postgres.git / summary / log / commit / refs

commit    f2db5f3bb0b5d429e9fa5b093936f8524e4192b1
Author:   Thomas Munro <tmunro@postgresql.org>
Date:     Tue Jul 24 22:58:44 2018 +0000
First released in: 12.0, 10.5, 11.0

    Pad semaphores to avoid false sharing.
    
    In a USE_UNNAMED_SEMAPHORES build, the default on Linux and FreeBSD
    since commit ecb0d20a, we have an array of sem_t objects.  This
    turned out to reduce performance compared to the previous default
    USE_SYSV_SEMAPHORES on an 8 socket system.  Testing showed that the
    lost performance could be regained by padding the array elements so
    that they have their own cache lines.  This matches what we do for
    similar hot arrays (see LWLockPadded, WALInsertLockPadded).
    
    Back-patch to 10, where unnamed semaphores were adopted as the default
    semaphore interface on those operating systems.
    
    Author: Thomas Munro
    Reviewed-by: Andres Freund
    Reported-by: Mithun Cy
    Tested-by: Mithun Cy, Tom Lane, Thomas Munro
    Discussion: https://postgr.es/m/CAD__OugYDM3O%2BdyZnnZSbJprSfsGFJcQ1R%3De59T3hcLmDug4_w%40mail.gmail.com


 src/backend/port/posix_sema.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/src/backend/port/posix_sema.c b/src/backend/port/posix_sema.c
index a2cabe58fcb..5174550794b 100644
--- a/src/backend/port/posix_sema.c
+++ b/src/backend/port/posix_sema.c
@@ -41,13 +41,19 @@
 #error cannot use named POSIX semaphores with EXEC_BACKEND
 #endif
 
+typedef union SemTPadded
+{
+	sem_t		pgsem;
+	char		pad[PG_CACHE_LINE_SIZE];
+} SemTPadded;
+
 /* typedef PGSemaphore is equivalent to pointer to sem_t */
 typedef struct PGSemaphoreData
 {
-	sem_t		pgsem;
+	SemTPadded	sem_padded;
 } PGSemaphoreData;
 
-#define PG_SEM_REF(x)	(&(x)->pgsem)
+#define PG_SEM_REF(x)	(&(x)->sem_padded.pgsem)
 
 #define IPCProtection	(0600)	/* access/modify by user only */
 


[parent: e0a2a4c87f22]