Question Prime95 stops working on 32K FFT (stock settings and overclock)

sigod1

Junior Member
Jul 6, 2019
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My computer:
i7 950
Gigabyte X58A-UD3R-REV2
Kingston HyperX DDR3 1600
ZALMAN 750 W PSU
ZALMAN CNPS10X cooler
Gigabyte GTX 660


Hi, I've been trying to overclock my 10 year old computer for fun and I've discovered that it doesn't like 32K FFTs on both stock and overclocked settings. XMP is disabled. HT is disabled. Prime95 stops working after 10 minutes when I target that specific size. It has passed small fft and large fft tests because they don't use 32K.

Memtest86 passed 6 runs, so I'm not sure where the problem is. Is it the memory controller on the CPU or the motherboard?
 

Dave3000

Senior member
Jan 10, 2011
924
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Have you tried running your RAM at 1066 MHz. Going from 1066 to 1600 is a big jump and your i7-950's memory controller might not be able to handle DDR3-1600 even if the RAM itself has no issues with DDR3-1600 speeds. DDR3-1066 is the maximum officially supported DDR3 speed for the i7-950.

https://ark.intel.com/content/www/us/en/ark/products/37150/intel-core-i7-950-processor-8m-cache-3-06-ghz-4-80-gt-s-intel-qpi.html

It could be the RAM or CPU failing as well but more than likely I suspect it's because the RAM is running too fast for the memory controller on the CPU to handle.
 
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sigod1

Junior Member
Jul 6, 2019
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Thanks for your reply.

I just tried another custom test stock settings for the cpu and in place ffts and 32K with 1066 set and it still failed after 8 minutes.
Wouldn't an overclocked CPU be able to handle the faster RAM speeds anyway?
 

ehume

Golden Member
Nov 6, 2009
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Have you tried running your RAM at 1066 MHz. Going from 1066 to 1600 is a big jump and your i7-950's memory controller might not be able to handle DDR3-1600 even if the RAM itself has no issues with DDR3-1600 speeds. DDR3-1066 is the maximum officially supported DDR3 speed for the i7-950.

https://ark.intel.com/content/www/us/en/ark/products/37150/intel-core-i7-950-processor-8m-cache-3-06-ghz-4-80-gt-s-intel-qpi.html

It could be the RAM or CPU failing as well but more than likely I suspect it's because the RAM is running too fast for the memory controller on the CPU to handle.
I used to run 1333 DDR3 RAM OC'd to 1600 on my i7 860. I ran 1600 native DDR3 RAM on an i7 875k for years with no problems (2010-2017). I don't think the problem is that the i7 970 cannot handle 1600 DDR3.
 

Dave3000

Senior member
Jan 10, 2011
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Thanks for your reply.

I just tried another custom test stock settings for the cpu and in place ffts and 32K with 1066 set and it still failed after 8 minutes.
Wouldn't an overclocked CPU be able to handle the faster RAM speeds anyway?
Is your RAM's cmd rate running at 1T? Also setting cmd rate to Auto might be setting it to 1T. If it is then set it manually to 2T and see if the fixes the issue. I read somewhere that with 2 or more DIMMs installed it is recommended to set the cmd rate to 2T for most stability. Also you might want to test one stick at a time and see if it fails with each stick individually as maybe one of the sticks could be bad. Also how were the CPU temperatures when running Prime just before it it failed on you? As to your question, an overclocked CPU does not handle faster RAM more stable, in fact I once read that someone had to lower their RAM speeds MHz to get a more stable CPU overclock. By "handle" I meant handle with stability.
 

sigod1

Junior Member
Jul 6, 2019
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Dave, it was running at 1T and I changed it to 2T, prime95 lasted 10minutes longer and just crashed again :(. I see there is another setting turbo or standard, i tried switching from to turbo to standard without changing the command rate before and it didn't do anything. I'll try switching to standard with the changed command rate
 
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sigod1

Junior Member
Jul 6, 2019
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30 min stable with turbo off and counting. Hopefully I'm not jumping the gun again. Interesting that my computer has been running prime unstable for 10 years.

Update: 2 hours stable and counting thanks for your help :)
 
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sigod1

Junior Member
Jul 6, 2019
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So, I'm just wondering is the 1T issue a problem with prime95 or my computer. I mean since I've been running it for 10 years on 1T, should I change to 2T permanently (stock or overclocked).
 

Dave3000

Senior member
Jan 10, 2011
924
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1T command rate not running stable is not a Prime 95 issue. It's just that a command rate of 1T is not stable on your system if you are having issues in Prime 95 with the command rate set to 1T. Command rate is dependent on memory controller, quality of RAM, and number of DIMMS installed as far as I know. The more DIMMS installed the more likely it will be unstable at 1T command rate. If I run my Ballistix DDR3-1600 with a command rate of 1T then my system will sometimes reboot itself when I shut it down but setting the command rate to 2T fixes this issue so I run my RAM's command rate manually at 2T. I've never encountered RAM that had a specific command rate in it's JEDEC profiles but some DIMMS have a specified command rate in their XMP profiles when I checked using the CPU-Z program. MIne shows nothing for the command rate for it's JEDEC profiles but shows 2T for the command rate for it's XMP profile in CPU-Z.
 
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sigod1

Junior Member
Jul 6, 2019
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Ok, so I've started to overclock and I can get 4.0ghz stable for 24hours with small fft with a 1.325 vcore, 1.335VTT, 21x191 16xuncore 8xmemory multiplier, but when I run 32k tests on prime it crashes after about 1hour. I tried to increase QPI VTT from 1.1 to 1.3 and IOH vcore to 1.3 and it was still unstable. I'm not sure where to go from here to get my RAM stable. RAM voltage is 1.66. If I lower the bclk it seems to stabilize, but that is way less than my 4ghz target. Any ideas? RAM timings are on auto except for the 2T command rate.
 

ClockHound

Golden Member
Nov 27, 2007
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24 hours stable of power virus torment is pretty good for an elder 950. Just let it have an afternoon nap in that last hour.

Or... maybe it's time to spend the big bucks on a 5670 upgrade. $18 gets you 2 more cores and an easy 4.2Ghz. Since X58 is forever (Bumperstickers still available), when you amortize the cost it's virtually nothing.
 

sigod1

Junior Member
Jul 6, 2019
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I figured out what the problem was. I needed to set the timings manually (9-9-9-27, auto was wrong) and all the advanced memory settings to auto except for the command rate. The voltage has to be 1.5 at 1066 and 1.66 at 1528 for 4ghz. I'm pretty sure it isn't stable with 1600Mhz at stock cpu multiplier and frequency without increasing QPI/VTT and DRAM voltage.
 
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sigod1

Junior Member
Jul 6, 2019
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After some more diagnostics I discovered that the ram still fails 32K ffts at anything above 1066mhz at stock cpu settings even with relaxing the timings, a DRAM voltage increase and an increase in QPI VTT. I just don't know how to get it stable at 1333 and above.
I also noticed that leaving EIST and C states on would cause instability in prime95 too. This has been a really difficult problem. Maybe there could be another setting in Advanced Memory Settings that could stabilize things, I'm not sure which one though...
 
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sigod1

Junior Member
Jul 6, 2019
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I noticed on the xmp profile that qpi voltage for 1600 was 1.4 so I increased it to 1.395 and prime95 has been stable for over 2hours with 32k. I was using 1.335v qpi and it obviously wasn't enough. Then I got a rounding error so obviously it needs to be higher. What a mission
 

sigod1

Junior Member
Jul 6, 2019
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I tried all three sticks of ram in ddr_1 slot and then tried one stick in ddr_3 and prime still kept crashing at 32k. Since that didn't find a bad ram stick or a bad slot I'm thinking either this particular ram is incompatible with the motherboard or all the ram slots are faulty. I'm leaning more towards a bad cpu or psu.
 

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