Re: [PATCH 0/3] Virtual huge zero page

Andrea Arcangeli
2012-09-29T14:37:52Z

On Sat, Sep 29, 2012 at 07:30:06AM -0700, Andi Kleen wrote:
> On Sat, Sep 29, 2012 at 03:48:11PM +0200, Andrea Arcangeli wrote:
> > On Sat, Sep 29, 2012 at 02:37:18AM +0300, Kirill A. Shutemov wrote:
> > > Cons:
> > > - increases TLB pressure;
> >
> > I generally don't like using 4k tlb entries ever. This only has the
>
> From theory I would also prefer the 2MB huge page.
>
> But some numbers comparing between the two alternatives are definitely
> interesting. Numbers are often better than theory.

Sure good idea, just all standard benchmarks likely aren't using zero
pages so I suggest a basic micro benchmark:

some loop of() {
memcmp(uninitalized_pointer, (char *)uninitialized_pointer+4G, 4G)
barrier();
}

>
> > There would be a small cache benefit here... but even then some first
> > level caches are virtually indexed IIRC (always physically tagged to
>
> Modern x86 doesn't have virtually indexed caches.

With the above memcmp, I'm quite sure the previous patch will beat the
new one by a wide margin, especially on modern x86 with more 2M TLB
entries and >= 8MB L2 caches.

But I agree we need to verify it before taking a decision, and that
the numbers are better than theory, or to rephrase it "let's check the
theory is right" :)