Re: [PATCH v5 0/7] Per process reclaim

Andrew Morton
2013-05-21T23:16:59Z

On Thu, 9 May 2013 16:21:22 +0900 Minchan Kim <minchan@kernel.org> wrote:

> These day, there are many platforms avaiable in the embedded market
> and they are smarter than kernel which has very limited information
> about working set so they want to involve memory management more heavily
> like android's lowmemory killer and ashmem or recent many lowmemory
> notifier(there was several trial for various company NOKIA, SAMSUNG,
> Linaro, Google ChromeOS, Redhat).
>
> One of the simple imagine scenario about userspace's intelligence is that
> platform can manage tasks as forground and backgroud so it would be
> better to reclaim background's task pages for end-user's *responsibility*
> although it has frequent referenced pages.
>
> The patch[1] prepares that force_reclaim in shrink_page_list can
> handle anonymous pages as well as file-backed pages.
>
> The patch[2] adds new knob "reclaim under proc/<pid>/" so task manager
> can reclaim any target process anytime, anywhere. It could give another
> method to platform for using memory efficiently.
>
> It can avoid process killing for getting free memory, which was really
> terrible experience because I lost my best score of game I had ever
> after I switch the phone call while I enjoyed the game.
>
> Reclaim file-backed pages only.
> echo file > /proc/PID/reclaim
> Reclaim anonymous pages only.
> echo anon > /proc/PID/reclaim
> Reclaim all pages
> echo all > /proc/PID/reclaim

Oh boy. I think I do agree with the overall intent, but there are so
many ways of doing this.

- Do we reclaim the pages altogether, or should we just give them one
round of aging? If the latter then you'd need to run "echo anon >
/proc/PID/reclaim" four times to firmly whack the pages, but that's
more flexible.

- Why do it via the pid at all? Would it be better to instead do
this to a memcg and require that the admin put these processes into
memcgs? In fact existing memcg controls could get us at least
partway to this feature.

- I don't understand the need for "Enhance per process reclaim to
consider shared pages". If "echo file > /proc/PID/reclaim" causes
PID's mm's file-backed pte's to be unmapped (which seems to be the
correct effect) then we get this automatically: unshared file pages
will be freed and shared file pages will remain in core until the
other sharing process's also unmap them.

Overall, I'm unsure whether/how to proceed with this. I'd like to hear
from a lot of the potential users, and hear them say "yes, we can use
this".