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Date: Fri, 14 Feb 2014 10:17:42 +0000
From: Mel Gorman <mgorman@...e.de>
To: Weijie Yang <weijie.yang.kh@...il.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
Hugh Dickins <hughd@...gle.com>, Michal Hocko <mhocko@...e.cz>,
Linux-MM <linux-mm@...ck.org>,
LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: swap: Use swapfiles in priority order
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@...e.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@...e.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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