The patch titled
Subject: mm: swap: properly update readahead statistics in unuse_pte_range()
has been added to the -mm tree. Its filename is
mm-swap-properly-update-readahead-statistics-in-unuse_pte_range.patch
This patch should soon appear at
http://ozlabs.org/~akpm/mmots/broken-out/mm-swap-properly-update-readahead-statistics-in-unuse_pte_range.patch
and later at
http://ozlabs.org/~akpm/mmotm/broken-out/mm-swap-properly-update-readahead-statistics-in-unuse_pte_range.patch
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------------------------------------------------------
From: Andrea Righi <andrea.righi@xxxxxxxxxxxxx>
Subject: mm: swap: properly update readahead statistics in unuse_pte_range()
In unuse_pte_range() we blindly swap-in pages without checking if the swap
entry is already present in the swap cache.
By doing this, the hit/miss ratio used by the swap readahead heuristic is
not properly updated and this leads to non-optimal performance during
swapoff.
Tracing the distribution of the readahead size returned by the swap
readahead heuristic during swapoff shows that a small readahead size is
used most of the time as if we had only misses (this happens both with
cluster and vma readahead), for example:
r::swapin_nr_pages(unsigned long offset):unsigned long:$retval
COUNT EVENT
36948 $retval = 8
44151 $retval = 4
49290 $retval = 1
527771 $retval = 2
Checking if the swap entry is present in the swap cache, instead, allows
to properly update the readahead statistics and the heuristic behaves in
a better way during swapoff, selecting a bigger readahead size:
r::swapin_nr_pages(unsigned long offset):unsigned long:$retval
COUNT EVENT
1618 $retval = 1
4960 $retval = 2
41315 $retval = 4
103521 $retval = 8
In terms of swapoff performance the result is the following:
Testing environment
===================
- Host:
CPU: 1.8GHz Intel Core i7-8565U (quad-core, 8MB cache)
HDD: PC401 NVMe SK hynix 512GB
MEM: 16GB
- Guest (kvm):
8GB of RAM
virtio block driver
16GB swap file on ext4 (/swapfile)
Test case
=========
- allocate 85% of memory
- `systemctl hibernate` to force all the pages to be swapped-out to the
swap file
- resume the system
- measure the time that swapoff takes to complete:
# /usr/bin/time swapoff /swapfile
Result (swapoff time)
======
5.6 vanilla 5.6 w/ this patch
----------- -----------------
cluster-readahead 22.09s 12.19s
vma-readahead 18.20s 15.33s
Link: http://lkml.kernel.org/r/20200416180132.GB3352@xps-13
Signed-off-by: Andrea Righi <andrea.righi@xxxxxxxxxxxxx>
Cc: "Huang, Ying" <ying.huang@xxxxxxxxx>
Cc: Minchan Kim <minchan@xxxxxxxxxx>
Cc: Anchal Agarwal <anchalag@xxxxxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: Vineeth Remanan Pillai <vpillai@xxxxxxxxxxxxxxxx>
Cc: Kelley Nielsen <kelleynnn@xxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/swapfile.c | 12 ++++++++----
1 file changed, 8 insertions(+), 4 deletions(-)
--- a/mm/swapfile.c~mm-swap-properly-update-readahead-statistics-in-unuse_pte_range
+++ a/mm/swapfile.c
@@ -1937,10 +1937,14 @@ static int unuse_pte_range(struct vm_are
pte_unmap(pte);
swap_map = &si->swap_map[offset];
- vmf.vma = vma;
- vmf.address = addr;
- vmf.pmd = pmd;
- page = swapin_readahead(entry, GFP_HIGHUSER_MOVABLE, &vmf);
+ page = lookup_swap_cache(entry, vma, addr);
+ if (!page) {
+ vmf.vma = vma;
+ vmf.address = addr;
+ vmf.pmd = pmd;
+ page = swapin_readahead(entry, GFP_HIGHUSER_MOVABLE,
+ &vmf);
+ }
if (!page) {
if (*swap_map == 0 || *swap_map == SWAP_MAP_BAD)
goto try_next;
_
Patches currently in -mm which might be from andrea.righi@xxxxxxxxxxxxx are
mm-swap-properly-update-readahead-statistics-in-unuse_pte_range.patch
