Re: [PATCH] x86: bump default NR_CPUS for 64-bit configuration
From: Josh Boyer
Date: Tue May 12 2015 - 13:57:20 EST
On Fri, May 8, 2015 at 6:25 AM, Kirill A. Shutemov
> Default NR_CPUS==8 is not enough to cover high-end desktop
> configuration: Haswell-E has upto 16 threads.
> Let's increase default NR_CPUS to 64 on 64-bit configuration. With this
> value CPU bitmask will still fit into one unsinged long.
> Default for 32-bit configuration is still 8: it's unlikely anybody will
> run 32-bit kernel on modern hardware.
> As alternative we could bump NR_CPUS to 128 to cover all dual-processor
> servers with some margin.
> For reference: Debian and Suse build their kernels with NR_CPUS==512,
> Fedora -- 1024.
FWIW, we're carrying a patch that drops the "if DEBUG_PER_CPU_MAPS"
dependency for CPUMASK_OFFSTACK to allow us to set that to 1024
without bringing in debug junk. I sent this patch a long time ago and
Ingo and Linus said it was stupid and it should be auto-selected.
Ingo was going to poke at it IIRC how the thread ended, because I
couldn't see a clean way to make Kconfig do what we wanted.
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> arch/x86/Kconfig | 3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index 6aaf38ac417d..c3333e5be5d7 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -851,7 +851,8 @@ config NR_CPUS
> default "1" if !SMP
> default "8192" if MAXSMP
> default "32" if SMP && X86_BIGSMP
> - default "8" if SMP
> + default "8" if SMP && X86_32
> + default "64" if SMP
> This allows you to specify the maximum number of CPUs which this
> kernel will support. If CPUMASK_OFFSTACK is enabled, the maximum
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