idea: automatically use hyperthreading when it helps

ixfd64 · 2012-07-07, 05:50

One question that often gets asked is whether Prime95 can take advantage of hyperthreading. The answer is that it generally cannot. In fact, assigning two threads to one worker often slows it down as the threads will get in each other's way. But in some cases, hyperthreading does bump up the speed a little. For example, using both threads on my old Pentium 4 desktop makes each iteration about 3% faster.

This gave me an idea: Prime95 should use both threads in a core when there is a speed advantage. This could be done by running two benchmarks when a hyperthreaded processor is detected: one for a single thread and the other for both threads. If the latter case is noticeably faster, then both threads are enabled.

clarke · 2012-07-08, 07:21

I quite agree with the idea, at least notify the user about gainings or losses depending on HT on/off when computation starts. Probably, that notification could be generated by the server based on benchmark database. But silently turning on using HT cores isn't good for those who decided to not do it. Perhaps, a configurable option may help.

Dubslow · 2012-07-08, 19:30

Quote:

Originally Posted by clarke

Perhaps, a configurable option may help.

That is already the case, which is why it isn't automatic. Take a look through undoc.txt. Ctrl+F on "AffinityScramble", "NumCPUs", and "NumPhysicalCPUs".

ixfd64 · 2012-08-07, 00:27

It seems that hyperthreading may actually be far from useless. Some users have reported a noticeable increase in overall throughput by running two TF threads (or even one LL plus a TF thread) on a single physical core. In this case, it would be cool if Prime95 had the ability to automatically assign TF assignments to all virtual cores.

Most of the posts refer to P4's, though, so I don't know if newer processors would benefit in the same way.

ixfd64 · 2012-08-07, 18:47

I should mention that if Prime95 does include this feature in the future, then it may be a good idea to make hyperthreaded cores TF to three or four bits above normal levels as they will still be relatively fast compared to older hardware.

2012-07-07, 05:50	#1
ixfd64 Bemusing Prompter "Danny" Dec 2002 California 2·5·239 Posts	idea: automatically use hyperthreading when it helps One question that often gets asked is whether Prime95 can take advantage of hyperthreading. The answer is that it generally cannot. In fact, assigning two threads to one worker often slows it down as the threads will get in each other's way. But in some cases, hyperthreading does bump up the speed a little. For example, using both threads on my old Pentium 4 desktop makes each iteration about 3% faster. This gave me an idea: Prime95 should use both threads in a core when there is a speed advantage. This could be done by running two benchmarks when a hyperthreaded processor is detected: one for a single thread and the other for both threads. If the latter case is noticeably faster, then both threads are enabled.

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2012-07-08, 07:21	#2
clarke Feb 2009 2²·7 Posts	I quite agree with the idea, at least notify the user about gainings or losses depending on HT on/off when computation starts. Probably, that notification could be generated by the server based on benchmark database. But silently turning on using HT cores isn't good for those who decided to not do it. Perhaps, a configurable option may help.

2012-08-07, 00:27	#4
ixfd64 Bemusing Prompter "Danny" Dec 2002 California 2×5×239 Posts	It seems that hyperthreading may actually be far from useless. Some users have reported a noticeable increase in overall throughput by running two TF threads (or even one LL plus a TF thread) on a single physical core. In this case, it would be cool if Prime95 had the ability to automatically assign TF assignments to all virtual cores. Most of the posts refer to P4's, though, so I don't know if newer processors would benefit in the same way.

2012-08-07, 18:47	#5
ixfd64 Bemusing Prompter "Danny" Dec 2002 California 100101010110₂ Posts	I should mention that if Prime95 does include this feature in the future, then it may be a good idea to make hyperthreaded cores TF to three or four bits above normal levels as they will still be relatively fast compared to older hardware.