Messages in this thread | | | Date | Tue, 17 Jan 2012 19:05:12 +0900 | From | KAMEZAWA Hiroyuki <> | Subject | Re: [RFC 2/3] vmscan hook |
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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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