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    SubjectRe: [RFC 2/3] vmscan hook
    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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