[PATCH 2/6] ipc/sem.c: cacheline align the semaphore structures

[Manfred Spraul]

As now each semaphore has it's own spinlock and parallel operations
are possible, give each semaphore it's own cacheline.

On a i3 laptop, this gives up to 28% better performance:

#semscale 10 | grep "interleave 2"
- before:
Cpus 1, interleave 2 delay 0: 36109234 in 10 secs
Cpus 2, interleave 2 delay 0: 55276317 in 10 secs
Cpus 3, interleave 2 delay 0: 62411025 in 10 secs
Cpus 4, interleave 2 delay 0: 81963928 in 10 secs

-after:
Cpus 1, interleave 2 delay 0: 35527306 in 10 secs
Cpus 2, interleave 2 delay 0: 70922909 in 10 secs <<< + 28%
Cpus 3, interleave 2 delay 0: 80518538 in 10 secs
Cpus 4, interleave 2 delay 0: 89115148 in 10 secs <<< + 8.7%

i3, with 2 cores and with hyperthreading enabled.
Interleave 2 in order use first the full cores.
HT partially hides the delay from cacheline trashing, thus
the improvement is "only" 8.7% if 4 threads are running.

Andrew: Could you merge it into -akpm and then forward it towards Linus' tree?

Signed-off-by: Manfred Spraul <manfred@xxxxxxxxxxxxxxxx>
---
 ipc/sem.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/ipc/sem.c b/ipc/sem.c
index 70480a3..1afbc57 100644
--- a/ipc/sem.c
+++ b/ipc/sem.c
@@ -96,7 +96,7 @@ struct sem {
 	int	sempid;		/* pid of last operation */
 	spinlock_t	lock;	/* spinlock for fine-grained semtimedop */
 	struct list_head sem_pending; /* pending single-sop operations */
-};
+} ____cacheline_aligned_in_smp;
 
 /* One queue for each sleeping process in the system. */
 struct sem_queue {
-- 
1.8.1.4


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