Hyperthreading

[joblack]

Originally Posted by Primeinator

But using 2 threads each would be using hyperthreading, would it not? How would I go about setting up these different scenarios you've listed?

I'm sure the processor tries to distribute the load equally to all 'real' cpus'. Tesets have shown that hyperthreading will decrease your mprime performance even if you only use the real core count.

[Primeinator]

Using the setting "smart assignment" yields the fastest result. It is nearly 65% faster than assigning a core per CPU and is 2-3% faster than using the setting "Run on any CPU."

[Rhyled]

Originally Posted by lycorn

No. You would be running 2 tests, each using 2 cores. That is different from hyperthreading, where each core is used by 2 threads.
To set this up, go to Test->Worker windows, then choose Number of worker windows to run:2, and CPUs to use (multithreading): 2. You may leave the affinity setting at "Run on any CPU".

Try this versus other suggested settings, and draw your own conclusions. Keep us posted.

Being an engineer rather than a mathematician, I ran benchmarks to figure out the best configuration of cores and hyperthreading. My results indicate that the best use of a Core i7 is to turn Hyperthreading off, and use one core per prime candidate.

It's not a huge influence, however. We're talking performance drops in the 3-4% range. The following information applies to a Core i7-920 running Windows 7 64-bit OS. How far they extrapolate to other processors/systems is anyone's guess.

Hyperthreading slowed down computation by 3%. It also increased cpu power consumption by 15%. I.e., I could run with hyperthreading on to process 8 candidates simultaneously, but the total time would be 3% greater than doing 2 sets of 4 sequentially. Hyperthreading gets turned off.

Putting multiple physical cores on the same candidate sped up processing for that candidate considerably, at the expense of total calculation power for several candidates run simultaneously. Total drops are anywhere from 4% (using 2 cores/candidate) to 11% (all 4 cores) to 20+% (8 cores under hyperthreading). You can check this yourself by using the Prime95 benchmark task under the Options menu. The benchmarks are based on using 1-n cores on the same test candidate, not multiple versions.

Some tasks benefit from hyperthreading, Prime95 isn't one of them.