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

On Mon, Oct 01, 2012 at 10:03:53AM -0700, H. Peter Anvin wrote:

Something isn't quite right about that.  If you look at your numbers:

1,049,134,961 LLC-loads
        6,222 LLC-load-misses

This is another way of saying in your benchmark the huge zero page is
parked in your LLC - using up 2 MB of your LLC, typically a significant
portion of said cache.  In a real-life application that will squeeze out
real data, but in your benchmark the system is artificially quiescent.

Agreed. And that argument applies to the cache benefits of the virtual
zero page too: squeeze the cache just more aggressively so those 4k
got out of the cache too, and that 6% improvement will disappear
(while the TLB benefit of the physical zero page is guaranteed and is
always present no matter the workload, even if the TLB miss at the
same frequency, it'll get filled with one less cacheline access every
time).

It is well known that microbenchmarks can be horribly misleading.  What
led to Kirill investigating huge zero page in the first place was the
fact that some applications/macrobenchmarks benefit, and I think those
are the right thing to look at.

The whole point of the two microbenchmarks was to measure the worst
cases for both scenarios and I think that was useful. Real life using
zero pages are going to be somewhere in that range.

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