Re: [PATCH] mm: compaction: Abort compaction if too many pages areisolated and caller is asynchronous

[Mel Gorman]

On Mon, Jun 06, 2011 at 11:15:57AM +0100, Mel Gorman wrote:
> On Fri, Jun 03, 2011 at 08:01:44AM +0900, Minchan Kim wrote:
> > On Fri, Jun 3, 2011 at 7:32 AM, Andrea Arcangeli <aarcange@xxxxxxxxxx> wrote:
> > > On Fri, Jun 03, 2011 at 07:23:48AM +0900, Minchan Kim wrote:
> > >> I mean we have more tail pages than head pages. So I think we are likely to
> > >> meet tail pages. Of course, compared to all pages(page cache, anon and
> > >> so on), compound pages would be very small percentage.
> > >
> > > Yes that's my point, that being a small percentage it's no big deal to
> > > break the loop early.
> > 
> > Indeed.
> > 
> > >
> > >> > isolated the head and it's useless to insist on more tail pages (at
> > >> > least for large page size like on x86). Plus we've compaction so
> > >>
> > >> I can't understand your point. Could you elaborate it?
> > >
> > > What I meant is that if we already isolated the head page of the THP,
> > > we don't need to try to free the tail pages and breaking the loop
> > > early, will still give us a chance to free a whole 2m because we
> > > isolated the head page (it'll involve some work and swapping but if it
> > > was a compoundtranspage we're ok to break the loop and we're not
> > > making the logic any worse). Provided the PMD_SIZE is quite large like
> > > 2/4m...
> > 
> > Do you want this? (it's almost pseudo-code)
> > 
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index 7a4469b..9d7609f 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -1017,7 +1017,7 @@ static unsigned long isolate_lru_pages(unsigned
> > long nr_to_scan,
> >         for (scan = 0; scan < nr_to_scan && !list_empty(src); scan++) {
> >                 struct page *page;
> >                 unsigned long pfn;
> > -               unsigned long end_pfn;
> > +               unsigned long start_pfn, end_pfn;
> >                 unsigned long page_pfn;
> >                 int zone_id;
> > 
> > @@ -1057,9 +1057,9 @@ static unsigned long isolate_lru_pages(unsigned
> > long nr_to_scan,
> >                  */
> >                 zone_id = page_zone_id(page);
> >                 page_pfn = page_to_pfn(page);
> > -               pfn = page_pfn & ~((1 << order) - 1);
> > +               start_pfn = pfn = page_pfn & ~((1 << order) - 1);
> >                 end_pfn = pfn + (1 << order);
> > -               for (; pfn < end_pfn; pfn++) {
> > +               while (pfn < end_pfn) {
> >                         struct page *cursor_page;
> > 
> >                         /* The target page is in the block, ignore it. */
> > @@ -1086,17 +1086,25 @@ static unsigned long
> > isolate_lru_pages(unsigned long nr_to_scan,
> >                                 break;
> > 
> >                         if (__isolate_lru_page(cursor_page, mode, file) == 0) {
> > +                               int isolated_pages;
> >                                 list_move(&cursor_page->lru, dst);
> >                                 mem_cgroup_del_lru(cursor_page);
> > -                               nr_taken += hpage_nr_pages(page);
> > +                               isolated_pages = hpage_nr_pages(page);
> > +                               nr_taken += isolated_pages;
> > +                               /* if we isolated pages enough, let's
> > break early */
> > +                               if (nr_taken > end_pfn - start_pfn)
> > +                                       break;
> > +                               pfn += isolated_pages;
> 
> I think this condition is somewhat unlikely. We are scanning within
> aligned blocks in this linear scanner. Huge pages are always aligned
> so the only situation where we'll encounter a hugepage in the middle
> of this linear scan is when the requested order is larger than a huge
> page. This is exceptionally rare.
> 
> Did I miss something?
> 

I forgot to mention the "pfn += isolated_pages" but I'm also worried
about it. It's a performance gain iif we are encountering huge pages
during the linear scan which I think is rare but also, I think this
is now skipping pages in the linear scan because we now have

                for (; pfn < end_pfn; pfn++) {
			if (isolate page) {
                               	isolated_pages = hpage_nr_pages(page);
                                pfn += isolated_pages;
			}
		}

hpage_nr_pages is returning 1 for order-0 LRU pages so now the loop is
effectively 

		for (; pfn < end_pfn; pfn += 2)

Did you mean

				pfn += isolated_pages - 1;

?

-- 
Mel Gorman
SUSE Labs
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