Re: [PATCH v2] mm: swap: use fixed-size readahead during swapoff

Huang, Ying
2020-04-16T00:41:51Z

> On Wed, Apr 15, 2020 at 09:32:47AM +0200, Andrea Righi wrote:
>> On Wed, Apr 15, 2020 at 10:37:00AM +0800, Huang, Ying wrote:
>> > Andrea Righi <andrea.righi@canonical.com> writes:
>> >
>> > > On Tue, Apr 14, 2020 at 09:31:24AM +0800, Huang, Ying wrote:
>> > >> Andrea Righi <andrea.righi@canonical.com> writes:
>> > >>
>> > >> > On Mon, Apr 13, 2020 at 09:00:34PM +0800, Huang, Ying wrote:
>> > >> >> Andrea Righi <andrea.righi@canonical.com> writes:
>> > >> >>
>> > >> >> [snip]
>> > >> >>
>> > >> >> > diff --git a/mm/swap_state.c b/mm/swap_state.c
>> > >> >> > index ebed37bbf7a3..c71abc8df304 100644
>> > >> >> > --- a/mm/swap_state.c
>> > >> >> > +++ b/mm/swap_state.c
>> > >> >> > @@ -20,6 +20,7 @@
>> > >> >> > #include <linux/migrate.h>
>> > >> >> > #include <linux/vmalloc.h>
>> > >> >> > #include <linux/swap_slots.h>
>> > >> >> > +#include <linux/oom.h>
>> > >> >> > #include <linux/huge_mm.h>
>> > >> >> >
>> > >> >> > #include <asm/pgtable.h>
>> > >> >> > @@ -507,6 +508,14 @@ static unsigned long swapin_nr_pages(unsigned long offset)
>> > >> >> > max_pages = 1 << READ_ONCE(page_cluster);
>> > >> >> > if (max_pages <= 1)
>> > >> >> > return 1;
>> > >> >> > + /*
>> > >> >> > + * If current task is using too much memory or swapoff is running
>> > >> >> > + * simply use the max readahead size. Since we likely want to load a
>> > >> >> > + * lot of pages back into memory, using a fixed-size max readhaead can
>> > >> >> > + * give better performance in this case.
>> > >> >> > + */
>> > >> >> > + if (oom_task_origin(current))
>> > >> >> > + return max_pages;
>> > >> >> >
>> > >> >> > hits = atomic_xchg(&swapin_readahead_hits, 0);
>> > >> >> > pages = __swapin_nr_pages(prev_offset, offset, hits, max_pages,
>> > >> >>
>> > >> >> Thinks this again. If my understanding were correct, the accessing
>> > >> >> pattern during swapoff is sequential, why swap readahead doesn't work?
>> > >> >> If so, can you root cause that firstly?
>> > >> >
>> > >> > Theoretically if the pattern is sequential the current heuristic should
>> > >> > already select a big readahead size, but apparently it's not doing that.
>> > >> >
>> > >> > I'll repeat my tests tracing the readahead size during swapoff to see
>> > >> > exactly what's going on here.
>> > >>
>> > >> I haven't verify it. It may be helpful to call lookup_swap_cache()
>> > >> before swapin_readahead() in unuse_pte_range(). The theory behind it is
>> > >> to update the swap readahead statistics via lookup_swap_cache().
>> > >
>> > > I did more tests trying to collect some useful information.
>> > >
>> > > In particular I've been focusing at tracing the distribution of the
>> > > values returned by swapin_nr_pages() in different scenarios.
>> > >
>> > > To do so I made swapin_nr_pages() trace-able and I used the following
>> > > bcc command to measure the distrubution of the returned values:
>> > >
>> > > # argdist-bpfcc -c -C 'r::swapin_nr_pages(unsigned long offset):unsigned long:$retval'
>> > >
>> > > I've collected this metric in the following scenarios:
>> > > - 5.6 vanilla
>> > > - 5.6 + lookup_swap_cache() before swapin_readahead() in
>> > > unuse_pte_range()
>> > > - 5.6 + atomic_inc(&swapin_readahead_hits) before swapin_readahead()
>> > > in unuse_pte_range()
>> > > - 5.6 + swapin_readahead_hits=last_readahead_pages (in the atomic way)
>> > > before swapin_readahead() in unuse_pte_range()
>> > >
>> > > Each kernel has been tested both with swappiness=0 and swappiness=60.
>> > > Results are pretty much identical changing the swappiness, so I'm just
>> > > reporting the default case here (w/ swappiness=60).
>> > >
>> > > Result
>> > > ======
>> > >
>> > > = swapoff performance (elapsed time) =
>> > >
>> > > vanilla 22.09s
>> > > lookup_swap_cache() 23.87s
>> > > hits++ 16.10s
>> > > hits=last_ra_pages 8.81s
>> > >
>> > > = swapin_nr_pages() $retval distribution =
>> > >
>> > > 5.6 vanilla:
>> > > r::swapin_nr_pages(unsigned long offset):unsigned long:$retval
>> > > COUNT EVENT
>> > > 36948 $retval = 8
>> > > 44151 $retval = 4
>> > > 49290 $retval = 1
>> > > 527771 $retval = 2
>> > >
>> > > 5.6 lookup_swap_cache() before swapin_readahead():
>> > > r::swapin_nr_pages(unsigned long offset):unsigned long:$retval
>> > > COUNT EVENT
>> > > 13093 $retval = 1
>> > > 56703 $retval = 8
>> > > 123067 $retval = 2
>> > > 366118 $retval = 4
>> > >
>> > > 5.6 atomic_inc(&swapin_readahead_hits) before swapin_readahead():
>> > > r::swapin_nr_pages(unsigned long offset):unsigned long:$retval
>> > > COUNT EVENT
>> > > 2589 $retval = 1
>> > > 8016 $retval = 2
>> > > 40021 $retval = 8
>> > > 566038 $retval = 4
>> > >
>> > > 5.6 swapin_readahead_hits=last_readahead_pages before swapin_readahead():
>> > > r::swapin_nr_pages(unsigned long offset):unsigned long:$retval
>> > > COUNT EVENT
>> > > 785 $retval = 2
>> > > 1072 $retval = 1
>> > > 21844 $retval = 4
>> > > 644168 $retval = 8
>> > >
>> > > In the vanilla case, the readahead heuristic seems to choose 2 pages
>> > > most of the time. This is because we are not properly considering the
>> > > hits (hits are always 0 in the swapoff code path) and, as you correctly
>> > > pointed out, we can fix this by calling lookup_swap_cache() in
>> > > unuse_pte_range() before calling swapin_readahead().
>> > >
>> > > With this change the distribution of the readahead size moves more
>> > > toward 4 pages, but we still have some 2s. That looks good, however it
>> > > doesn't seem to speed up swapoff very much... maybe because calling
>> > > lookup_swap_cache() introduces a small overhead? (still need to
>> > > investigate about this theory).
>> > >
>> > > In the next test I've tried to always increment hits by 1 before calling
>> > > swapin_readahead() in unuse_pte_range(). This is basically cheating,
>> > > because I'm faking the hit ratio, forcing the heuristic to use a larger
>> > > readahead size; in fact, the readahead size moves even more toward 4
>> > > pages and swapoff performance are a little better now.
>> > >
>> > > Pushing even more the "cheating" I can pretend that the previous
>> > > readahead was all hits (swapin_readahead_hits=last_readahead_pages), so
>> > > I'm forcing the heuristic to move toward the max size and keep using it.
>> > > The result here is pretty much identical to my fixed-size patch, because
>> > > swapin_nr_pages() returns the max readahead size pretty much all the
>> > > time during swapoff (8 pages or, more in general, vm.page-cluster).
>> > >
>> > > Personally I don't like very much forcing the heuristic in this way,
>> > > it'd be nice if it would just work by accounting the proper hit ratio
>> > > (so just by adding lookup_swap_cache() as you correctly suggested), but
>> > > this solution doesn't seem to improve performance in reality. For this
>> > > reason I still think we should consider the swapoff scenario like a
>> > > special one and somehow bypass the readahead heuristic and always return
>> > > the max readahead size.
>> > >
>> > > Looking at the hits of the previous step in the swapoff case just
>> > > doesn't work, because we may have some misses, but they will become hits
>> > > very soon, since we are reading all the swapped out pages back into
>> > > memory. This is why using the max readahead size gives better
>> > > swapoff performance.
>> > >
>> > > What do you think?
>> >
>> > >From your description, it appears that you are using cluster readahead
>> > instead of vma readahead. Can you verify this via,
>> >
>> > # cat /sys/kernel/mm/swap/vma_ra_enabled
>>
>> # cat /sys/kernel/mm/swap/vma_ra_enabled
>> true
>>
>> However, it's still using the cluster readahead because I'm using a
>> swap file and nr_rotate_swap=1, so, according to the following, it's
>> never using the vma readahead:
>>
>> static inline bool swap_use_vma_readahead(void)
>> {
>> return READ_ONCE(enable_vma_readahead) && !atomic_read(&nr_rotate_swap);
>> }
>>
>> I'll investigate more on this, I think there's no reason to prevent the
>> usage of vma readahead if the underlying device is non-rotational.
>
> Few more details about this.
>
> Even if I have vma_ra_enabled (in /sys/kernel/mm/swap/vma_ra_enabled) it
> looks like my virtio_blk device is considered rotational by default:
>
> $ cat /sys/block/vda/queue/rotational
> 1
>
> Therefore the vma readahead is not used. If I change this to
> "rotational" then the vma readahead is used; this is confirmed by the
> fact that swapin_nr_pages isn't called anymore:
>
> r::swapin_nr_pages(unsigned long offset):unsigned long:$retval
> COUNT EVENT
> 13 $retval = 1
> 18 $retval = 2
> 23 $retval = 4
> 29 $retval = 8
>
> swapoff time: 16.44s
>
> In terms of swapoff performance vma readahead works better than the
> vanilla cluster readahead (~21s), but the "improved" cluster readahead
> (with lookup_swap_cache() in unuse_pte_range()) still gives slightly
> better results (~12s).

Thanks for testing. Can you check the return value of
__swapin_nr_pages() to verify whether vma readahead works as expected?

Best Regards,
Huang, Ying

> -Andrea