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Old 2018-03-08, 03:31   #1
albyva
 
"Alby"
Mar 2017
Ashburn, VA

11 Posts
Default Only 50% on i7-7820HQ @ 2.90GHz

I just recently bought a top end Macbook Pro (i7-7820HQ @ 2.90GHz) and I'm running Prime95 (v29.4 Build 7). It runs great and has so far knocked out M47965471 and M47963281 in 3 days each. Now its working on M84710237 which is expected to take 10 days.

In any case, here my question..... When I run (htop), I only see 4 of 8 CPU cores being utilized for Prime95. I then look at (uptime) and get back around 4.00 - 4.50, which
divided by 8 says my CPU is only about 50%-56% utilized. So my question is, why isn't Prime95 using nearly 100% of my 8 Core processor?
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Old 2018-03-08, 03:35   #2
albyva
 
"Alby"
Mar 2017
Ashburn, VA

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Oh wait, I think I see the problem (my assumption). I was thinking the 7820HQ had 8 Cores, but what it has is 4 Cores and 8 threads. Doh!!
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Old 2018-03-08, 06:08   #3
VBCurtis
 
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"Curtis"
Feb 2005
Riverside, CA

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Quote:
Originally Posted by albyva View Post
Oh wait, I think I see the problem (my assumption). I was thinking the 7820HQ had 8 Cores, but what it has is 4 Cores and 8 threads. Doh!!
You've nailed the central part of the reason. Some distributed-computing tasks multithread in a way that benefits from using the hyperthreaded cores, but Prime95 is hand-tuned assembly code that fully uses all 4 cores (or fully uses the memory bandwidth, sometimes showing htop with just under 4.00 because cores occasionally wait on memory requests). Adding more than 4 threads doesn't make tests any faster, so those hyperthreads are left open.
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