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

[Huang\, Ying]

Minchan Kim <minchan@xxxxxxxxxx> writes:

> On Fri, Apr 21, 2017 at 08:29:30PM +0800, Huang, Ying wrote:
>> "Huang, Ying" <ying.huang@xxxxxxxxx> writes:
>> 
>> > Minchan Kim <minchan@xxxxxxxxxx> writes:
>> >
>> >> On Wed, Apr 19, 2017 at 04:14:43PM +0800, Huang, Ying wrote:
>> >>> Minchan Kim <minchan@xxxxxxxxxx> writes:
>> >>> 
>> >>> > Hi Huang,
>> >>> >
>> >>> > On Fri, Apr 07, 2017 at 02:49:01PM +0800, Huang, Ying wrote:
>> >>> >> From: Huang Ying <ying.huang@xxxxxxxxx>
>> >>> >> 
>> >>> >>  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.
>> 
>> I tested the patch with 1 swap device and 1 process to eat memory
>> (remove the "if (nr_swapfiles > 1)" for test).  I think this is the
>> worse case because there is no lock contention.  The memory freeing time
>> increased from 1.94s to 2.12s (increase ~9.2%).  So there is some
>> overhead for some cases.  I change the algorithm to something like
>> below,
>> 
>>  void swapcache_free_entries(swp_entry_t *entries, int n)
>>  {
>>  	struct swap_info_struct *p, *prev;
>>  	int i;
>> +	swp_entry_t entry;
>> +	unsigned int prev_swp_type;
>>  
>>  	if (n <= 0)
>>  		return;
>>  
>> +	prev_swp_type = swp_type(entries[0]);
>> +	for (i = n - 1; i > 0; i--) {
>> +		if (swp_type(entries[i]) != prev_swp_type)
>> +			break;
>> +	}
>
> That's really what I want to avoid. For many swap usecases,
> it adds unnecessary overhead.
>
>> +
>> +	/* Sort swap entries by swap device, so each lock is only taken once. */
>> +	if (i)
>> +		sort(entries, n, sizeof(entries[0]), swp_entry_cmp, NULL);
>>  	prev = NULL;
>>  	p = NULL;
>>  	for (i = 0; i < n; ++i) {
>> -		p = swap_info_get_cont(entries[i], prev);
>> +		entry = entries[i];
>> +		p = swap_info_get_cont(entry, prev);
>>  		if (p)
>> -			swap_entry_free(p, entries[i]);
>> +			swap_entry_free(p, entry);
>>  		prev = p;
>>  	}
>>  	if (p)
>> 
>> With this patch, the memory freeing time increased from 1.94s to 1.97s.
>> I think this is good enough.  Do you think so?
>
> What I mean is as follows(I didn't test it at all):
>
> With this, sort entries if we found multiple entries in current
> entries. It adds some condition checks for non-multiple swap
> usecase but it would be more cheaper than the sorting.
> And it adds a [un]lock overhead for multiple swap usecase but
> it should be a compromise for single-swap usecase which is more
> popular.
>

How about the following solution?  It can avoid [un]lock overhead and
double lock issue for multiple swap user case and has good performance
for one swap user case too.

Best Regards,
Huang, Ying