mersenneforum.org  

Go Back   mersenneforum.org > Great Internet Mersenne Prime Search > Software

Reply
 
Thread Tools
Old 2011-06-12, 10:24   #1
dmoran
 
Jan 2009

3 Posts
Default Sandy Bridge CPU Usage only 50 percent

I noticed that 2 Sandy Bridge CPUs(E3-1230,2600K), both 4 cores w Hyperthreading enabled are only using 50% of CPU.(Taskmgr Windows7 x64 v26.6 Prime95) Both show pairs of logical CPUs(1-2,3-4,5-6,7-8) for each physical CPU. If I run torture test with 8 workers, CPU usage is 100%. I believe CPU usage was 100% before v26.6 upgrade, which reduced 8 workers down to 4. Any ideas?
dmoran is offline   Reply With Quote
Old 2011-06-12, 13:17   #2
LaurV
Romulan Interpreter
 
LaurV's Avatar
 
Jun 2011
Thailand

2·3·7·233 Posts
Default

i think that is right, you have 4 "physical" cores, but 8 "logical", because of HT. I had same problem in the past with an atom cpu, doing factoring, saw that only 50% is busy and it was no way to convince it to start 4 threads of factoring (2 physical cores, taskmgr reporting 4 because of HT). After some playing around with the settings I convinced prime95 to start 4 workers, cpu became 100% busy, but the speed of each worker decreased to about 55%, so in fact you won't get much gain in the speed, and beside of it, your computer (talking here only about my atom, no idea how your computer will behave) became a bit slower (longer response for other programs). So, I believe the Prime95 correctly detected my 2 physical cores, and the best way should be to let it run only 2 workers, in this case I have the HT ("rest of 50%") free to do my daily stuff. Forcing the prime95 to open 4 workers is a bit of "too much" for 2 physical cores, either if taskman reports 4. Same as running two or more workers on a single core, you won't get much gain, with or without HT, the difference would be quite small.
LaurV is offline   Reply With Quote
Old 2011-06-12, 14:45   #3
Mini-Geek
Account Deleted
 
Mini-Geek's Avatar
 
"Tim Sorbera"
Aug 2006
San Antonio, TX USA

10000101011002 Posts
Default

LaurV is right: the extra HT logical cores don't significantly help LL testing, and in this case the task manager saying it's 50% is still using your CPU as much as possible. I think the HT detection was improved recently, so the new version decided to go down from 8 to 4 worker threads because you only have 4 physical cores.

Last fiddled with by Mini-Geek on 2011-06-12 at 14:46
Mini-Geek is offline   Reply With Quote
Old 2011-06-14, 21:21   #4
Colt45ws
 
Colt45ws's Avatar
 
Jun 2010

17 Posts
Default

Yes. I dont know if it is the same as on Nehalem, but enabling the helper threads on that arch caused the per iteration time to be about 5ms longer, dependent on FFT size.
It also burned 15W more power and ran hotter as a result.
Colt45ws is offline   Reply With Quote
Reply

Thread Tools


Similar Threads
Thread Thread Starter Forum Replies Last Post
So Sandy Bridge Xeons are now launched fivemack Hardware 6 2012-03-14 11:27
Overclocking, Sandy Bridge-E : Don't firejuggler Hardware 6 2012-03-08 19:38
2 disabled cores in new Sandy Bridge-E :( stars10250 Hardware 8 2011-11-16 13:55
New Sandy Bridge Computer Help (Built - WOW!) Prime95 Hardware 104 2011-05-24 00:32
Sandy Bridge benchmarks are out. nucleon Hardware 0 2011-01-04 11:41

All times are UTC. The time now is 23:24.


Fri Oct 22 23:24:49 UTC 2021 up 91 days, 17:53, 0 users, load averages: 1.16, 1.10, 1.23

Powered by vBulletin® Version 3.8.11
Copyright ©2000 - 2021, Jelsoft Enterprises Ltd.

This forum has received and complied with 0 (zero) government requests for information.

Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation.
A copy of the license is included in the FAQ.