From 46201d603f9c79d99e56ce62fb71aae31a99303d Mon Sep 17 00:00:00 2001 From: Thomas Munro Date: Wed, 25 Jul 2018 10:58:54 +1200 Subject: [PATCH] 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 5719caf9b56..445f34bb452 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 */