Migration from Core 2 Quad to i7 965

[Freightyard]

Copied my Prime95 directory over to my new machine, but Prime95 is only running at 50% CPU load (not using 8 threads for hyperthreading).

What is the setting to change this?

[Uncwilly]

Are you using v25.8? It is better than 25.6 for this.

[Freightyard]

Yes, 25.8. It reports "setting affinity to run worker on logical CPUs 0,1".. "2,3"... "4,5"... "6,7"

But notice the CPU usage for each pair below -- they are mirror images of each other. Where "0" is busy, "1" is idle, and vise-versa. The total is only 50%.

[Freightyard]

Ah--- but if I run TWO copies of v25.8 I attain 100%!

[Prime95]

Originally Posted by Freightyard

Ah--- but if I run TWO copies of v25.8 I attain 100%!

You never need to run two copies of v25.8. Use Test/Worker Threads dialog box. Either increase the number of workers or (probably better for you because of hyperthreading) set each worker to use 2 CPUs for each LL test.

[Freightyard]

Well 2 CPUs fixed it. Funny, I tried both of those previously and Prime95 ignored it--8 threads created four new empty worker windows that ignored extra lines in worktodo; and the CPUs box was shaded-out or changed back to 1 last time I tried that.

No matter. It's at 100% now. Thank you.

[Freightyard]

Or, maybe not...

Is TF work single-threaded only? I decided to try a day's worth of TF for fun. I absolutely could not get CPU load over 50% with only 4 workers (1, 2, or even 4 CPUs). With 8 workers... 100%. (Core i7 965)

So, does # of threads depend on the type of work? I realize the "normal" situation wouldn't bounce between TF and LL, but ideally one wouldn't have to tweak to keep CPU load at 100%--the program would just select what is best. I suppose this could get complicated if one had 2 workers (x1 thread) doing TF and 3 workers (x2 threads) doing LL.

No complaints... Just learning by doing here...

[joblack]

Originally Posted by Freightyard

Or, maybe not...

Is TF work single-threaded only? I decided to try a day's worth of TF for fun. I absolutely could not get CPU load over 50% with only 4 workers (1, 2, or even 4 CPUs). With 8 workers... 100%. (Core i7 965)
No complaints... Just learning by doing here...

I think it depends how Windows interprets the hyperthreaded cpus. They are no real cpus.

But let it work with 8 threads - that will be fine.

[lycorn]

Originally Posted by Freightyard

Is TF work single-threaded only?

Yes.

[Freightyard]

Originally Posted by lycorn

Yes.

So, then a Core i7 has to use 8 workers during TF to keep busy, but one must switch back to 4 workers for LL--assuming it is more efficient to assign one hyperthreading core (2 threads) per exponent than to test 8 exponents at a time.

Fair enough.

Originally Posted by joblack

I think it depends how Windows interprets the hyperthreaded cpus. They are no real cpus.

Windows interprets a quad-core with H.T. as 8 CPUs. But if you try do LL testing on 8 exponents (8 workers) simultaneously--anyone correct me if I'm wrong here--each core is then working on 2 exponents (which causes great competition for cache/memory) and the iteration time increases substantially. It makes more sense to have each hyperthreaded core (2 "CPUs") work on one exponent -- thus 4 workers having 2 threads each. LL testing can assign multiple threads to work on the same exponent concurrently.

However, Trial Factoring cannot utilize multiple threads for each exponent. So with 4 workers (4 exponents) there are only 4 threads operating and the hyperthreaded core is only 50% utilized. EVEN IF YOUR SETTINGS SPECIFY 2 CPUs. Hence, you apparently need to run 8 workers for TF work on a Hyperthreading quad-core.

[S485122]

Originally Posted by Freightyard

However, Trial Factoring cannot utilize multiple threads for each exponent. So with 4 workers (4 exponents) there are only 4 threads operating and the hyperthreaded core is only 50% utilized. EVEN IF YOUR SETTINGS SPECIFY 2 CPUs. Hence, you apparently need to run 8 workers for TF work on a Hyperthreading quad-core.

You must take the 50 % CPU utilisation with huge boulder of salt : Windows sees 8 CPU's but you know there are only 4, Hyperthreading lets you try to use the wasted cycles of each CPU, this results, on older architectures, in a 0 to 5 % increase of throughput. This means that the 50 % CPU utilisation that you see is really 100% of the real core and 0% of the 5% that could still be squeezed in => 95%. (I am not shure about the 5% on i7 architecture, but it is certainely not 100%.)

Jacob