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

Andrea Righi
2020-04-15T12:07:28Z

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).

-Andrea