Re: [PACTH v2 0/3] Implement /proc/<pid>/totmaps

[Robert Foss <robert.foss@collabora.com>]

On 2016-08-16 03:12 AM, Michal Hocko wrote:
> On Mon 15-08-16 12:25:10, Robert Foss wrote:
>>
>>
>> On 2016-08-15 09:42 AM, Michal Hocko wrote:
> [...]
>>> The use case is to speed up monitoring of
>>> memory consumption in environments where RSS isn't precise.
>>>
>>> For example Chrome tends to many processes which have hundreds of VMAs
>>> with a substantial amount of shared memory, and the error of using
>>> RSS rather than PSS tends to be very large when looking at overall
>>> memory consumption.  PSS isn't kept as a single number that's exported
>>> like RSS, so to calculate PSS means having to parse a very large smaps
>>> file.
>>>
>>> This process is slow and has to be repeated for many processes, and we
>>> found that the just act of doing the parsing was taking up a
>>> significant amount of CPU time, so this patch is an attempt to make
>>> that process cheaper.
>
> Well, this is slow because it requires the pte walk otherwise you cannot
> know how many ptes map the particular shared page. Your patch
> (totmaps_proc_show) does the very same page table walk because in fact
> it is unavoidable. So what exactly is the difference except for the
> userspace parsing which is quite trivial e.g. my currently running Firefox
> has
> $ awk '/^[0-9a-f]/{print}' /proc/4950/smaps | wc -l
> 984
>
> quite some VMAs, yet parsing it spends basically all the time in the kernel...
>
> $ /usr/bin/time -v awk '/^Rss/{rss+=$2} /^Pss/{pss+=$2} END {printf "rss:%d pss:%d\n", rss, pss}' /proc/4950/smaps
> rss:1112288 pss:1096435
>         Command being timed: "awk /^Rss/{rss+=$2} /^Pss/{pss+=$2} END {printf "rss:%d pss:%d\n", rss, pss} /proc/4950/smaps"
>         User time (seconds): 0.00
>         System time (seconds): 0.02
>         Percent of CPU this job got: 91%
>         Elapsed (wall clock) time (h:mm:ss or m:ss): 0:00.02
>
> So I am not really sure I see the performance benefit.
>

I did some performance measurements of my own, and it would seem like 
there is about a 2x performance gain to be had. To me that is 
substantial, and a larger gain than commonly seen.

There naturally also the benefit that this is a lot easier to interact 
with programmatically.

$ ps aux | grep firefox
robertfoss   5025 24.3 13.7 3562820 2219616 ?     Rl   Aug15 277:44 
/usr/lib/firefox/firefox https://allg.one/xpb
$ awk '/^[0-9a-f]/{print}' /proc/5025/smaps | wc -l
1503


$ /usr/bin/time -v -p zsh -c "(repeat 25 {cat /proc/5025/totmaps})"
[...]
	Command being timed: "zsh -c (repeat 25 {cat /proc/5025/totmaps})"
	User time (seconds): 0.00
	System time (seconds): 0.40
	Percent of CPU this job got: 90%
	Elapsed (wall clock) time (h:mm:ss or m:ss): 0:00.45


$ /usr/bin/time -v -p zsh -c "repeat 25 { awk '/^Rss/{rss+=\$2} 
/^Pss/{pss+=\$2} END {printf \"rss:%d pss:%d\n\", rss, pss}\' 
/proc/5025/smaps }"
[...]
	Command being timed: "zsh -c repeat 25 { awk '/^Rss/{rss+=$2} 
/^Pss/{pss+=$2} END {printf "rss:%d pss:%d\n", rss, pss}\' 
/proc/5025/smaps }"
	User time (seconds): 0.37
	System time (seconds): 0.45
	Percent of CPU this job got: 92%
	Elapsed (wall clock) time (h:mm:ss or m:ss): 0:00.89