Re: [PATCH] mm: swap: Use swapfiles in priority order

[Mel Gorman <mgorman@suse.de>]

On Thu, Feb 13, 2014 at 11:58:05PM +0800, Weijie Yang wrote:
> On Thu, Feb 13, 2014 at 6:42 PM, Mel Gorman <mgorman@suse.de> wrote:
> > According to the swapon documentation
> >
> >         Swap  pages  are  allocated  from  areas  in priority order,
> >         highest priority first.  For areas with different priorities, a
> >         higher-priority area is exhausted before using a lower-priority area.
> >
> > A user reported that the reality is different. When multiple swap files
> > are enabled and a memory consumer started, the swap files are consumed in
> > pairs after the highest priority file is exhausted. Early in the lifetime
> > of the test, swapfile consumptions looks like
> >
> > Filename                                Type            Size    Used    Priority
> > /testswap1                              file            100004  100004  8
> > /testswap2                              file            100004  23764   7
> > /testswap3                              file            100004  23764   6
> > /testswap4                              file            100004  0       5
> > /testswap5                              file            100004  0       4
> > /testswap6                              file            100004  0       3
> > /testswap7                              file            100004  0       2
> > /testswap8                              file            100004  0       1
> >
> > This patch fixes the swap_list search in get_swap_page to use the swap files
> > in the correct order. When applied the swap file consumptions looks like
> >
> > Filename                                Type            Size    Used    Priority
> > /testswap1                              file            100004  100004  8
> > /testswap2                              file            100004  100004  7
> > /testswap3                              file            100004  29372   6
> > /testswap4                              file            100004  0       5
> > /testswap5                              file            100004  0       4
> > /testswap6                              file            100004  0       3
> > /testswap7                              file            100004  0       2
> > /testswap8                              file            100004  0       1
> >
> > Signed-off-by: Mel Gorman <mgorman@suse.de>
> > ---
> >  mm/swapfile.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/mm/swapfile.c b/mm/swapfile.c
> > index 4a7f7e6..6d0ac2b 100644
> > --- a/mm/swapfile.c
> > +++ b/mm/swapfile.c
> > @@ -651,7 +651,7 @@ swp_entry_t get_swap_page(void)
> >                 goto noswap;
> >         atomic_long_dec(&nr_swap_pages);
> >
> > -       for (type = swap_list.next; type >= 0 && wrapped < 2; type = next) {
> > +       for (type = swap_list.head; type >= 0 && wrapped < 2; type = next) {
> 
> Does it lead to a "schlemiel the painter's algorithm"?
> (please forgive my rude words, but I can't find a precise word to describe it
> because English is not my native language. My apologize.)
> 
> How about modify it like this?
> 

I blindly applied your version without review to see how it behaved and
found it uses every second swapfile like this

Filename                                Type            Size    Used    Priority
/testswap1                              file            100004  100004  8
/testswap2                              file            100004  16      7
/testswap3                              file            100004  100004  6
/testswap4                              file            100004  8       5
/testswap5                              file            100004  100004  4
/testswap6                              file            100004  8       3
/testswap7                              file            100004  100004  2
/testswap8                              file            100004  23504   1

I admit I did not review the swap priority search algorithm in detail
because the fix superficially looked straight forward but this
alternative is not the answer either.

-- 
Mel Gorman
SUSE Labs

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