From: Huang Ying <[email protected]>
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 <[email protected]>
Acked-by: Tim Chen <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Shaohua Li <[email protected]>
Cc: Minchan Kim <[email protected]>
Cc: Rik van Riel <[email protected]>
v2:
- Avoid sort swap entries if there is only one swap device.
---
mm/swapfile.c | 10 ++++++++++
1 file changed, 10 insertions(+)
diff --git a/mm/swapfile.c b/mm/swapfile.c
index 90054f3c2cdc..b91b0b0088c5 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 +1083,8 @@ void swapcache_free_entries(swp_entry_t *entries, int n)
prev = NULL;
p = NULL;
+ 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)
--
2.11.0
On Wed, 2017-04-05 at 15:10 +0800, Huang, Ying wrote:
> 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.
That makes a lot of sense.
> @@ -1075,6 +1083,8 @@ void swapcache_free_entries(swp_entry_t
> *entries, int n)
>
> prev = NULL;
> p = NULL;
> + if (nr_swapfiles > 1)
> + sort(entries, n, sizeof(entries[0]), swp_entry_cmp,
> NULL);
But it really wants a comment in the code, so people
reading the code a few years from now can see why
we are sorting things we are about to free.
Maybe something like:
/* Sort swap entries by swap device, so each lock is only taken
once. */
> for (i = 0; i < n; ++i) {
> p = swap_info_get_cont(entries[i], prev);
> if (p)
--
All rights reversed
Rik van Riel <[email protected]> writes:
> On Wed, 2017-04-05 at 15:10 +0800, Huang, Ying wrote:
>> 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.
>
> That makes a lot of sense.
>
>> @@ -1075,6 +1083,8 @@ void swapcache_free_entries(swp_entry_t
>> *entries, int n)
>>
>> prev = NULL;
>> p = NULL;
>> + if (nr_swapfiles > 1)
>> + sort(entries, n, sizeof(entries[0]), swp_entry_cmp,
>> NULL);
>
> But it really wants a comment in the code, so people
> reading the code a few years from now can see why
> we are sorting things we are about to free.
>
> Maybe something like:
> /* Sort swap entries by swap device, so each lock is only taken
> once. */
Good suggestion! I will add it in the next version.
Best Regards,
Huang, Ying
>> for (i = 0; i < n; ++i) {
>> p = swap_info_get_cont(entries[i], prev);
>> if (p)