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Subject[PATCH -mm -v3] mm, swap: Sort swap entries before free
From: Huang Ying <>

To reduce the lock contention of swap_info_struct->lock when freeing
swap entry. The freed swap entries will be collected in a per-CPU
buffer firstly, and be really freed later in batch. During the batch
freeing, if the consecutive swap entries in the per-CPU buffer belongs
to same swap device, the swap_info_struct->lock needs to be
acquired/released only once, so that the lock contention could be
reduced greatly. But if there are multiple swap devices, it is
possible that the lock may be unnecessarily released/acquired because
the swap entries belong to the same swap device are non-consecutive in
the per-CPU buffer.

To solve the issue, the per-CPU buffer is sorted according to the swap
device before freeing the swap entries. Test shows that the time
spent by swapcache_free_entries() could be reduced after the patch.

Test the patch via measuring the run time of swap_cache_free_entries()
during the exit phase of the applications use much swap space. The
results shows that the average run time of swap_cache_free_entries()
reduced about 20% after applying the patch.

Signed-off-by: Huang Ying <>
Acked-by: Tim Chen <>
Cc: Hugh Dickins <>
Cc: Shaohua Li <>
Cc: Minchan Kim <>
Cc: Rik van Riel <>


- Add some comments in code per Rik's suggestion.


- Avoid sort swap entries if there is only one swap device.
mm/swapfile.c | 12 ++++++++++++
1 file changed, 12 insertions(+)

diff --git a/mm/swapfile.c b/mm/swapfile.c
index 90054f3c2cdc..f23c56e9be39 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -37,6 +37,7 @@
#include <linux/swapfile.h>
#include <linux/export.h>
#include <linux/swap_slots.h>
+#include <linux/sort.h>

#include <asm/pgtable.h>
#include <asm/tlbflush.h>
@@ -1065,6 +1066,13 @@ void swapcache_free(swp_entry_t entry)

+static int swp_entry_cmp(const void *ent1, const void *ent2)
+ const swp_entry_t *e1 = ent1, *e2 = ent2;
+ return (long)(swp_type(*e1) - swp_type(*e2));
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);
for (i = 0; i < n; ++i) {
p = swap_info_get_cont(entries[i], prev);
if (p)
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