[PATCH v2] mm: introduce reference pages

[Matthew Wilcox]

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 ;-(