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

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Date:	Fri, 14 Feb 2014 21:33:17 +0800
From:	Weijie Yang <weijie.yang.kh@...il.com>
To:	Mel Gorman <mgorman@...e.de>
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 Fri, Feb 14, 2014 at 6:17 PM, Mel Gorman <mgorman@...e.de> wrote:
> 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

I am sorry to waste your time, I should have tested it.
I will review the code more carefully, and send a tested patch if I find a
better way.
Apologize again.

> 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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