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Old 2023-01-23, 22:17   #1
fogolol
 
Jan 2023

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Default How do i get Prime95 to use all 16 threads?

I have a Ryzen 7 1800x and wanted to use it to contribute but I am unable to get it to run over 8 cores, it will allow me to go under 8 but anything over will not allow me, while the cpu is an 8 core chip it does have amd's equivalent to hyperthreading therefore 16 threads but when using prime95 i am confined to using 50% of my cpu which leads to subpar performance, currently getting 8 MS per iteration while my secondary pc is doing 6 MS with a inferior CPU (i5 6400) I did try to edit the local.txt file but nothing changed, is there anything i can do?
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Old 2023-01-23, 23:17   #2
kriesel
 
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Quote:
Originally Posted by fogolol View Post
I have a Ryzen 7 1800x and wanted to use it to contribute but I am unable to get it to run over 8 cores, it will allow me to go under 8 but anything over will not allow me, while the cpu is an 8 core chip it does have amd's equivalent to hyperthreading therefore 16 threads but when using prime95 i am confined to using 50% of my cpu which leads to subpar performance, currently getting 8 MS per iteration while my secondary pc is doing 6 MS with a inferior CPU (i5 6400) I did try to edit the local.txt file but nothing changed, is there anything i can do?
Welcome to the forum. You might find some of the reference info useful.
Generally, there is no advantage to enabling hyperthreading in prime95 for PRP, P-1, or LLDC. (These are usually memory bound not processor bound.) By default, hyperthreading is off for PRP, LL, CERT, P-1, P+1, ECM, because performance is usually better with one thread per core than two or more. It is on by default for TF, but that really should be left to GPUs.
If you'd like to experiment, try benchmarking with and without hyperthreading, for the fft length you are currently using. (Options, Benchmark...)
It's hard to tell whether your ms timings are good, medium, or poor, without knowing what exponent or fft length they are from, or how many workers you have configured. (Four cores/worker is default.)
If you plan to do any P-1, P+1, or ECM, check (Options, Resource Limits, then find memory resources in the middle) and set it to a lot, as much as you can reasonably spare, for both day and night.
Have fun!

(Some tech background on SMT/hyperthreading: https://www.anandtech.com/show/16261...amd-ryzen-5000)

Last fiddled with by kriesel on 2023-01-23 at 23:29
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Old 2023-01-24, 08:52   #3
fogolol
 
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I benchmarked my cpu with and without hyperthreading and yes you are right, I am getting worse performance using 100% of my cpu versus 50%, that is a waste of power... I will continue doing it without hyperthreading now, but i might disable the option on the bios maybe i'll get better performance? Only one way to find out! Anyway thank you for your reply you were very helpful!
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Old 2023-01-24, 09:21   #4
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Quote:
Originally Posted by fogolol View Post
i might disable the option on the bios maybe i'll get better performance? Only one way to find out! Anyway thank you for your reply you were very helpful!
Depending on what other apps you run, SMT may be beneficial to them. It may help Mlucas run in WSL, for example, or garden variety apps.
And you're welcome.
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Old 2023-01-24, 13:14   #5
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Quote:
Originally Posted by fogolol View Post
I am getting worse performance using 100% of my cpu versus 50%, that is a waste of power...
A waste of power isn't a good way to look at it. Your CPU really does have only 8 cores, so once you have 8 threads pushing those 8 cores to 100% utilization each you really are at 100% CPU utilization. Those other 8 threads are only pretending to be cores, so when you don't use them it makes the operating system think you have 8 idle cores, thus it reports only 50% utilization.

But programs that are written to drive cores to 100% to begin with (like Prime 95) can't benefit from, and in fact are actually hindered by, extra threads running on pretend cores that don't exist.

Just remember that with Prime 95, if hyperthreading is turned on, 50% utilization as shown by the OS is actually 100% utilization of your physical cores, which means you have your processor properly maxed out.

Last fiddled with by PhilF on 2023-01-24 at 13:18
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Old 2023-01-24, 13:47   #6
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A handful of years ago when I was still working at the hospital, we wondered this very question.
With a roomful of racks of servers and zillions of desktops, we're talking about a significant difference in resources,
Although most programs were server applications, there was a good representation of client applications that were
real hogs.
The result was as previous posters have stated: leave hyper threading off, period.
We only found 1 instance in which it helped: Microsoft SQL. This is a server application.
I believe it was around the year 2000 when processors started to outrun memory bandwidth by significant margins.
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Old 2023-01-24, 16:09   #7
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One advantage of leaving hyperthreading on, at least on Linux, is that it can keep the system more responsive.
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Old 2023-01-25, 00:52   #8
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Quote:
Originally Posted by fogolol View Post
...i am confined to using 50% of my cpu...
This number is misleading. Prime95 is using all your cores, but none of the threads to determine this number. So, it is actually 100%. In my many years of using Prime95, it has always shown me 50% regardless of the CPU type. Prime95 is designed to be a background process and will regulate itself depending on what else is running at the same time. The goal is for it to be transparent to other programs.
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