Number of joob on very modern CPU

[fbeth]

Hello everybody,

After 8 years with my old laptop I decided it was time for an upgrade, I received the new computer today that has an Intel Core Ultra 7 155H. I looked at the specs online and it says that this CPU has 2 different type of cores : 6 Performance-cores and 8 Efficient-cores, for a total of 22 threads. I'm wondering if it's better to compile on 12 threads to only use the performance-cores or 22 to use all of them. If someone has experience with that kind of CPU I would be delighted if you could share about how you handle compilation. Please note that the computer has 32G of RAM so it won't be a problem for most packages.

[Banana]

Not with the CPU, but keep in mind the RAM usage

[pietinger]

fbeth,

Your CPU has 6 P-cores (12 logical cores with Intel "Hyperthreading" == SMT), 8 E-cores and 2 Low Power E-cores:
https://www.intel.com/content/www/us/en/products/sku/236847/intel-core-ultra-7-processor-155h-24m-cache-up-to-4-80-ghz/specifications.html

Normally the Linux kernel decides on which core it starts a new thread. Since I also have an i9-13900k (with 8 P-cores and 16 E-cores), I am very interested in how the kernel handles this. From the looks of it, it SHOULD be better under 6.11:
https://www.phoronix.com/news/Intel-P-State-Asymmetric-Cap
_________________
https://wiki.gentoo.org/wiki/User:Pietinger

[fbeth]

I've settled with 20 threads, everything seems good. I'm using kernel 6.11 currently because it's the first version that supports my sound card. I might do a few benchmarks later, but no promise.

[spica]

fbeth,

If you run -march=native, try to re-emerge gcc. If it succeds you're good now, otherwise you'll see something like this one https://www.reddit.com/r/Gentoo/comments/1bqjtyo/gcc_inconsistent_paraml1cachesize_on_12th_gen/

[pietinger]

[fbeth]

So I did a small benchmark, nothing to fancy or complex. The CPU is an Intel Core Ultra 7 155H and I compiled my current kernel (6.11.0-rc6) two times, the first time with -j12 and the second time with -j20.
I was only running alacritty inside KDE Plasma 6 with the ppower cord plugged in. If you need more info or if you have any ideas in order to make better tests, feel free to share it with me.

-j12: 6min 34s
-j20: 5min 55s

That's almost a 10% time difference, it's not huge considering I used 8 more cores, but it shows that the E-cores are not that useful during heavy CPU tasks. pietinger mentioned that the scheduler should be better under kernel 6.11, it looks like the scheduler is prioritizing the P-cores.