuff are in the conversation?

Is there a way to know how to get precisely 256?

Aslo is the L3 cache used?
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Old 2013-04-19, 14:54   #8
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Quote:
Originally Posted by Zerowalker View Post
Is there a way to know how to get precisely 256?
No. And it wouldn't help to find an FFT that used exactly 256KB of data. Such an FFT would not fit in a 256KB cache because of the way caches are organized.

Yes, data that is evicted from the L2 cache ends up in the L3 cache.

Why the great concern over whether or not an FFT fits in the L2 cache?
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Old 2013-04-19, 15:01   #9
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I see.

Okay so, it goes like, L1 first then L2 last L3, and then RAM?

Well, i just wanted to run a stress test that used the CPUs all L-Cache to full extent, without using RAM (except for the RAM used for loading the application).
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