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Old 2015-04-24, 16:20   #1
ZFR
 
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Default Using older version of Prime95

Hi,

I just got a new PC and installed Prime95 v28. I wanted to do stress testing first before I start looking for primes, however I did get some double checks assigned. No problem I thought, I'll get working on them soon as I finished stress tests.

It turned out v28 was overheating my CPU. According to Intel and overclocking forums, this is common in Haswell CPUs using newer versions of Prime95 and it was suggested that v26 be used.

I downloaded v26, and deleted the v28, forgetting that I had those double checks assigned.

Turned out v26 was indeed not overheating. When I started looking for primes again, I got new LL tests assigned. When I checked my account:
http://www.mersenne.org/cpus/
my new PC was listed twice. I merged them. Now however my account page still shows my the old double check test under "Current exponent", but my worktodo file shows only the new LL tests I'm doing now.

Any idea how to fix this? I don't mind doing either, I just don't want those double tests to be left unassigned. I tried manually communicating using Prime95, but it didn't solve it.

Thanks.
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Old 2015-04-24, 19:08   #2
James Heinrich
 
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If you look at the bottom of http://www.mersenne.org/workload/ you should see all your assignments, you can copy-paste the missing entries into your worktodo.txt (stop/close Prime95 before editing worktodo.txt, or alternately put the missing assignments into a file call worktodo.add in the Prime95 folder and it will get automatically merged into worktodo.txt after a few (~30?) minutes).
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Old 2015-04-24, 20:13   #3
S485122
 
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Quote:
Originally Posted by ZFR View Post
It turned out v28 was overheating my CPU. According to Intel and overclocking forums, this is common in Haswell CPUs using newer versions of Prime95 and it was suggested that v26 be used.
It means your cooling is not sufficient.

If you have not made significant progress on your assignments you can always unreserve them on the page James indicated.

Jacob
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Old 2015-04-24, 20:35   #4
Prime95
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Quote:
Originally Posted by ZFR View Post
It turned out v28 was overheating my CPU. According to Intel and overclocking forums, this is common in Haswell CPUs using newer versions of Prime95 and it was suggested that v26 be used.
That was terrible advice.

Use v28 and either:

a) fix your cooling or reduce your overclock
or b) as a last resort, put CpuSupportsFMA3=0 in local.txt


BTW, prime95 loves fast memory for best throughput.

P.S. Welcome to GIMPS!
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Old 2015-04-24, 21:29   #5
ZFR
 
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Quote:
Originally Posted by Prime95 View Post
P.S. Welcome to GIMPS!
Thanks :) I've been here since 2008. This is my third PC running GIMPS.

Quote:
Originally Posted by S485122 View Post
It means your cooling is not sufficient.
Quote:
Originally Posted by Prime95 View Post
a) fix your cooling or reduce your overclock
My cooling is good. I use a bequeit cooling system with Arctic Silver paste and I'm getting temps of 64-66 C when running stress tests, including AIDA64 and RealBench. 66-67 C when running Prime95 small FFTs using v26. And I'm not overclocking.

The problem is as soon as I run Prime95 v28 I my Vcore skyrockets (it's set on Auto in BIOS) reaching up to 1.4 with temperatures in 90s + CPU throttling. No amount of cooling can work with such voltages. Manually limiting Vcore in BIOS doesn't help. What finally worked was manually limiting TDP in BIOS, but that means the CPU gets underclocked to around 3.6GHz when Prime95 run (with temperatures in low 70s)

Using i7 4790K and Gigabyte Z97X-GT motherboard

https://communities.intel.com/thread/54422
https://communities.intel.com/docs/DOC-23517

I know that if Prime95 can bring about such voltages, then other programs can do this too. Should other programs cause this, then with TDP limit, the worst than can happen is I get underclocked to 3.6GHz. However, I really cannot have this done 24/7 with Prime95 running.

I'm really open to suggestions, because I've spent the last week playing with BIOS and running stress tests, but until Gigabyte/Intel somehow find a way to fix the voltage jumps, I can't see myself using v28 of Prime95.
If me using v26 can have any detrimental effects on Primenet, let me know so I won't use it.

Or if any 4790K users have any suggestions of what worked for them, let me know. I'm already using latest BIOS.

Thanks.

Quote:
Originally Posted by Prime95 View Post
or b) as a last resort, put CpuSupportsFMA3=0 in local.txt
Tried this, but it didn't have any effect :(

Last fiddled with by ZFR on 2015-04-24 at 21:31
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Old 2015-04-24, 22:03   #6
ZFR
 
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Quote:
Originally Posted by Prime95 View Post
or b) as a last resort, put CpuSupportsFMA3=0 in local.txt

Actually, this did have some effect. Now it's only underclocked to 4100 GHz.

When I add CpuSupportsAVX=0 to local.txt, I get the following message:
"No FFT lengths available in range specified."

However when I add both CpuSupportsFMA3=0 and CpuSupportsAVX=0 then it works fine. I get 69-73 C temperatures without any underclocking (at full 4.4GHz) when running small FFTs. 66-68 when running Blend.

The question then is: should I run GIMPS with FMA and AVX disabled? Other than slower speeds, are there any negative effects here?

What would you suggest?

Last fiddled with by ZFR on 2015-04-24 at 22:33
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Old 2015-04-25, 00:14   #7
Prime95
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Quote:
Originally Posted by ZFR View Post
However when I add both CpuSupportsFMA3=0 and CpuSupportsAVX=0 then it works fine. I get 69-73 C temperatures without any underclocking (at full 4.4GHz) when running small FFTs. 66-68 when running Blend.
This is the same as running v26, but at least you get any bug fixes since v26. There are 3 sets of FFTs, from slowest to fastest: SSE2, AVX, and FMA. AVX FFTs were introduced in v27 and FMA3 FFTs in v28.

Quote:
The question then is: should I run GIMPS with FMA and AVX disabled? Other than slower speeds, are there any negative effects here?
What would you suggest?
There are no negative effects other than slower speed.

If it were me and I'd paid Intel for a chip that is rated to run at 4GHz and it couldn't, then I would return it. I did that with one of my 4570Ks and was much happier with the replacement.

I was surprised setting CpuSupportsFMA3=0 made no difference. I thought adaptive voltage kicked in for FMA3 FFTs, but some quick googling found that AVX FFTs will trigger adaptive voltage.

You did mention that your vCore climbs to 1.4V. This seems very high to me. I run my two 4570Ks at about 1.25V fixed (not adaptive). If you are planning on running prime95 24/7, then you may as well select the lowest fixed vCore that passes the torture test (well, plus a smidge just to be safe).

Other readers may have some suggestions for you.
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Old 2015-04-25, 01:13   #8
ZFR
 
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Quote:
Originally Posted by Prime95 View Post
then you may as well select the lowest fixed vCore that passes the torture test (well, plus a smidge just to be safe).
I tried selecting fixed voltage, but it would still go above it...

Quote:
Originally Posted by Prime95 View Post
If it were me and I'd paid Intel for a chip that is rated to run at 4GHz and it couldn't, then I would return it.
Right. This made me realize that 4790K's base frequency is actually only 4GHz and not 4.4. That 4.4GHz is the "Turbo Boost" frequency. And it is that Turbo Boost which was causing the voltage to jump all over the place when running Prime95 (and causing it to underclock to 3500s when I limited Turbo Boost power - which is what I meant by limiting TDP in my previous post).

Disabling Turbo Boost in BIOS did the trick! I can now nicely run Prime95 with AVX and FMA enabled at 4GHz without underclocking and at temperature of only 55-56 Blend, 64-65 smal FFTs and with a steady Vcore of 1.08 :)

Thank you thank you thank you! You have no idea how much time I spent last week messing with BIOS to try and solve this.

Now I can probably manually increase the voltage and the frequency to 4.1GHz or 4.2GHz at least and still stay within safe temperatures, but for the moment I've had enough playing with BIOS to last me for some time.
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Old 2015-04-25, 06:59   #9
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Turbo boost is a funny beast. There are a bunch of factors at play which, like you saw, make a "Turbo Boost" to 4.4 GHz inferior in every way than a manual overclock to 4.4 GHz.

Turbo boost follows some basic voltage tables which are like 99.9+% guaranteed to work because it kicks in automatically. At least someone who overclocks and overvolts is accountable for their own tweaks. The Intel CPU in your grandma's laptop "overclocks" itself but granny doesn't know jack about overclocking. If the CPU bricks itself every time it enables turbo boost then she'll be angry so the voltage tables have to be ROCK solid AKA often more than 0.1V above the necessary amount.

Then there's the extra boost for AVX workloads which means if you're turbo boosted to the max and have the AVX increment then things go nuts. I believe there's a way to disable or change that increment on some motherboards, but it IS there for a reason... If you set an overclock manually, Turbo Boost doesn't do anything. If Turbo Boost is disabled, the extra voltage to feed the overclock is stopped as well, and you're relying on whatever voltage profile you've manually set

You can manually overclock with 1.2V or you can let the Turbo Boost get you there with 1.3V. On more recent CPU's that last 0.1V makes one helluva difference.
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Old 2015-04-25, 07:03   #10
ZFR
 
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Quote:
Originally Posted by TheMawn View Post
Turbo boost is a funny beast. There are a bunch of factors at play which, like you saw, make a "Turbo Boost" to 4.4 GHz inferior in every way than a manual overclock to 4.4 GHz.
Absolutely. I just tried manually overclocking to 4.4 and got there easily with 1.2V and just 8 degrees incerase in temperature. Didin't run much stress tests though. For the moment I'm leaving it to 4.0 without overclocking, but it's good to know the extra 400MHz is there if I ever need it :)
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