On Thu, 2008-11-27 at 10:33 +0100, Peter Zijlstra wrote:
> On Thu, 2008-11-27 at 00:32 +0100, Eric Dumazet wrote:
> > Avoids cache line ping pongs between cpus and prepare next patch,
> > because updates of nr_inodes metric dont need inode_lock anymore.
> >
> > (socket8 bench result : 25s to 20.5s)
> >
> > Signed-off-by: Eric Dumazet <dada1@xxxxxxxxxxxxx>
> > ---
>
> > @@ -96,9 +96,40 @@ static DEFINE_MUTEX(iprune_mutex);
> > * Statistics gathering..
> > */
> > struct inodes_stat_t inodes_stat;
> > +static DEFINE_PER_CPU(int, nr_inodes);
> >
> > static struct kmem_cache * inode_cachep __read_mostly;
> >
> > +int get_nr_inodes(void)
> > +{
> > + int cpu;
> > + int counter = 0;
> > +
> > + for_each_possible_cpu(cpu)
> > + counter += per_cpu(nr_inodes, cpu);
> > + if (counter < 0)
> > + counter = 0;
> > + return counter;
> > +}
>
> It would be good to get a cpu hotplug handler here and move to
> for_each_online_cpu(). People are wanting distro's to be build with
> NR_CPUS=4096.
Also, this trade-off between global vs per_cpu only works if
get_nr_inodes() is called significantly less than nr_inodes is changed.
With it being called from writeback that might not be true for all
workloads. One thing you can do about it is use the regular per-cpu
counter stuff, which allows you to do an approximation of the global
number (it also does all the hotplug stuff for you already).
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