[PATCH][B][aws] UBUNTU SAUCE: mm: swap: improve swap readahead heuristic

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[PATCH][B][aws] UBUNTU SAUCE: mm: swap: improve swap readahead heuristic

Andrea Righi
BugLink: https://bugs.launchpad.net/bugs/1831940

Apply a more aggressive swapin readahead policy to improve swapoff
performance.

The idea is to start with no readahead (only read one page) and linearly
increment the amount of readahead pages each time swapin_readahead() is
called, up to the maximum cluster size (defined by vm.page-cluster),
then go back to one page to give the disk enough time to prefetch the
requested pages and avoid re-requesting them multiple times.

Also increase the default vm.page-cluster size to 8 (that seems to work
better with this new heuristic).

Signed-off-by: Andrea Righi <[hidden email]>
---
 mm/swap.c       |  2 +-
 mm/swap_state.c | 60 ++++++++-----------------------------------------
 2 files changed, 10 insertions(+), 52 deletions(-)

diff --git a/mm/swap.c b/mm/swap.c
index abc82e6c14d1..5603bc987ef0 100644
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -1022,7 +1022,7 @@ void __init swap_setup(void)
  if (megs < 16)
  page_cluster = 2;
  else
- page_cluster = 3;
+ page_cluster = 8;
  /*
  * Right now other parts of the system means that we
  * _really_ don't want to cluster much more
diff --git a/mm/swap_state.c b/mm/swap_state.c
index 6dac8c6ee6d9..a2246bcebc77 100644
--- a/mm/swap_state.c
+++ b/mm/swap_state.c
@@ -472,62 +472,21 @@ struct page *read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
  return retpage;
 }
 
-static unsigned int __swapin_nr_pages(unsigned long prev_offset,
-      unsigned long offset,
-      int hits,
-      int max_pages,
-      int prev_win)
-{
- unsigned int pages, last_ra;
-
- /*
- * This heuristic has been found to work well on both sequential and
- * random loads, swapping to hard disk or to SSD: please don't ask
- * what the "+ 2" means, it just happens to work well, that's all.
- */
- pages = hits + 2;
- if (pages == 2) {
- /*
- * We can have no readahead hits to judge by: but must not get
- * stuck here forever, so check for an adjacent offset instead
- * (and don't even bother to check whether swap type is same).
- */
- if (offset != prev_offset + 1 && offset != prev_offset - 1)
- pages = 1;
- } else {
- unsigned int roundup = 4;
- while (roundup < pages)
- roundup <<= 1;
- pages = roundup;
- }
-
- if (pages > max_pages)
- pages = max_pages;
-
- /* Don't shrink readahead too fast */
- last_ra = prev_win / 2;
- if (pages < last_ra)
- pages = last_ra;
-
- return pages;
-}
-
 static unsigned long swapin_nr_pages(unsigned long offset)
 {
- static unsigned long prev_offset;
- unsigned int hits, pages, max_pages;
- static atomic_t last_readahead_pages;
+ static unsigned int prev_pages;
+ unsigned long pages, max_pages;
 
  max_pages = 1 << READ_ONCE(page_cluster);
  if (max_pages <= 1)
  return 1;
 
- hits = atomic_xchg(&swapin_readahead_hits, 0);
- pages = __swapin_nr_pages(prev_offset, offset, hits, max_pages,
-  atomic_read(&last_readahead_pages));
- if (!hits)
- prev_offset = offset;
- atomic_set(&last_readahead_pages, pages);
+ pages = READ_ONCE(prev_pages) + 1;
+ if (pages > max_pages) {
+ WRITE_ONCE(prev_pages, 0);
+ pages = max_pages;
+ } else
+ WRITE_ONCE(prev_pages, pages);
 
  return pages;
 }
@@ -684,8 +643,7 @@ struct page *swap_readahead_detect(struct vm_fault *vmf,
  pfn = PFN_DOWN(SWAP_RA_ADDR(swap_ra_info));
  prev_win = SWAP_RA_WIN(swap_ra_info);
  hits = SWAP_RA_HITS(swap_ra_info);
- swap_ra->win = win = __swapin_nr_pages(pfn, fpfn, hits,
-       max_win, prev_win);
+ swap_ra->win = win = swapin_nr_pages(fpfn);
  atomic_long_set(&vma->swap_readahead_info,
  SWAP_RA_VAL(faddr, win, 0));
 
--
2.17.1


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ACK/Cmnt: [PATCH][B][aws] UBUNTU SAUCE: mm: swap: improve swap readahead heuristic

Connor Kuehl
On 12/3/19 2:58 AM, Andrea Righi wrote:

> BugLink: https://bugs.launchpad.net/bugs/1831940
>
> Apply a more aggressive swapin readahead policy to improve swapoff
> performance.
>
> The idea is to start with no readahead (only read one page) and linearly
> increment the amount of readahead pages each time swapin_readahead() is
> called, up to the maximum cluster size (defined by vm.page-cluster),
> then go back to one page to give the disk enough time to prefetch the
> requested pages and avoid re-requesting them multiple times.
>
> Also increase the default vm.page-cluster size to 8 (that seems to work
> better with this new heuristic).
>
> Signed-off-by: Andrea Righi <[hidden email]>

Was this patch was included in the AMI that received positive test
results? I remember having a small chat about a recent round of testing
for this on IRC but also wanted the mailing list to be informed of the
positive results too :-)

Acked-by: Connor Kuehl <[hidden email]>

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Re: ACK/Cmnt: [PATCH][B][aws] UBUNTU SAUCE: mm: swap: improve swap readahead heuristic

Andrea Righi
On Fri, Dec 06, 2019 at 12:53:19PM -0800, Connor Kuehl wrote:

> On 12/3/19 2:58 AM, Andrea Righi wrote:
> > BugLink: https://bugs.launchpad.net/bugs/1831940
> >
> > Apply a more aggressive swapin readahead policy to improve swapoff
> > performance.
> >
> > The idea is to start with no readahead (only read one page) and linearly
> > increment the amount of readahead pages each time swapin_readahead() is
> > called, up to the maximum cluster size (defined by vm.page-cluster),
> > then go back to one page to give the disk enough time to prefetch the
> > requested pages and avoid re-requesting them multiple times.
> >
> > Also increase the default vm.page-cluster size to 8 (that seems to work
> > better with this new heuristic).
> >
> > Signed-off-by: Andrea Righi <[hidden email]>
>
> Was this patch was included in the AMI that received positive test results?
> I remember having a small chat about a recent round of testing for this on
> IRC but also wanted the mailing list to be informed of the positive results
> too :-)
>
> Acked-by: Connor Kuehl <[hidden email]>

Thanks Connor, good point, I should have mentioned it in the comment.
This patch has been tested a lot and it is included in the AMI that
received positive results.

-Andrea

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