Re: [RFC] [PATCH] Scalable Statistics Counters

Rusty Russell (rusty@rustcorp.com.au)
Sat, 08 Dec 2001 18:38:09 +1100

In message <20011207182214.D15810@in.ibm.com> you write:
> On Thu, Dec 06, 2001 at 02:18:26PM +1100, Rusty Russell wrote:
> > On Wed, 05 Dec 2001 12:08:57 -0800
> > Andrew Morton <akpm@zip.com.au> wrote:
> >
> > > http://www.zipworld.com.au/~akpm/linux/2.4/2.4.7/
> >
> > Oops, guess I should have read this thread first (still catching up on mail
).
> >
> > Please see my per-cpu patch (just posted under [PATCH] 2.5.1-pre5: per-cpu
> > areas), and my previous /proc patch. Combining the two into convenient for
m
> > is left as an exercise for the reader...
>
> Hi Rusty,
>
> Your per-cpu area patch looks like a good solution with a very simple
> implementation. BTW, some OSes map the per-cpu data areas
> to the same virtual address for each CPU avoiding the per-cpu data
> array lookup. I am not sure if this really saves much, we are ourselves
> trying to understand the overhead of such array lookup with
> statctrs.

I'd be interested in the results: it'd certainly be neater. Another
option would be to use the per-cpu region pointer where architectures
currently hold smp_processor_id(), and derive the current CPU from the
per-CPU area instead of vice versa.

> IIUC, we can declare statically allocated per-cpu data using
> this allocator (kstat, apic_timer_irqs etc.). For things that
> are a part of dynamically allocated structure, we would still
> need to use a dynamic per-cpu allocator, right ?

Yep... Someone Else's Problem 8)

> Another interesting question is how we can load different
> per-cpu sections to different areas in memory. I would suspect
> that for NUMA, we would want to locate the per-cpu sections closest
> to the corresponding CPUs.

It could possibly be done with linker tricks in vmlinux.lds, and yes,
definitely worth doing.

> I couldn't find the /proc patch. Any pointers ?

Hmm... I'm working on a rewrite, but the interface should stay the
same:
http://lists.insecure.org/linux-kernel/2001/Nov/0087.html

Cheers!
Rusty.

--
  Anyone who quotes me is an idiot. -- Rusty Russell.
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