Re: page fault scalability patch V11 [0/7]: overview

[William Lee Irwin III]

William Lee Irwin III wrote:
>> Split counters easily resolve the issues with both these approaches
>> (and apparently your co-workers are suggesting it too, and have
>> performance results backing it).

On Sat, Nov 20, 2004 at 01:18:22PM +1100, Nick Piggin wrote:
> Split counters still require atomic operations though. This is what
> Christoph's latest effort is directed at removing. And they'll still
> bounce cachelines around. (I assume we've reached the conclusion
> that per-cpu split counters per-mm won't fly?).

Split != per-cpu, though it may be. Counterexamples are
as simple as atomic_inc(&mm->rss[smp_processor_id()>>RSS_IDX_SHIFT]);
Furthermore, see Robin Holt's results regarding the performance of the
atomic operations and their relation to cacheline sharing.

And frankly, the argument that the space overhead of per-cpu counters
is problematic is not compelling. Even at 1024 cpus it's smaller than
an ia64 pagetable page, of which there are numerous instances attached
to each mm.


-- wli
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