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Old 2003-11-25, 23:31   #1
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Default Program only running at 50%? (Hyperthreading-related)

Hello,
Prime95 seems to cause significant slowdowns on P4s with HT enabled. I think this is becuase to the OS there appears to be 2 physical CPUs, but in reality you have one full speed "CPU" and another with only 30% or so to give a little boost to multithreaded apps.

However with prime95, the OS schedules it to run 100% on for example CPU0, leaving only the 30% "real power" left on CPU1 for applications to run. Running two copies of prime95 and setting affinity makes no difference either.

Is there some way to work around this problem? I'm unfortunately thinking no due to the way the OS is scheduling the CPUs and they aren't actually physical CPUs thus the OS is comlpetely unaware that 100% of "CPU1" when 100% of "CPU0" is in use is really only about 30% of the CPUs performance.

Does this make sense? Has anyone else seen this behaviour or am I alone? Thanks :).
 
Old 2003-11-26, 00:17   #2
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We talked about it a bit...

This is the addressed thread:

http://www.mersenneforum.org/showthr...hyperthreading

HTH

Luigi
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Old 2003-11-26, 02:42   #3
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If you run HT on win2k you will have signifant slow down.

If you use HT on WinXP or above you wont have the slow down problem.

Last fiddled with by xtreme2k on 2003-11-26 at 02:42
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Old 2003-11-26, 07:20   #4
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Thank you for the replies, I have read the linked thread but it didn't really have much information regarding the slowdown issues.

xtreme2k, you say Win2k will have the slowdown but WinXP won't? What is the difference between these? I am currently using Windows Server 2003 and I am experiencing this slow down problem. Is it only XP that somehow "fixes" this? I'd be interested to hear more about the techniques involved if you know how it works. Thank you.
 
Old 2003-11-26, 12:04   #5
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From what I know only Win2k has this slow down. Win2k3 server and WinXP shouldn't have slow down at all.

Try do some benchmarks and post the results and let us compare :)

THe reason Win2k has to slow down is because Win2k does not understand HT is different from true 2 cpu and doing stuff it shouldn't be doing hence the slow down
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Old 2003-11-26, 16:43   #6
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Well it's probably not the best benchmark in the world but try getting .fr-08 demo from http://www.theproduct.de/ and load it up a few times. It plays music and progress during loading, without prime95 I get about 2-3 secs worth, with prime95 the music I hear lasts longer and load time is definitely increase.

HOWEVER I run Sandra 'CPU Arithmetic' benchmark and it comes out the same regardless if prime95 is running or not! Which makes me think, maybe this .fr-08 demo is using RAM intensively? Maybe hyperthreading and intensive RAM usage don't go together so well.

Still, the fact it causes noticable slowdown with certain things is unfortunate enough to make me a little nervous to run it on my new HT system :(. It was all so easy back with one CPU and a simple 'Idle time' :P
 
Old 2003-11-26, 19:22   #7
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Try using the benchmark that comes with Prime95:
Test|Stop
Options|Benchmark
We all understand the figures produced using that.....
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Old 2003-11-27, 08:39   #8
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Yes you need to show us the prime benchmarks.

Under WinXP/2k3 you might only see your overall CPU at 50% but I can assure you it should be running at full speed for Prime95.

However this is not the case for Win2k.
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Old 2003-11-27, 09:37   #9
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As I understand it, the problem is not that Prime95 slows down, but that everything else does.

As the Sandra CPU mark is the same with and without p95, problems with the priority setting is probably ruled out.

Could it be cache contention between the two threads? Just guessing...
MemMark or a similar memory benchmark could perhaps provide some pointers.
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Old 2003-11-28, 01:40   #10
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Yes, the issue is that prime95 is CAUSING slowdown as noted in initial post, not that prime95 is running slow! I think some other shared resource such as memory or cache is being taken up, on which the OS priority has no effect.

This also happens with distributed.net and Folding@HOME so I don't think it is specific problem to prime95. Is anyone else on a P4 hyperthreaded able to confirm this? Try various benchmarks, not just CPU arithmetic... as I said, I don't know what that .fr08 demo stresses the most, but it definitely takes much longer to load when prime95 is running...
 
Old 2003-11-28, 12:27   #11
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Oh. I get it now.

I think you are right.

Instead of idle priority letting other apps to run at 100% speed, apps is ran in the 'OTHER' cpu whilst prime95 keeps running.
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