Re: [PATCH -mm -v3] mm, swap: Sort swap entries before free

["Huang\, Ying" <ying.huang@intel.com>]

Minchan Kim <minchan@kernel.org> writes:

> On Wed, Apr 19, 2017 at 04:14:43PM +0800, Huang, Ying wrote:
>> Minchan Kim <minchan@kernel.org> writes:
>> 
>> > Hi Huang,
>> >
>> > On Fri, Apr 07, 2017 at 02:49:01PM +0800, Huang, Ying wrote:
>> >> From: Huang Ying <ying.huang@intel.com>
>> >> 
>> >> To reduce the lock contention of swap_info_struct->lock when freeing
>> >> swap entry.  The freed swap entries will be collected in a per-CPU
>> >> buffer firstly, and be really freed later in batch.  During the batch
>> >> freeing, if the consecutive swap entries in the per-CPU buffer belongs
>> >> to same swap device, the swap_info_struct->lock needs to be
>> >> acquired/released only once, so that the lock contention could be
>> >> reduced greatly.  But if there are multiple swap devices, it is
>> >> possible that the lock may be unnecessarily released/acquired because
>> >> the swap entries belong to the same swap device are non-consecutive in
>> >> the per-CPU buffer.
>> >> 
>> >> To solve the issue, the per-CPU buffer is sorted according to the swap
>> >> device before freeing the swap entries.  Test shows that the time
>> >> spent by swapcache_free_entries() could be reduced after the patch.
>> >> 
>> >> Test the patch via measuring the run time of swap_cache_free_entries()
>> >> during the exit phase of the applications use much swap space.  The
>> >> results shows that the average run time of swap_cache_free_entries()
>> >> reduced about 20% after applying the patch.
>> >> 
>> >> Signed-off-by: Huang Ying <ying.huang@intel.com>
>> >> Acked-by: Tim Chen <tim.c.chen@intel.com>
>> >> Cc: Hugh Dickins <hughd@google.com>
>> >> Cc: Shaohua Li <shli@kernel.org>
>> >> Cc: Minchan Kim <minchan@kernel.org>
>> >> Cc: Rik van Riel <riel@redhat.com>
>> >> 
>> >> v3:
>> >> 
>> >> - Add some comments in code per Rik's suggestion.
>> >> 
>> >> v2:
>> >> 
>> >> - Avoid sort swap entries if there is only one swap device.
>> >> ---
>> >>  mm/swapfile.c | 12 ++++++++++++
>> >>  1 file changed, 12 insertions(+)
>> >> 
>> >> diff --git a/mm/swapfile.c b/mm/swapfile.c
>> >> index 90054f3c2cdc..f23c56e9be39 100644
>> >> --- a/mm/swapfile.c
>> >> +++ b/mm/swapfile.c
>> >> @@ -37,6 +37,7 @@
>> >>  #include <linux/swapfile.h>
>> >>  #include <linux/export.h>
>> >>  #include <linux/swap_slots.h>
>> >> +#include <linux/sort.h>
>> >>  
>> >>  #include <asm/pgtable.h>
>> >>  #include <asm/tlbflush.h>
>> >> @@ -1065,6 +1066,13 @@ void swapcache_free(swp_entry_t entry)
>> >>  	}
>> >>  }
>> >>  
>> >> +static int swp_entry_cmp(const void *ent1, const void *ent2)
>> >> +{
>> >> +	const swp_entry_t *e1 = ent1, *e2 = ent2;
>> >> +
>> >> +	return (long)(swp_type(*e1) - swp_type(*e2));
>> >> +}
>> >> +
>> >>  void swapcache_free_entries(swp_entry_t *entries, int n)
>> >>  {
>> >>  	struct swap_info_struct *p, *prev;
>> >> @@ -1075,6 +1083,10 @@ void swapcache_free_entries(swp_entry_t *entries, int n)
>> >>  
>> >>  	prev = NULL;
>> >>  	p = NULL;
>> >> +
>> >> +	/* Sort swap entries by swap device, so each lock is only taken once. */
>> >> +	if (nr_swapfiles > 1)
>> >> +		sort(entries, n, sizeof(entries[0]), swp_entry_cmp, NULL);
>> >
>> > Let's think on other cases.
>> >
>> > There are two swaps and they are configured by priority so a swap's usage
>> > would be zero unless other swap used up. In case of that, this sorting
>> > is pointless.
>> >
>> > As well, nr_swapfiles is never decreased so if we enable multiple
>> > swaps and then disable until a swap is remained, this sorting is
>> > pointelss, too.
>> >
>> > How about lazy sorting approach? IOW, if we found prev != p and,
>> > then we can sort it.
>> 
>> Yes.  That should be better.  I just don't know whether the added
>> complexity is necessary, given the array is short and sort is fast.
>
> Huh?
>
> 1. swapon /dev/XXX1
> 2. swapon /dev/XXX2
> 3. swapoff /dev/XXX2
> 4. use only one swap
> 5. then, always pointless sort.

Yes.  In this situation we will do unnecessary sorting.  What I don't
know is whether the unnecessary sorting will hurt performance in real
life.  I can do some measurement.

Best Regards,
Huang, Ying

> Do not add such bogus code.
>
> Nacked.