Re: [PATCH -mm -v2] mm, swap: Sort swap entries before free

[Huang, Ying]

Rik van Riel  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)

Re: [PATCH -mm -v2] mm, swap: Sort swap entries before free

[Huang, Ying]

Rik van Riel  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)

Re: [PATCH -mm -v2] mm, swap: Sort swap entries before free

[Rik van Riel]

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


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Re: [PATCH -mm -v2] mm, swap: Sort swap entries before free

[Rik van Riel]

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


signature.asc
Description: This is a digitally signed message part

[PATCH -mm -v2] mm, swap: Sort swap entries before free

[Huang, Ying]

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 

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 
 #include 
 #include 
+#include 
 
 #include 
 #include 
@@ -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

[PATCH -mm -v2] mm, swap: Sort swap entries before free

[Huang, Ying]

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 

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 
 #include 
 #include 
+#include 
 
 #include 
 #include 
@@ -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