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Old 2007-04-06, 15:46   #1
JuanTutors
 
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Default Dual core: three copies or just two?

Has anyone tried to answer the question of whether it's better to run three or four copies of prime95 on a dual core PC instead of just two?
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Old 2007-04-06, 16:26   #2
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As far as I know, none of the dual cores are hyperthreaded, this means that you have only two processing units. It would be terrible for the performance to run more copies than 2 : you would have two instance competing for memory and cache access. If there is a hyperthreade dualcore, you would hit the same bottleneck as quadcores : memory access, but whithout the benefit of have one real processor per instance.

It would be like running two instances of prime95 on a single core.
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Old 2007-04-07, 01:37   #3
jasong
 
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If any hyperthreaded dual-core or more core processors come out, I would advise at least experimenting by trying 50% more Prime95 instances than cores. I'm not saying it will be beneficial, but it's usually a good idea for me to act on ideas that "feel" a certain way.

It sounds like magical thinking, but the reason for these ideas sometimes reveals itself later on. I may have read something and not be consciously aware of the memory.

edit: I've thought about it, and seem to remember reading that the third instance "floats" between cpus. I'm not sure what happens with more than two cores.

Last fiddled with by jasong on 2007-04-07 at 01:39
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Old 2007-04-09, 17:51   #4
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To date, only the Pentium D Extreme Edition 840 (smithfield) processor has (well had) HT.

http://www.xbitlabs.com/articles/cpu...ium-840_3.html
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Old 2007-04-12, 01:40   #5
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I can't see how running more Primes then Cores can be of any benefit.

If you consider, you will either have 1 prime using 100% of each core or 2 primes each using 50% (and thereby going half the speed) - thereby making it a moot point. However, if you consider the memory and caching limitations, as well as the overhead required for the OS to switch between each program - running one copy would be far superior

Also, on the point of HT, HyperThreading only gives an advantage if the two processes are using two DIFFERENT parts of the CPU - the two primes you are running are still going to compete against each other for the same part of the processor...
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Old 2007-04-12, 02:18   #6
jasong
 
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Quote:
Originally Posted by TehPenguin View Post
Also, on the point of HT, HyperThreading only gives an advantage if the two processes are using two DIFFERENT parts of the CPU - the two primes you are running are still going to compete against each other for the same part of the processor...
Are you sure about that? I seem to remember reading somewhere that the pipeline on Pentium-4 was so long that portions of it weren't being 100% utilized. The hyperthreading was basically adding instructions on the heels of other instructions. I'm not even sure that it matters at all what part of the processor each process uses.
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Old 2007-04-12, 11:32   #7
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Default most Prime95 instances than cores

In higher core numbers machines, you may not even be able to productively run as many instances as cores (4 or 8). I have access to a 8 core, 2 Xeon cpu machine that I will post results on soon that shows this quite clearly.

I think that memory becomes a major bottleneck with multiple instances running. In any case, there is clearly no role for running 2 instances per core with hyperthreading. You can try this, and watch your iteration times drop by more than 2-fold by running 2 instances on 1 core.
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Old 2007-04-12, 11:33   #8
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Default more Prime95 instances than cores

In higher core numbers machines, you may not even be able to productively run as many instances as cores (4 or 8). I have access to a 8 core, 2 Xeon cpu machine that I will post results on soon that shows this quite clearly.

I think that memory becomes a major bottleneck with multiple instances running. In any case, there is clearly no role for running 2 instances per core with hyperthreading. You can try this, and watch your iteration times drop by more than 2-fold by running 2 instances on 1 core.
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Old 2007-04-12, 11:34   #9
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Default sorry for the double post

guess you cant edit the subject header :-)
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Old 2007-04-12, 13:09   #10
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Quote:
Originally Posted by sgrupp View Post
guess you cant edit the subject header :-)
Would you mind testing my multi-thread factoring program when you'll have time?

Luigi
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Old 2007-04-13, 01:43   #11
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I think the solution to this, for the people addicted to running their cpus at 100%, is to run more than one project to attempt to accomplish as much computing as possible.

I have a feeling projects that don't take up a lot of L2 cache are going to become a lot more popular to people like me. :)

Last fiddled with by jasong on 2007-04-13 at 01:43
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