Prime95 Not using 100%

[Thratrun]

Hi, I'm using prime95 with a i7 6.7k 3.4ghz, with 4 workers (1 core each), and my cpu usage for prime95 is 56% most of the time, with 60-65% total CPU usage. Each core (including HT ones) is at 50% more or less.

[VBCurtis]

That would be because hyperthread cores aren't real cores, but Windows doesn't know the difference. 8 * 50% = 4 * 100% = all 4 cores fully used.

[Thratrun]

Originally Posted by VBCurtis

That would be because hyperthread cores aren't real cores, but Windows doesn't know the difference. 8 * 50% = 4 * 100% = all 4 cores fully used.

Oh so everything is fine then? It is working at max?

[kladner]

You are running fine. If you turned off hyperthreading usage would be 100%. I keep HT on, but set affinity to run on one of each adjacent pair of cores. This assigns all physical cores.
EDIT:

A 6700k should be running at 4GHz by default

Good point. I had not even noticed that detail. I have a 6700K which does quite well at 4.3GHz.

[henryzz]

A 6700k should be running at 4GHz by default

[kladner]

Oops. Should have edited my previous post.

[Thratrun]

Originally Posted by kladner

You are running fine. If you turned off hyperthreading usage would be 100%. I keep HT on, but set affinity to run on one of each adjacent pair of cores. This assigns all physical cores.
EDIT:

Good point. I had not even noticed that detail. I have a 6700K which does quite well at 4.3GHz.

How do I set affinity? In each worker at the beginning it says "setting affinity worker on cpu core #1" (the #1 is the number of the worker)

Oh, mine is a 6700 not a 6700k, I said it wrong

EDIT: I didnt have 100% because i didnt have HT for LL, P-1, ECM (it said it wasn't recommended) but i activated it and now it shows 100%

[Thratrun]

However, after activating HT in prime95, the ms/iter for 3 of the workers are higher and the other stays more or less the same, so it looks like it works better without HT

[Dubslow]

Originally Posted by Thratrun

However, after activating HT in prime95, the ms/iter for 3 of the workers are higher and the other stays more or less the same, so it looks like it works better without HT

This is true. Prime95 is very rare software that is written in hand-tuned x86 assembly -- the very, very large majority of software is not, and the compiled-to-assembly software generally isn't optimized enough to make full use of the hardware, which is the purpose of hyperthreading -- a second process can make use of silicon on one core that a first process simply can't use. Prime95 is the exception to the rule: its hand-tuned assembly is designed to use every bit of silicon to its maximum possible extent, so hyperthreading offers no additional value.

Even other software in use on other parts of this forum -- software with inline assembly included in the most critical computational loops , but still mostly written in C -- see a ~30% throughput gain with hyperthreading. Prime95 is very much different than nearly any other software in the world in this respect.

[kladner]

Originally Posted by Thratrun

How do I set affinity? In each worker at the beginning it says "setting affinity worker on cpu core #1" (the #1 is the number of the worker)

Oh, mine is a 6700 not a 6700k, I said it wrong

EDIT: I didnt have 100% because i didnt have HT for LL, P-1, ECM (it said it wasn't recommended) but i activated it and now it shows 100%

In local.txt:

[Worker #1]
Affinity=1,3,5,7
# Affinity=0,2,4,6

The second "Affinity" line is a commented example of another set of 'cores' that would use only physical cores.
Note that I run a single worker with 4 cores. If you have more workers, add sections as appropriate: [Worker #2] etc. Also that, in Windows at least, core numbers start with 0.

[GP2]

Originally Posted by Dubslow

This is true. Prime95 is very rare software that is written in hand-tuned x86 assembly -- the very, very large majority of software is not, and the compiled-to-assembly software generally isn't optimized enough to make full use of the hardware, which is the purpose of hyperthreading -- a second process can make use of silicon on one core that a first process simply can't use. Prime95 is the exception to the rule: its hand-tuned assembly is designed to use every bit of silicon to its maximum possible extent, so hyperthreading offers no additional value.

Even other software in use on other parts of this forum -- software with inline assembly included in the most critical computational loops , but still mostly written in C -- see a ~30% throughput gain with hyperthreading. Prime95 is very much different than nearly any other software in the world in this respect.

Except Prime95 hasn't (yet) been tuned for Skylake, and therefore if you use Skylake you should try turning on hyperthreading (by adding HyperthreadLL=1 to local.txt) and doing benchmarks with and without this setting. For me it did make a difference.