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SubjectRe: [RFC 2/3] vmscan hook
On Tue, Jan 17, 2012 at 07:05:12PM +0900, KAMEZAWA Hiroyuki wrote:
> On Tue, 17 Jan 2012 18:13:56 +0900
> Minchan Kim <minchan@kernel.org> wrote:
>
> > On Tue, Jan 17, 2012 at 05:39:32PM +0900, KAMEZAWA Hiroyuki wrote:
> > > On Tue, 17 Jan 2012 17:13:57 +0900
> > > Minchan Kim <minchan@kernel.org> wrote:
> > >
> > >
> > > > + /*
> > > > + * We want to avoid dropping page cache excessively
> > > > + * in no swap system
> > > > + */
> > > > + if (nr_swap_pages <= 0) {
> > > > + free = zone_page_state(mz->zone, NR_FREE_PAGES);
> > > > + file = zone_page_state(mz->zone, NR_ACTIVE_FILE) +
> > > > + zone_page_state(mz->zone, NR_INACTIVE_FILE);
> > > > + /*
> > > > + * If we have very few page cache pages,
> > > > + * notify to user
> > > > + */
> > > > + if (file < free)
> > > > + low_mem = true;
> > > > + }
> > >
> > > I can't understand why you think you can check lowmem condition by "file < free".
> >
> > The reason I thought so is I want to maintain some page cache to some degree.
> > But I admit It's very naive heuristic and should be improved.
> >
> > > And I don't think using per-zone data is good.
> > > (I'm not sure how many zones embeded guys using..)
> >
> > Agree. In case of swapless system, we need another heuristic.
> >
> > >
> > > Another idea:
> > > 1. can't we use some technique like cleancache to detect the condition ?
> >
> > I totally forgot cleancache approach. Could you remind that?
> >
>
> Similar to 'victim cache'. Then, cache some clean pages somewhere when
> vmscan pageout it.
>
> page -> vmscan's pageout -> cleancache -> may be discarded.
>
> If a filesystem look up a page which is in a cleancache, cache-hit and
> bring it back to radix-tree. If not, read from disk again.
> And cleancache for swap(frontswap) was posted, too.

I am not sure this can prevent swapout.
I think it ends up evicting pages into swap devices.

>
>
> > > 2. can't we measure page-in/page-out distance by recording something ?
> >
> > I can't understand your point. What's relation does it with swapout prevent?
> >
>
> If distance between pageout -> pagein is short, it means thrashing.
> For example, recoding the timestamp when the page(mapping, index) was
> paged-out, and check it at page-in.

Our goal is prevent swapout. When we found thrashing, it's too late.

>
>
> > > 3. NR_ANON + NR_FILE_MAPPED can't mean the amount of core memory if we can
> > > ignore the data file cache ?
> >
> > It's good but how do we define some amount?
> > It's very vague but I guess we can get a good idea from that.
> > Perhaps, you already has it.
> >
>
> Hm, a rough idea is...
>
> - we now have rss counter per mm.
> - mapped anon
> - mapped file
> - swapents
>
> Ok, here, add one more counter.
>
> - paged-out file. (I think this can be recorded in pte.)
> +1 when try_to_unmap_file() unmaps it.
> -1 when a page is back or unmapped.
>
> Then, scanning all tasks. Then,
>
> mapped_anon + mapped_file
> active_map_ratio = ----------------------------------------------------- * 100
> mapped_anon + mapped_file + swapents + paged_out_file
>
> Ok, how to use this value...
>
> Like memcg's threshold notify interface, you can change the mem_notify interface
> to use eventfd() as
>
> <event_fd, fd of /dev/mem_notify, threshold of active_map_ratio>
>
> This will inform you an event when active_map_ratio crosses passed threshold.
>
> complicated ?

Yes. :)
I want to make simple if possible.

>
>
> > > 4. how about checking kswapd's busy status ?
> >
> > Could you elaborate on your idea?
> >
>
> I just thought kswapd may not stop when the situation is very bad.

As I said eariler, the goal is prevent swap.
When we found kswapd is busy, it might many pages are already swapped-out so it's too late.

>
> Thanks,
> -Kame
>
>
>
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