We found the Missing Performance: Zen 5 Tested with SMT Disabled

[W1zzard]

Reviews of AMD’s Zen 5 processors this week surprised many, with lower-than-expected results. After some investigation, we discovered that turning off Simultaneous Multithreading (SMT) can yield notable performance gains, particularly in gaming. This article presents our findings, including comparisons with the 7800X3D and 7700X with SMT turned off.

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[dgianstefani]

I've run my 7800X3D with SMT disabled for security reasons for a good while now, I noticed some improvements in frame rate stability and memory OC too.

 

[Daven]

So enable SMT for applications and disable for games. Got it!

Thanks for investigating this!

Edit: Looking at Cinebench Multi, you get 27% more performance with SMT than without for no additional power use. This dispels the idea that you would save power by getting rid of HT unless the particular implementation is severely borked.

 

[dont whant to set it"']

Skip to comments section, like review, jump to page one!

 

[Wirko]

So enable SMT for applications and disable for games. Got it!

Thanks for investigating this!

Or set thread affinity to 0, 2, 4, 6, 8, 10, 12, 14 for games and apps that seem to hate HT. Wouldn't that be almost as good?

 

[izy]

Interesting , maybe they could fix this with an AGESA BIOS update tho pretty weird they didnt do anything about this from the start.

Or set thread affinity to 0, 2, 4, 6, 8, 10, 12, 14 for games and apps that seem to hate HT. Wouldn't that be almost as good?

It should work by default without us doing tricks

 

[napata]

This isn't anything new. A lot of games even ignore SMT/HT threads as an optimization unless they detect you're on a low core count. A pure gaming CPU would not have SMT/HT.

Interesting , maybe they could fix this with an AGESA BIOS update tho pretty weird they didnt do anything about this from the start.


It should work by default without us doing tricks

It would mess up game specific optimzations so they can't do that. Just look at how global settings don't even work for the 7950X3D in some games. There's no real fix for it as that's just a cost of SMT/HT.

 

[izy]

This isn't anything new. A lot of games even ignore SMT/HT threads as an optimization unless they detect you're on a low core count. A pure gaming CPU would not have SMT/HT.


It would mess up game specific optimzations so they can't do that. Just look at how global settings don't even work for the 7950X3D in some games. There's no real fix for it as that's just a cost of SMT/HT.

Are you sure about that? Even W1zzard said, quote: "In the briefings we heard some vague comments that AMD is using AI-trained networks inside the CPU to optimize operation, maybe that mechanism could be used for better thread placement? Just to clarify, there is no training while the CPU is running in your system, the network is generated during design-time in AMD's labs. However, I would expect that it is upgradeable through microcode somehow. In the past AMD has made changes to the behavior of their processors through AGESA BIOS updates, I think they will address the scheduling similarly."

 

[cerulliber]

according to Intel overclockers:
-we have to disable cool & quiet
-PBO
-XMP
-pch
-rebar
-IGD/shared memmory
-ErP
-power fault protection
-usb standby power at s4/s5
-ccpt
-cpu overheat protection
-legacy usb
-CSM
-UEFI
-secure boot
-fTPM
-core isolation
-spectre/meltdown protection
-chasis intruder
-SMT
-we'll end up disabling AMD

but we'll gain 10 cinebench points /s

 

[Macro Device]

we'll end up disabling AMD

Oh no, don't you dare!

 

[brink]

very interesting! thanks for the tests
i see in the results that amd should have already for zen4 start to investigate and optimize workload placement on cores/threads, especially for games, together with microsoft.
there are for sure many reasons why this have not happened in the way it'd been necessary. amd, microsoft, mobo manufacturers, game (engines) developers..
i'm very curious for the next week :-D

 

[SN2716057]

7800X3D FTW! I'll keep everything as is, no disabling smt, xmp expo on memory, and slight negative on pbo. Zero issues.

 

[AusWolf]

Games run better with SMT disabled? What!? This does not compute!

 

[RootinTootinPootin]

so everytime I would game, I have to hit the bios and do this? that looks not so practical.

 

[SL2]

Much appreciated, I've been waiting for something like this, but didn't expect it already.

I do think there's something else that also impedes it tho.

 

[AusWolf]

so everytime I would game, I have to hit the bios and do this? that looks not so practical.

It's not that big of a difference. Unless you have a 4090 and you game at 1080p or lower, I wouldn't bother.

Still interesting results, though.

 

[SL2]

so everytime I would game, I have to hit the bios and do this? that looks not so practical.

No, you don't have to.

Also, there might be a fix for this later on.

 

[RootinTootinPootin]

It's not that big of a difference. Unless you have a 4090 and you game at 1080p or lower, I wouldn't bother.

Still interesting results, though.

well, that has been a given, some peeps on Intel disables HT for gaming, but nonetheless the multi-core performance ain't regressing.

 

[izy]

so everytime I would game, I have to hit the bios and do this? that looks not so practical.

Right and we dont know if this improvements with SMT OFF will translate into real use performance increase , usually people have some apps opened in the background while gamming like discord , chrome tabs etc...

 

[R0H1T]

Games run better with SMT disabled? What!? This does not compute!

Not always games/chips/settings is an extremely complicated relationship!

 

[blu3dragon]

With a 5800x3d there is a measurable gain in 3dmark's 8 thread CPU test from disabling CPPC preferred cores in the bios. That seems like a windows scheduler bug. I wonder if something similar is going on here?

On Zen 3:
AMD CBS > NBIO > SMU > CPPC Preferred Cores Disabled

See https://www.overclock.net/threads/5800x3d-owners.1798046/?post_id=28975266#post-28975266

I have not found the equivalent for Zen 4

 

[Dsirius]

Great article and very usefull, keep posting man.

 

[Crackong]

Some games runs better without SMT
Always true for both AMD and Intel CPUs for a while.

Zen5 is no different.

 

[Metroid]

I've run my 7800X3D with SMT disabled for security reasons for a good while now, I noticed some improvements in frame rate stability and memory OC too.

Mind to explain why it improves security with smt off? maybe is related to this https://www.tomshardware.com/news/new-vulnerability-affects-all-amd-zen-cpus

 

[Chrispy_]

How much of the blame for the scheduler doing stupid things is on Microsoft, AMD, and application developers - proportionally?

Mind to explain why it improves security with smt off? maybe is related to this https://www.tomshardware.com/news/new-vulnerability-affects-all-amd-zen-cpus

Exploits exist where a process can read data in the SMT pipeline from the other process sharing the core, despite the two processes not supposed to have access to the data from the other.

SMT exploits where the security barrier fell down due to the shared nature of two instructions running through a core at the same time were the working principle behind Spectre and Meltdown.