Kernel config for multicore CPUs

Mr. Alex · 2010-09-05 11:01:25

Do I understand it correct - although Arch's Linux kernel supports multicore CPUs, it compiles the kernel while you install Arch and the final kernel config will support as many CPU cores as the CPU had while installation?
So if I installed Arch on 1-core CPU and then change CPU to 4-core, Arch and all its programs will be able to use only 1 core of this CPU and 3 other won't be equipped? If I want them to be equipped, I should either recompile the kernel or reinstall Arch?

flamelab · 2010-09-05 11:06:06

All of the cores will be available to the system, not just one or two.

Mr. Alex · 2010-09-05 12:23:57

But when I install Arch, does it compile the kernel depending on my hardware? Or I get the same kernel configuration on every machine?

Awebb · 2010-09-05 12:29:50

It's the first time I hear that the arch installer compiles the kernel. Arch takes about 20mins to install on my box, there is no way this might be including kernel compile time...

Jookia · 2010-09-05 12:33:23

If Arch compiled the kernel, you'd know about it.

graysky · 2010-09-05 12:56:40

@OP - With Arch, you are not compiling the kernel; you are downloading a pre-compiled binary. as you do with all of the official Arch packages. The default config allows for up to 16 CPUs/Cores to be function (with hyperthreading). Perhaps you're confusing making the bootable kernel image from the kernel. For example, mkinitcpio does run when the kernel is updated/installed. You very well may need to re-run it if you drop in a new MB. I haven't tried this before.

Stebalien · 2010-09-05 13:32:25

If you want to double check, this should spit out the maximum number of CPUs your computer can have with the current kernel configuration.

zcat /proc/config.gz|sed -ne 's/^CONFIG_NR_CPUS=//p'

Skripka · 2010-09-05 14:12:20

graysky wrote:

@OP - With Arch, you are not compiling the kernel; you are downloading a pre-compiled binary. as you do with all of the official Arch packages. The default config allows for up to 16 CPUs/Cores to be function (with hyperthreading). Perhaps you're confusing making the bootable kernel image from the kernel. For example, mkinitcpio does run when the kernel is updated/installed. You very well may need to re-run it if you drop in a new MB. I haven't tried this before.

IIRC, the last time I swapped mainboards/CPUs-Arch was smart enough to realize it was plugged into new gear-and booted fine. Only thing that really needed reconfiguring/reinstalling was ALSA.

trn · 2010-09-06 13:05:39

Stebalien wrote:

If you want to double check, this should spit out the maximum number of CPUs your computer can have with the current kernel configuration.
zcat /proc/config.gz|sed -ne 's/^CONFIG_NR_CPUS=//p'

I'm also having this problem, the kernel config only supports 8 cpu's. I also tried the kernel26-lts and same results; 8 cpu's only.

Dogmeat · 2010-09-06 13:47:15

i686 kernel has set CONFIG_NR_CPUS=8, while x86_64 kernel CONFIG_NR_CPUS=16.

R00KIE · 2010-09-06 14:05:42

@graysky
I've been using a usb flash drive with arch with several different computers and it has always booted fine with the "normal" boot image. If that doesn't work the fallback image will most probably work I believe.

The 8 CPU limit is for the 32bit flavor of Arch. If you have a half decent recent machine and more than 8 logical/physical cpu cores probably you should look into using 64bit arch (unless you have been really cheap and bought less that 4GB of ram, which doesn't make much sense if you have so much cpu horsepower on tap). Also when booting linux will use all available cpus up to the CONFIG_NR_CPUS limit, there is no need to recompile anything (assuming the programs support and have been compiled with MP support in the first place).

trn · 2010-09-06 15:31:43

Aww SOB! Just another total N00b error on my part miss-understanding the naming scheme. I thought i686 was AMD64 with no x86 backward compatibility only 64bit. Sounds like x86_64 is the 64bit version that I should be using; i'm installing on 980x's 970's and Westmere's... so yeah good systems. Thanks for the help!

Arch Linux

#1 2010-09-05 11:01:25

Kernel config for multicore CPUs

#2 2010-09-05 11:06:06

Re: Kernel config for multicore CPUs

#3 2010-09-05 12:23:57

Re: Kernel config for multicore CPUs

#4 2010-09-05 12:29:50

Re: Kernel config for multicore CPUs

#5 2010-09-05 12:33:23

Re: Kernel config for multicore CPUs

#6 2010-09-05 12:56:40

Re: Kernel config for multicore CPUs

#7 2010-09-05 13:32:25

Re: Kernel config for multicore CPUs

#8 2010-09-05 14:12:20

Re: Kernel config for multicore CPUs

#9 2010-09-06 13:05:39

Re: Kernel config for multicore CPUs

#10 2010-09-06 13:47:15

Re: Kernel config for multicore CPUs

#11 2010-09-06 14:05:42

Re: Kernel config for multicore CPUs

#12 2010-09-06 15:31:43

Re: Kernel config for multicore CPUs

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