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P cores and E cores

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  P cores and E cores Andy Burns <usenet@andyburns.uk> - 2025-11-24 17:50 +0000
    Re: P cores and E cores Paul <nospam@needed.invalid> - 2025-11-25 02:32 -0500
      Re: P cores and E cores Andy Burns <usenet@andyburns.uk> - 2025-11-25 09:49 +0000
    Re: P cores and E cores Brian Gregory <void-invalid-dead-dontuse@email.invalid> - 2026-01-02 22:02 +0000
      Re: P cores and E cores Andy Burns <usenet@andyburns.uk> - 2026-01-03 00:33 +0000

#27518 — P cores and E cores

On a Win11 machine with an i5-1335U (2x P cores with hyperthreading, and 
8x E cores without HT)

For manually setting CPU affinity, is it guaranteed that CPU0 to CPU3 
are the P cores and CPU4 to CPU11 are E cores?

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#27526

On Mon, 11/24/2025 12:50 PM, Andy Burns wrote:
> On a Win11 machine with an i5-1335U (2x P cores with hyperthreading, and 8x E cores without HT)
> 
> For manually setting CPU affinity, is it guaranteed that CPU0 to CPU3 are the P cores and CPU4 to CPU11 are E cores?

We can look at two different models for inspiration.

https://www.techpowerup.com/cpu-specs/core-i5-1235u.c2591

https://www.techpowerup.com/cpu-specs/core-i5-1335u.c3065

Now, one of the pretty basic questions for modern CPUs,
is do they use mesh bus or ring bus. You would be surprised
to find they're still using ring bus, although "it is a
bit of a loser" on performance. On my 4930K, it turns
a 6 core CPU, into 5 cores of performance (one cores worth
lost to the ring bus).

This article claims you can look at /proc/cpuinfo
for some sort of information like you are seeking.
Yet, Linux simply does not do Task Manager displays for
its CPU activity, so there is hardly a chance of
transferring the info visually from one OS to another.

   https://sites.utexas.edu/jdm4372/2021/05/27/locations-of-cores-and-l3-slices-on-xeon-scalable-processors/

You could try running SuperPI, change the affinity setting
and see where the "lump" goes. SuperPI runs on a single
thread, and using affinity would force it onto a single
"core" if you want.

If you did whole SuperPI runs by setting Affinity before
the run starts, you could do a "bench per each core".
You should get four results which are better
than the other eight results. In the Affinity mask, you'd
only enable one CPU core in each case, for the run. Then
you could plot the SuperPI times on a representative diagram
of Task Manager that you were watching during each Affinity
run. That would not "number the diagram" necessarily,
but it should help identify which are a (hyperthreaded) P.

   Paul

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#27529

Paul wrote:

> This article claims you can look at /proc/cpuinfo
> for some sort of information like you are seeking.
> Yet, Linux simply does not do Task Manager displays for
> its CPU activity, so there is hardly a chance of
> transferring the info visually from one OS to another.

Yeah, not sure Id trust what I could learn from Linux to Windows, nt 
even from Win10 to Win11.

I managed to remote onto one of the devices, the complaint is about the 
fan noise it produces when someone sends it "some work" to do, so by 
changing affinity I can certainly get the core temperature down from 
95°C to 70°C and fan speed down from 6500 to 3700 rpm.

The supplier claims they can't reproduce it, but I've learned they're 
testing with an unknown i7 instead of an i5, and 32GB instead of 16GB; 
no doubt the real cause is poor software but I doubt I'll impress that 
on them, so have to treat the symptoms rather than the cause ...

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#28946

On 24/11/2025 17:50, Andy Burns wrote:
> On a Win11 machine with an i5-1335U (2x P cores with hyperthreading, and 
> 8x E cores without HT)
> 
> For manually setting CPU affinity, is it guaranteed that CPU0 to CPU3 
> are the P cores and CPU4 to CPU11 are E cores?

I don't know but on my Win 11 with 12th gen Intel PC it's the same, P 
cores starting from 0 then E cores.

-- 
Brian Gregory (in England).

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#28947

Brian Gregory wrote:

> Andy Burns wrote:
> >> On a Win11 machine with an i5-1335U (2x P cores with hyperthreading,
>> and 8x E cores without HT)
>>
>> For manually setting CPU affinity, is it guaranteed that CPU0 to CPU3 
>> are the P cores and CPU4 to CPU11 are E cores?
> 
> I don't know but on my Win 11 with 12th gen Intel PC it's the same, P 
> cores starting from 0 then E cores.
Thanks, spent a couple of days messing with CPU affinity and using Intel 
extreme tuning utility (just for monitoring) and although the software 
seemed to be dividing it's load over multiple cores wuite well, it was 
hitting thermal throttling and quite soon afterwards power throttling, 
making it run choppily.

Hoping to re-test on Core Ultra 5 instead of Core i5 soon ... more P 
cores and no hyperthreading at all ...

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