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

[Minchan Kim]

On Fri, Apr 28, 2017 at 09:09:53AM +0800, Huang, Ying wrote:
> Minchan Kim <minchan@xxxxxxxxxx> writes:
> 
> > On Wed, Apr 26, 2017 at 08:42:10PM +0800, Huang, Ying wrote:
> >> 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.
> >
> > How worse with approach I suggested compared to as-is?
> 
> The performance difference between your version and my version is small
> for my testing.

If so, why should we add code to optimize further?

> 
> > Unless it's too bad, let's not add more complicated thing to just
> > enhance the minor usecase in such even *slow* path.
> > It adds code size/maintainance overead.
> > With your suggestion, it might enhance a bit with speicific benchmark
> > but not sure it's really worth for real practice.
> 
> I don't think the code complexity has much difference between our latest
> versions.  As for complexity, I think my original version which just

What I suggested is to avoid pointless overhead for *major* usecase
and the code you are adding now is to optimize further for *minor*
usecase. And now I dobut the code you are adding is really worth
unless it makes a meaningful output.
If it doesn't, it adds just overhead(code size, maintainance, power and
performance). You might argue it's really *small* so it would be okay
but think about that you would be not only one in the community so
kernel bloats day by day with code to handle corner cases.

> uses nr_swapfiles to avoid sort() for single swap device is simple and
> good enough for this task.  Maybe we can just improve the correctness of

But it hurts *major* usecase.

> swap device counting as Tim suggested.

I don't know what Tim suggested. Anyway, my point is that minor
usecase doesn't hurt major usecase and justify the benefit
if you want to put more. So I'm okay with either solution to
meet it.