Re: [PATCH] percpu data: only iterate over possible CPUs

[Eric Dumazet]

Andrew Morton a écrit :
> Andi Kleen <ak@xxxxxx> wrote:
> > On Thursday 09 February 2006 19:04, Andrew Morton wrote:
> > > Ashok Raj <ashok.raj@xxxxxxxxx> wrote:
> > > > The problem was with ACPI just simply looking at the namespace doesnt
> > > >  exactly give us an idea of how many processors are possible in this platform.
> > > We need to fix this asap - the performance penalty for HOTPLUG_CPU=y,
> > > NR_CPUS=lots will be appreciable.
> > What is this performance penalty exactly? 
>
> All those for_each_cpu() loops will hit NR_CPUS cachelines instead of
> hweight(cpu_possible_map) cachelines.

You mean NR_CPUS bits, mostly all included in a single cacheline, and even in 
a single long word :) for most cases (NR_CPUS <= 32 or 64)



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