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Core i5 6600T Cache Speed reduced when using SkyOC
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MageTank View Drop Down
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    Posted: 05 Jan 2016 at 12:26pm
Greetings! So I've had the Asrock Z170 Gaming ITX/AC Fatal1ty motherboard for 2 months now, and it is by far my favorite ITX motherboard to date. Very premium feel and features to it, and so far, overclocking my CPU's has been easy and reliable. However, with the recent Sky OC being released, i wanted to test some of the locked Intel SKU's, most notably the Pentium G4400 and the Core i5 6600T. 

The Pentium G4400 was simple to overclock, and i managed to reach a stable 4.7ghz on the stock intel cooler under prime95 and linpack load. Everything was amazing, even overclocked my ram to 3200mhz CL15-15-15-30-CR1. Everything ran fine. After that test, i decided to test a Core i5 6600T, since they were cheap, and I wanted to see what the voltage tolerances of the T series processors would be like. Needless to say, i was very impressed. The CPU made it to 4.5ghz without breaking a sweat, at 166.7bclk. Temps were fine, and managed to pass my previous stability tests. However, I noticed one very significant difference between the Pentium G4400 and Core i5 6600T. 

When overclocking the Core i5 6600T, it's L1 cache speed drops by over 75%. Thinking this was a random fluke, i retested my G4400 at stock and overclocked speeds. My pentium saw a performance boost across the board when overclocking, as its cache speed actually increased at 4.6ghz compared to its 3.3ghz stock, but the Core i5 6600T went from 820GB/s, to less than 240GB/s. I have tried absolutely everything I could think of to remedy this, and I am completely out of idea's. Is it the board forcing the CPU Cache to some sort of default low cache multiplier that I cannot touch? Or is it the nature of the 6600T to lose cache speed upon overclocking the core? If anyone else has had this problem, I would appreciate any insight you could offer.

Screenshots of my G4400 and 6600T below.

G4400 Before:

G4400 After:

Core i5 6600T Before:

Core i5 6600T After:
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MageTank View Drop Down
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I now know this problem is not unique to me. Someone with a Core i5 6500 has the exact same problem with his cache speed degrading after using SkyOC. He even has the exact same board as I do.



Is it a problem with the board itself? Or is it a bios issue that ASRock can solve with an update? I would appreciate any comment that can help figure this out, as I am completely out of ideas.
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Xaltar View Drop Down
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Could you post some CPUz screens so we can see the frequency your cache is running at both at stock and overclocked? I suspect your cache multiplier is being lowered as you increase the BCLK to prevent issues. It may be that the scaling is too aggressive. 
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Originally posted by Xaltar Xaltar wrote:

Could you post some CPUz screens so we can see the frequency your cache is running at both at stock and overclocked? I suspect your cache multiplier is being lowered as you increase the BCLK to prevent issues. It may be that the scaling is too aggressive. 

Sure thing. 

Stock: (Turbo Boost 3.5ghz, not overclocked)

Overclocked: (50%, 150 BCLK)

Thank you for taking the time to help me out with this. 

EDIT: More validation of cache speed, from XTU:

Ignore the 100C, after overclocking these locked Skylake CPU's, temperature reporting is not available. Package temps are still at 35C idle, under 70C at load.


Edited by MageTank - 07 Jan 2016 at 3:08am
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Can you set your cache multi in XTU? if so try setting it 1 or 2 lower than your core multi and run your benchmark again. 1 to 1 core/cache ratios have been known to cause performance issues on haswell, it may be the same on skylake.
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Originally posted by Xaltar Xaltar wrote:

Can you set your cache multi in XTU? if so try setting it 1 or 2 lower than your core multi and run your benchmark again. 1 to 1 core/cache ratios have been known to cause performance issues on haswell, it may be the same on skylake.

The options to touch cache multi in XTU are grayed out. I can only touch the voltage, and even 1.4v has not helped. 
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I got this same thing going on with an AsRock OC Formula board and i5-6400.  Pretty much has to be something going on inside the processor to cause this.  I have no clue if it is fixable :(
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I guess you can try upping the BCLK in small increments and see when the deficit kicks in. If it occurs the moment you increase the BCLK regardless of the size of the increase then it is likely an inherent flaw. If the deficit grows as the frequency is increased then it may be some kind of mechanism to ensure stability. Either way, given the issue did not effect the G4400 it may be correctable via a BIOS update. 

edit: In fact now that I think about it I remember reading that benchmarks were not reflecting the benefit of the overclock on certain CPUs with BCLK overclocking and the posts were not exclusive to ASRock boards either. It may be an inherent flaw with the overclocking method. There still may be hope for a fix though. I won't be able to confirm until I have a SKY OC enabled board to test with my 6600k.

Let us know what happens with the incremental increase test I suggested above.


Edited by Xaltar - 07 Jan 2016 at 9:01pm
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will do - also fyi - skyoc locks core and uncore 1 to 1 as far as i can see.
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If that is the case then the issue may well be correctable.
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