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-   -   How to use all threads (i7-920) (https://www.mersenneforum.org/showthread.php?t=18264)

petsoukos 2013-06-04 09:25

How to use all threads (i7-920)
 
Hi,

I downloaded the software (p95v279.linux64.tar.gz) and used all the defaults settings, but the program uses only 4 threads out of 8 possible threads.

I'm under Linux-64 (ubuntu 12.04)
CPU: i7 920@3.05GHz
RAM: 6GB Triple Channel ~1250MHz

How can I restart/reset the program to use all threads?

Thanks!

P.S.
I searched for an answer, but couldn't find any. Maybe I didn't see it, if a similar thread exists please provide a link and lock this one.

LaurV 2013-06-04 11:49

That CPU has only 4 physical cores. For the most of the work types you can do (LL included) you will not get any speed improvement from running 8 threads (i.e. instead of finishing 4 exponents in 10 days, you will finish 8 in 20 days), contrarily, your computer will be slower for the other tasks you do, for still no gain on P95 work.

Search the forum for "HT versus not HT" there are thousands of discussions. Or look the web for "hyper threading" and see what it is and how it works. HT takes advantage of the fact that most of the application software you use is not "optimized" for the CPU (i.e. to squeeze its calculus capacity to the maximum), so they work few milliseconds and wait few milliseconds. HT doubles the number of threads and runs a double number of tasks/applications in the same time (or more) in such a way that more applications use the same CPU core, and when one waits, the other takes over. It is not the case for P95, which is very optimized application. One task (worker) for one core is enough to use it to the maximum. Other task requesting the same core will (mostly) wait, or produce no [STRIKE]useful[/STRIKE] additional work.

petsoukos 2013-06-04 20:33

[QUOTE=LaurV;342462]That CPU has only 4 physical cores. For the most of the work types you can do (LL included) you will not get any speed improvement from running 8 threads (i.e. instead of finishing 4 exponents in 10 days, you will finish 8 in 20 days), contrarily, your computer will be slower for the other tasks you do, for still no gain on P95 work.

Search the forum for "HT versus not HT" there are thousands of discussions. Or look the web for "hyper threading" and see what it is and how it works. HT takes advantage of the fact that most of the application software you use is not "optimized" for the CPU (i.e. to squeeze its calculus capacity to the maximum), so they work few milliseconds and wait few milliseconds. HT doubles the number of threads and runs a double number of tasks/applications in the same time (or more) in such a way that more applications use the same CPU core, and when one waits, the other takes over. It is not the case for P95, which is very optimized application. One task (worker) for one core is enough to use it to the maximum. Other task requesting the same core will (mostly) wait, or produce no [STRIKE]useful[/STRIKE] additional work.[/QUOTE]

OK, thank you! I'll leave it alone to run on 4 cores,


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