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Pad semaphores to avoid false sharing.

In a USE_UNNAMED_SEMAPHORES build, the default on Linux and FreeBSDsince commitecb0d20, we have an array of sem_t objects. Thisturned out to reduce performance compared to the previous defaultUSE_SYSV_SEMAPHORES on an 8 socket system. Testing showed that thelost performance could be regained by padding the array elements sothat they have their own cache lines. This matches what we do forsimilar hot arrays (see LWLockPadded, WALInsertLockPadded).Back-patch to 10, where unnamed semaphores were adopted as the defaultsemaphore interface on those operating systems.Author: Thomas MunroReviewed-by: Andres FreundReported-by: Mithun CyTested-by: Mithun Cy, Tom Lane, Thomas MunroDiscussion:https://postgr.es/m/CAD__OugYDM3O%2BdyZnnZSbJprSfsGFJcQ1R%3De59T3hcLmDug4_w%40mail.gmail.com

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src/backend/port
- posix_sema.c

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`‎src/backend/port/posix_sema.c`

Lines changed: 8 additions & 2 deletions

Original file line number	Diff line number	Diff line change
`@@ -41,13 +41,19 @@`
`41`	`41`	`#error cannot use named POSIX semaphores with EXEC_BACKEND`
`42`	`42`	`#endif`
`43`	`43`
	`44`	`+typedefunionSemTPadded`
	`45`	`+{`
	`46`	`+sem_tpgsem;`
	`47`	`+charpad[PG_CACHE_LINE_SIZE];`
	`48`	`+}SemTPadded;`
	`49`	`+`
`44`	`50`	`/* typedef PGSemaphore is equivalent to pointer to sem_t */`
`45`	`51`	`typedefstructPGSemaphoreData`
`46`	`52`	`{`
`47`		`-sem_tpgsem;`
	`53`	`+SemTPaddedsem_padded;`
`48`	`54`	`}PGSemaphoreData;`
`49`	`55`
`50`		`-#definePG_SEM_REF(x)(&(x)->pgsem)`
	`56`	`+#definePG_SEM_REF(x)(&(x)->sem_padded.pgsem)`
`51`	`57`
`52`	`58`	`#defineIPCProtection(0600)/* access/modify by user only */`
`53`	`59`

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Commit2d30675

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`‎src/backend/port/posix_sema.c`

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