On Fri, Feb 14, 2014 at 6:17 PM, Mel Gorman <mgorman@xxxxxxx> 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@xxxxxxx> 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@xxxxxxx>
>> > ---
>> > 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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