Re: [patch] mm: unlockless reclaim

[Nick Piggin <npiggin@suse.de>]

On Thu, Jul 12, 2007 at 12:43:39AM -0700, Andrew Morton wrote:
> On Thu, 12 Jul 2007 06:11:15 +0200 Nick Piggin <npiggin@suse.de> wrote:
> 
> > unlock_page is pretty expensive. Even after my patches to optimise the
> > memory order and away the waitqueue hit for uncontended pages, it is
> > still a locked operation, which may be anywhere up to hundreds of cycles
> > on some CPUs.
> > 
> > When we reclaim a page, we don't need to "unlock" it as such, because
> > we know there will be no contention (if there was, it would be a bug
> > because the page is just about to get freed).
> > 
> > Signed-off-by: Nick Piggin <npiggin@suse.de>
> > 
> > Index: linux-2.6/include/linux/page-flags.h
> > ===================================================================
> > --- linux-2.6.orig/include/linux/page-flags.h
> > +++ linux-2.6/include/linux/page-flags.h
> > @@ -115,6 +115,8 @@
> >  		test_and_set_bit(PG_locked, &(page)->flags)
> >  #define ClearPageLocked(page)		\
> >  		clear_bit(PG_locked, &(page)->flags)
> > +#define __ClearPageLocked(page)		\
> > +		__clear_bit(PG_locked, &(page)->flags)
> >  #define TestClearPageLocked(page)	\
> >  		test_and_clear_bit(PG_locked, &(page)->flags)
> >  
> > Index: linux-2.6/mm/vmscan.c
> > ===================================================================
> > --- linux-2.6.orig/mm/vmscan.c
> > +++ linux-2.6/mm/vmscan.c
> > @@ -576,7 +576,7 @@ static unsigned long shrink_page_list(st
> >  			goto keep_locked;
> >  
> >  free_it:
> > -		unlock_page(page);
> > +		__ClearPageLocked(page);
> >  		nr_reclaimed++;
> >  		if (!pagevec_add(&freed_pvec, page))
> >  			__pagevec_release_nonlru(&freed_pvec);
> 
> mutter.
> 
> So why does __pagevec_release_nonlru() check the page refcount?

It doesn't, although it will have to return the count to zero of course.

I don't want to submit that because the lockless pagecache always needs
the refcount to be checked :) And which I am actually going to submit to
you after you chuck out a few patches.

But unlock_page is really murderous on my powerpc (with all the
unlock-speeup patches, dd if=/dev/zero of=/dev/null of a huge sparse file
goes up by 10% throughput on the G5!!).

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