difference (from Kernel view) between Pentium M and P4

[OPelerin]

I've recently installed Gentoo on my new laptop IBM T42.

According to Dmesg, it gets a Pentium M

Intel machine check architecture supported.
Intel machine check reporting enabled on CPU#0.
CPU: Intel(R) Pentium(R) M processor 1.70GHz stepping 06
Enabling fast FPU save and restore... done.
Enabling unmasked SIMD FPU exception support... done.

My kernel is setup as:


( ) Pentium M
(X) Pentium-4/Celeron(P4-based)/Pentium-4 M/Xeon

I did'nt choose Pentium M ( I imported my .config from my home PC) and the PC works without issues.


So my question is: What is the advantage of choosing Pentium-M versus P4 in my case?

Kind regards,
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Olivier PELERIN

[ben-xo]

As far as i'm aware, the Pentium M in the kernel config is a Pentium-III with SpeedStep. If your laptop is relatively new, then it will be a "Pentium 4-M", which is a Pentium-IV with speedstep. It's likely that you chose the right option if it works fine. (If you really had a Pentium-III M and had chosen options for a Pentium-IV M, then your kernel would probably crash).
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Ben XO

[mark_alec]

[OPelerin]

yeah. This laptop is new ( my new company just bought it early july).

Any clue why dmesg do not show it as PIV M?
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Olivier PELERIN

[colo]

Becaue it is none. Understood?

Pentium III Mobile, Pentium 4 Mobile and Pentium M are THREE different types of architectures, sharing just certain parts of their design.
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Free Software. Free Sociecty. Better Lives.

[OPelerin]

Mark,

Then I'm back to my original question.

What advantages of compiling the kernel in Pentium M? What is enabled, what is the performance impact? Currently everything runs well in PIV compiled kernel....
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Olivier PELERIN

[whitesouls]

greeting...

[colo]

SSE3 is supported on Pentium 4 Prescott and Athlon 64 Venice/Manchester/Toledo Cores only, as well as the Turion series of AMD mobile chips. Pentium 4 Mobile and previous series of processors from Intel lack SSE3 support; though the offer MMX, MMXEXT, SSE and SSE2 (just as the Pentium M, up to the Dothan core, does right now).

Furthermore, for f*ck's sake, it's already been clearly stated that he does not use NEITHER a "P4-M", NOR a "P3-M", BUT a "Pentium M". I hope you succeed in understanding that correctly this time.

BTT, though: I don't know for sure, but I suspect the architecture optimization directly impacts the CFLAGS being passed to gcc when running make - so if your Pentium 4 choice does not break the kernel for you, it may be possible your kernel is not optimized to the maximum possible extent.
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Free Software. Free Sociecty. Better Lives.

[whitesouls]

[OPelerin]

Dear colo

"Furthermore, for f*ck's sake, it's already been clearly stated that he does not use NEITHER a "P4-M", NOR a "P3-M", BUT a "Pentium M". I hope you succeed in understanding that correctly this time. "

Yes that's correct the help file inside the kernel says 'dont use it'. But there is no technical explanation that explain why.

So back to my initial question. What is the added value?
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Olivier PELERIN

[bollucks]

The advantage is code micro-optimisation for the architecture.

[nxsty]

When you select Pentium M from menuconfig the kernel will be compiled with -march=i686 -mtune=pentium3 and when you select Pentium4 the kernel will be compiled with -march=i686 -mtune=pentium4. So it's only a matter of compiler optimizations actually.

[Jerem]

If you select Pentium 4 in the kernel configuration, it just tells what compilation options it should use, not what processor it should detect.

I greatly advise you to choose pentium-M, so the kernel can use pentium-M specific behaviour to save battery life.

[OPelerin]

I've compiled my kernel as Pentium M. From perf standpoint, there is not much differences...

In both cases, if my laptop boot on battery

/proc/cpuinfo still reports 600 MHZ instead of 1700 MHZ.


Either 2 things

1) /proc/cpuinfo is static and is not updated in real time

2) CPU frequency switchover do not work when the laptop is back from battery to power and vice versa.. It's stay at the frequency the CPU got when it was booting.


Anybody experienced this?
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Olivier PELERIN

[Jerem]

The /proc pseudo-filesystem is not the correct place to look for dynamically changing information. Have a look at /sys. It's updated instantly.

[Tsuna]

let's make it clear once and for all, as said above, you have a Pentium M, which is NOT a pentium3-M, nor a pentium4-M

The Pentium-M is a very optimized plateform based on Pentium3-Pro, whose cores are codenamed Banias, Dothan or Sonoma. The Pentium-M is way more efficiant and powerful than the Pentium4-M and uses much less power. If you have a Pentium-M, then choose Pentium-M in the kernel, that's all. It doesn't matter if it's also working if you choose Pentium4 or i386, don't bother and pick up the best choice for your arch.

More infos @ http://en.wikipedia.org/wiki/Pentium_M
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[bollucks]