Re: [RFC 2/3] vmscan hook

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

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.


> > 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.


> > 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 ? 


> > 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.

Thanks,
-Kame



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