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.
Yes. What I concerned is that one swap device may be locked twice
instead of once during the freeing. I will give it some test.
Best Regards,
Huang, Ying
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index f23c56e9be39..0d76a492786f 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -1073,30 +1073,40 @@ static int swp_entry_cmp(const void *ent1, const void *ent2)
> return (long)(swp_type(*e1) - swp_type(*e2));
> }
>
> -void swapcache_free_entries(swp_entry_t *entries, int n)
> +void swapcache_free_entries(swp_entry_t *entries, int nr)
> {
> - struct swap_info_struct *p, *prev;
> int i;
> + struct swap_info_struct *cur, *prev = NULL;
> + bool sorted = false;
>
> - if (n <= 0)
> + if (nr <= 0)
> return;
>
> - 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);
> - for (i = 0; i < n; ++i) {
> - p = swap_info_get_cont(entries[i], prev);
> - if (p)
> - swap_entry_free(p, entries[i]);
> - else
> + for (i = 0; i < nr; i++) {
> + cur = swap_info_get_cont(entries[i], prev);
> + if (!cur)
> break;
> - prev = p;
> + if (cur != prev && !sorted && prev) {
> + spin_unlock(&cur->lock);
> + /*
> + * Sort swap entries by swap device,
> + * so each lock is only taken once.
> + */
> + sort(entries + i, nr - i,
> + sizeof(swp_entry_t),
> + swp_entry_cmp, NULL);
> + sorted = true;
> + prev = NULL;
> + i--;
> + continue;
> + }
> +
> + swap_entry_free(cur, entries[i]);
> + prev = cur;
> }
> - if (p)
> - spin_unlock(&p->lock);
> +
> + if (cur)
> + spin_unlock(&cur->lock);
> }
>
> /*
--
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