Re: [PATCH v2] mm: introduce reference pages

[Peter Collingbourne <pcc@google.com>]

On Thu, Aug 13, 2020 at 3:31 PM Matthew Wilcox <willy@infradead.org> wrote:
>
> On Thu, Aug 13, 2020 at 03:09:37PM -0700, Peter Collingbourne wrote:
> > In order to measure the performance and RSS impact of reference pages,
> > a version of this patch backported to kernel version 4.14 was tested on
> > a Pixel 4 together with a modified [2] version of the Scudo allocator
> > that uses reference pages to implement pattern initialization. A
> > PDFium test program was used to collect the measurements like so:
> >
> > $ wget https://static.docs.arm.com/ddi0487/fb/DDI0487F_b_armv8_arm.pdf
> > $ /system/bin/time -v ./pdfium_test --pages=1-100 DDI0487F_b_armv8_arm.pdf
> >
> > and the median of 100 runs measurement was taken with three variants
> > of the allocator:
> >
> > - "anon" is the baseline (no pattern init)
> > - "memset" is with pattern init of allocator pages implemented by
> >   initializing anonymous pages with memset
> > - "refpage" is with pattern init of allocator pages implemented
> >   by creating reference pages
> >
> > All three variants are measured using the patch that I linked. "anon"
> > is without the patch, "refpage" is with the patch and "memset" is
> > with a previous version of the patch [3] with "#if 0" in place of
> > "#if 1" in linux.cpp. The measurements are as follows:
> >
> >           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.
>
> Umm.  These numbers aren't all /that/ compelling.  memset takes 0.7%
> longer than the baseline and consumes 4.8% more memory.  I was expecting
> better, to be honest ;-(

It really depends on your environment and the program that you're
running. 5% can go a long way on memory constrained hardware like
low-end phones. And the number will go higher if the program makes
sparse use of its allocations.

Peter