Re: [PATCH v3] mm: introduce reference pages

[Apologies for the delay in getting back to you; other work ended up
taking priority and now I'm back to looking at this.]

On Tue, Aug 18, 2020 at 11:25 AM John Hubbard [off-list ref] wrote:

On 8/17/20 8:00 PM, Matthew Wilcox wrote:

quoted

On Mon, Aug 17, 2020 at 07:31:39PM -0700, John Hubbard wrote:

quoted

quoted

            Real time (s)    Max RSS (KiB)
anon        2.237081         107088
memset      2.252241         112180
refpage     2.243786         107128

We can see that RSS for refpage is almost the same as anon, and real
time overhead is 44% that of memset.

Are some of the numbers stale, maybe? Try as I might, I cannot combine
anything above to come up with 44%. :)

You're not trying hard enough ;-)

(2.252241 - 2.237081) / 2.237081 = .00677668801442594166
(2.243786 - 2.237081) / 2.237081 = .00299720930981041812
.00299720930981041812 / .00677668801442594166 = .44228232189973614648

tadaa!

haha, OK then! :) Next time I may try harder, but on the other hand my
interpretation of the results is still "this is a small effect", even
if there is a way to make it sound large by comparing the 3rd significant
digits of the results...

quoted

As I said last time this was posted, I'm just not excited by this.  We go
from having a 0.68% time overhead down to an 0.30% overhead, which just
doesn't move the needle for me.  Maybe there's a better benchmark than
this to show benefits from this patchset.

Remember that this is a "realistic" benchmark, so it's doing plenty of
other work besides faulting pages. So I don't think we should expect
to see a massive improvement here.

I ran the pdfium benchmark again but I couldn't see the same
improvements that I got last time. This seems to be because pdfium has
since switched to its own allocator, bypassing the system allocator. I
think the gains should be larger with the memset optimization that
I've implemented, but I'm still in the process of finding a suitable
realistic benchmark that uses the system allocator.

But I would find a 0.4% perf improvement convincing enough,
personally, given that the workload is realistic. Consider a certain
large company which spends $billions annually on data centers. In that
environment a 0.4% performance improvement on realistic workloads can
translate to $millions of savings. And that's not taking into account
the memory savings which are important both in mobile environments and
in data centers.

Yes, I wonder if there is an artificial workload that just uses refpages
really extensively, maybe we can get some good solid improvements shown
with that? Otherwise, it seems like we've just learned that memset is
actually pretty good in this case. :)

Yes, it's possible to see the performance improvement here more
clearly with a microbenchmark. I've updated the commit message in v4
to include a microbenchmark program and some performance numbers from
it.

Peter

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