Re-enabling hidden cores on a Ryzen?

Lamberto Vitali · Apr 28, 2023

I came across "The Ryzen 9 3900XT has 2 CCDs with each 2 CCXs. Each CCX has 1 out of the 4 cores disabled. Adding up all the cores that are left, 3 times 4, makes 12 cores" on SkatterBencher #13: AMD Ryzen 9 3900XT Overclocked to 4550 MHz - SkatterBencher

What's that? DISABLED?! I've got a 16 core CPU here with 4 turned off? Why are they off? Can I turn them on?

Ghot · Apr 28, 2023

Lamberto Vitali said:
I came across "The Ryzen 9 3900XT has 2 CCDs with each 2 CCXs. Each CCX has 1 out of the 4 cores disabled. Adding up all the cores that are left, 3 times 4, makes 12 cores" on SkatterBencher #13: AMD Ryzen 9 3900XT Overclocked to 4550 MHz - SkatterBencher

What's that? DISABLED?! I've got a 16 core CPU here with 4 turned off? Why are they off? Can I turn them on?

You're not gonna like this.
If it was a good idea to turn them on, AMD would have done so.

They could have charged a lot more if they could claim 16 cores. ^^

On a side note: The 3900X already has a boost clock of 4.6Ghz, so a 4550Mhz overclock... ain't that impressive.

Lamberto Vitali · Apr 28, 2023

Ghot said:
You're not gonna like this.
If it was a good idea to turn them on, AMD would have done so.

They could have charged a lot more if they could claim 16 cores. ^^

Yes. Mine would overheat and use too much power (for the current carrying capabilities of various tracks) if I just turned them on, I assume I'd need to lower voltages too and stick on liquid nitrogen cooling. I'm not going to bother. But out of interest, why do AMD do it? Why put in cores (which cost money) and turn them off?

I have two theories:

1) There's a heat/power limit which means they can only use 12. But when they make them, they're not always perfect, so they select the best 12 out of 16.

2) I've read if one breaks, it will disable it automatically. Can it perhaps act like a rust spinner hard disk and reallocate an unused core to replace a worn out one?

Ghot said:
On a side note: The 3900X already has a boost clock of 4.6Ghz, so a 4550Mhz overclock... ain't that impressive.

Yeah but I assume they got that speed with all cores running. At stock, it only uses that speed when you're running a single core. Mine is currently running 24 threads on Asteroids@Home (AVX) - this is making it run at the stock clock of 3.8.

Ghot · Apr 28, 2023

Lamberto Vitali said:
Why put in cores (which cost money) and turn them off?

Could be a power/heat issue
Could be trouble when they made the CPU die.

The point is... if they could claim 16 cores... they would. Any CPU manufacturer would.

I don't know what motherboard you have, so I can't look up your BIOS settings... but most AMD CPUs can be overclocked by just turning up the multiplier and adding a bit of voltage.
I got an AMD FX-4170 to boot at 5Ghz. LOL
It wasn't stable by any means, but it was just a quick multiplier tweak.

https://www.cpu-world.com/CPUs/Bulldozer/AMD-FX-Series%20FX-4170%20FD4170FRW4KGU.html

World's fastest multiplier overclock. Took about 5 minutes total. Booted every time.
1st pic is stock, ofc.

Then I got an FX-8350, then the Ryzen 3700X...

Well... it's past my bedtime.
I wouldn't mess with the hidden cores, but that's me.

Lamberto Vitali · Apr 28, 2023

Ghot said:
Could be a power/heat issue

I have a problem with the use of your word issue.

Ghot said:
Could be trouble when they made the CPU die.

But they're ALL like that.

Ghot said:
The point is... if they could claim 16 cores... they would. Any CPU manufacturer would.

But I'm interested in WHY.

Ghot said:
I don't know what motherboard you have, so I can't look up your BIOS settings... but most AMD CPUs can be overclocked by just turning up the multiplier and adding a bit of voltage.
I got an AMD FX-4170 to boot at 5Ghz. LOL
It wasn't stable by any means, but it was just a quick multiplier tweak.

As I'm running at 85C (25C ambient) with one of those big 6 inch cubed Zalman coolers, I doubt I can do that. I'd have to LOWER voltage.

Ghot said:
World's fastest multiplier overclock. Took about 5 minutes total. Booted every time.
1st pic is stock, ofc.

What is an ofc? OFC shows many possibilities.

Ghot said:
View attachment 58961

25/21 isn't much faster....

Ghot said:
Then I got an FX-8350, then the Ryzen 3700X...

View attachment 58962

Well... it's past my bedtime.
I wouldn't mess with the hidden cores, but that's me.

As I already said, I'm not going to due to temperature limits, but I still want to know WHY they put them there.

Helmut · Apr 28, 2023

The reason is to maximise the yield of CPUs produced.

Those with 16 fully functional cores will be one model of CPU, and those with up to 4 partially functional cores will be another CPU with 12 cores. The faulty cores being disabled.

Obviously there is a load of other quality controls as well.

Fewer production lines required to produce a range of sellable CPUs.

Enabling partially functional cores is just stoopid.

Lamberto Vitali · Apr 28, 2023

Ah, a sensible answer, thanks. So it's as I thought, they sell what happens to work after testing. And what I have here is 12 working cores and 4 broken ones.

Although.... if mine had passed the test for all 16 cores, there's no way that could have run without overheating.

I like what I read about it automatically turning off some more if they should ever break.

Bree · Apr 28, 2023

Lamberto Vitali said:
What's that? DISABLED?! I've got a 16 core CPU here with 4 turned off? Why are they off? Can I turn them on?

Helmut said:
The reason is to maximise the yield of CPUs produced.....
....Enabling partially functional cores is just stoopid.

Helmut is absolutely right.....

....the disabled cores are turned off for a reason: they failed factory tests. Cores can fail for any number of reasons, including defects in the silicon, problems running at full frequency, or a bug introduced during manufacturing.

Both AMD and Intel disable CPU cores for this very reason.

Unlocking AMD CPU Cores Safe Say Mobo Makers

While browsing through motherboards at Computex, several manufacturers were quick to tell us some details about the recently released dual-core Phenom II CPUs from AMD.

www.tomshardware.com

Lamberto Vitali · Apr 28, 2023

Bree said:
....the disabled cores are turned off for a reason: they failed factory tests. Cores can fail for any number of reasons, including defects in the silicon, problems running at full frequency, or a bug introduced during manufacturing.

Both AMD and Intel disable CPU cores for this very reason.

I'm surprised they can test them thoroughly enough to be sure which bits work.

I'm also surprised it doesn't cost an enormous amount to run the tests on every single individual CPU. It defeats the purpose of mass production. Do they run these tests on the cheaper CPUs? Or are those made with wider safety margins and always work?

Bree · Apr 28, 2023

Lamberto Vitali said:
I'm surprised they can test them thoroughly enough to be sure which bits work.

I'm also surprised it doesn't cost an enormous amount to run the tests on every single individual CPU. It defeats the purpose of mass production. Do they run these tests on the cheaper CPUs? Or are those made with wider safety margins and always work?

The tests are fully automated. The wafers are first tested before they're even diced into chips. The diced chips are then tested again to classify them.

Although it passed the Wafer Test, the chip is still bad.

Packaging damage.

The chip is partially damaged. For example, the CPU has 2 cores damaged, the GPU is damaged, the display interface is damaged, etc.

The chip is good, with no faults.

At this point, engineers need to work with the marketing department to decide how to classify these chips. For example, if the GPU is broken, it is treated as a “Celeron” series processor without a display core. For example, a CPU with two broken CPUs is treated as a “Core i3” series processor. If the chip is working properly, but the operating frequency is not high, it is treated as a “Core i5” series processor. No problem at all, as the “Core i7” processor.

How do semiconductor manufacturers do factory chip testing?

Once the chip is produced by the fab, it enters the Wafer Test stage. This phase of testing may be performed in the fab.

www.diskmfr.com

Lamberto Vitali · Apr 28, 2023

I wondered why there were so many models of chips, it seemed like a waste of money having so many models. But this makes more sense, they make one, then it gets a score.

However I don't think we can apply the same to cars. A Renault Scenic for example has 32 different submodels. All combinations of: Mk1-4, small/medium/large petrol engine or diesel engine, and within each Mk they redesign stuff like driveshafts. It's almost impossible to get the right replacement part!

Senecio · Apr 28, 2023

There's very little point in overclocking a 3900x. It is quite difficult to do, and retain stability - because AMD have already tuned the processor to give the best boost clocks they can. You're better off getting more/faster RAM, which does produce a noticeable improvement in day to day use.

Lamberto Vitali · Apr 28, 2023

Actually, changing from single channel to dual channel RAM makes hardly any difference. Reviews suggest an average of 15%. I run many different science programs on my computers, which use the CPU flat out. Some produced a noticeable (but not huge) benefit from dual channel, the others showed none at all. Either most programs don't use the RAM that often, or they use a small enough amount the Ryzen's cache takes care of it.

Ghot · Apr 28, 2023

Lamberto Vitali · Apr 28, 2023

Ghot said:
View attachment 58970

Unable to parse between lines. Please extrapolate insult.

Ghot · Apr 28, 2023

Lamberto Vitali said:
Unable to parse between lines. Please extrapolate insult.

It's not an insult.
That's the famous "I can haz cheeseburger" cat.
This topic just made me think of it.

Really... no insult intended.

TairikuOkami · Apr 28, 2023

Undervolting and overclocking with a decent cooling should do, unless the piece is too faulty. People automatically assume increasing the voltage, but I managed to undervolt and overclock CPU on a laptop, I increased it by 400MHz with a lower voltage and 30C lower temperature. Manufacturers just do not have the time to tinker with each piece, each is unique. All it takes is time and patience.

JAM83 · Apr 28, 2023

I have the Ryzen 7 1800X. . .It's fast enough for me. I don't dare overclocking anything.

JAM83 · Apr 28, 2023

TairikuOkami said:
Undervolting and overclocking with a decent cooling should do, unless the piece is too faulty. People automatically assume increasing the voltage, but I managed to undervolt and overclock CPU on a laptop, I increased it by 400MHz with a lower voltage and 30C lower temperature. Manufacturers just do not have the time to tinker with each piece, each is unique. All it takes is time and patience.

Overclocking is so dangerous! I have no idea why people do it! If you want faster parts, buy a better one. . Don't screw with the default settings.

tinmar49 · Apr 28, 2023

I have had several older AMD processors, AM3 which had unlockable extra cores, either 3 to 4 or 2 to 4 in my case. I tried unlocking them with complete success, but reverted to the original configuration shortly after. At the time, the main theory was because AMD had got so good at producing multicore processors that they often locked a percentage to be able to sell enough lower spec processors.

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