Re: page table lock patch V15 [0/7]: overview

[Nick Piggin]

Andrew Morton wrote:
> Nick Piggin <nickpiggin@xxxxxxxxxxxx> wrote:
>
> > So my patches cost about 7% in lmbench fork benchmark.
>
>
> OK, well that's the sort of thing we need to understand fully.  What sort
> of CPU was that on?

That was on a P4, although I've seen pretty similar results on ia64 and
other x86 CPUs.

Note that this was with my ptl removal patches. I can't see why Christoph's
would have _any_ extra overhead as they are, but it looks to me like they're
lacking in atomic ops. So I'd expect something similar for Christoph's when
they're properly atomic.

> Look, -7% on a 2-way versus +700% on a many-way might well be a tradeoff we
> agree to take.  But we need to fully understand all the costs and benefits.

I think copy_page_range is the one to keep an eye on.

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