Re: [PATCH 4/8] mm: free swp_entry in madvise_free

[Minchan Kim]

On Fri, Oct 30, 2015 at 01:28:14PM +0100, Michal Hocko wrote:
> On Fri 30-10-15 16:01:40, Minchan Kim wrote:
> > When I test below piece of code with 12 processes(ie, 512M * 12 = 6G
> > consume) on my (3G ram + 12 cpu + 8G swap, the madvise_free is siginficat
> > slower (ie, 2x times) than madvise_dontneed.
> > 
> > loop = 5;
> > mmap(512M);
> > while (loop--) {
> >         memset(512M);
> >         madvise(MADV_FREE or MADV_DONTNEED);
> > }
> > 
> > The reason is lots of swapin.
> > 
> > 1) dontneed: 1,612 swapin
> > 2) madvfree: 879,585 swapin
> > 
> > If we find hinted pages were already swapped out when syscall is called,
> > it's pointless to keep the swapped-out pages in pte.
> > Instead, let's free the cold page because swapin is more expensive
> > than (alloc page + zeroing).
> > 
> > With this patch, it reduced swapin from 879,585 to 1,878 so elapsed time
> > 
> > 1) dontneed: 6.10user 233.50system 0:50.44elapsed
> > 2) madvfree: 6.03user 401.17system 1:30.67elapsed
> > 2) madvfree + below patch: 6.70user 339.14system 1:04.45elapsed
> > 
> > Acked-by: Hugh Dickins <hughd@xxxxxxxxxx>
> > Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx>
> 
> Yes this makes a lot of sense.
> 
> Acked-by: Michal Hocko <mhocko@xxxxxxxx>

Thanks!

> 
> One nit below.
> 
> > ---
> >  mm/madvise.c | 26 +++++++++++++++++++++++++-
> >  1 file changed, 25 insertions(+), 1 deletion(-)
> > 
> > diff --git a/mm/madvise.c b/mm/madvise.c
> > index 640311704e31..663bd9fa0ae0 100644
> > --- a/mm/madvise.c
> > +++ b/mm/madvise.c
> > @@ -270,6 +270,8 @@ static int madvise_free_pte_range(pmd_t *pmd, unsigned long addr,
> >  	spinlock_t *ptl;
> >  	pte_t *pte, ptent;
> >  	struct page *page;
> > +	swp_entry_t entry;
> 
> This could go into !pte_present if block

Sure, I fixed.
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