linux.conf.au
Optimization

From: Andrew Morton <akpm@zip.com.au>
To: Rusty Russell <rusty@rustcorp.com.au>
Cc: torvalds@transmeta.com, linux-kernel@vger.kernel.org, k-suganuma@mvj.biglobe.ne.jp
Subject: Re: [PATCH] 2.5.21 Nonlinear CPU support

Rusty Russell wrote:
> Linus, please apply.  Tested on my dual x86 box.
> 
> This patch removes smp_num_cpus, cpu_number_map and cpu_logical_map
> from generic code, and uses cpu_online(cpu) instead, in preparation
> for hotplug CPUS.

umm.  This patch does introduce a non-zero amount of bloat:
 
> ...
> -       ntfs_compression_buffers =  (u8**)kmalloc(smp_num_cpus * sizeof(u8*),
> +       ntfs_compression_buffers =  (u8**)kmalloc(NR_CPUS * sizeof(u8*),

and slowdown:

> ...
>         busiest = NULL;
>         max_load = 1;
> -       for (i = 0; i < smp_num_cpus; i++) {
> -               int logical = cpu_logical_map(i);
> +       for (i = 0; i < NR_CPUS; i++) {
> +               if (!cpu_online(i))
> +                       continue;

and for the majority of SMP machines it gives nothing back, yes?

Is there some way of optimising all that?