From daa7527afc2274432094ebe7ceb03aa41f916607 Mon Sep 17 00:00:00 2001 From: Robert Haas Date: Wed, 8 Jan 2014 18:49:14 -0500 Subject: [PATCH] Reduce the number of semaphores used under --disable-spinlocks. Instead of allocating a semaphore from the operating system for every spinlock, allocate a fixed number of semaphores (by default, 1024) from the operating system and multiplex all the spinlocks that get created onto them. This could self-deadlock if a process attempted to acquire more than one spinlock at a time, but since processes aren't supposed to execute anything other than short stretches of straight-line code while holding a spinlock, that shouldn't happen. One motivation for this change is that, with the introduction of dynamic shared memory, it may be desirable to create spinlocks that last for less than the lifetime of the server. Without this change, attempting to use such facilities under --disable-spinlocks would quickly exhaust any supply of available semaphores. Quite apart from that, it's desirable to contain the quantity of semaphores needed to run the server simply on convenience grounds, since using too many may make it harder to get PostgreSQL running on a new platform, which is mostly the point of --disable-spinlocks in the first place. Patch by me; review by Tom Lane. --- src/backend/postmaster/postmaster.c | 9 ++++++ src/backend/storage/ipc/ipci.c | 1 + src/backend/storage/ipc/shmem.c | 21 ++++++++++-- src/backend/storage/lmgr/spin.c | 50 ++++++++++++++++++----------- src/include/pg_config_manual.h | 8 +++++ src/include/storage/s_lock.h | 2 +- src/include/storage/spin.h | 6 ++++ 7 files changed, 74 insertions(+), 23 deletions(-) diff --git a/src/backend/postmaster/postmaster.c b/src/backend/postmaster/postmaster.c index 377fa591a8..a7e40cc4a5 100644 --- a/src/backend/postmaster/postmaster.c +++ b/src/backend/postmaster/postmaster.c @@ -471,6 +471,9 @@ typedef struct slock_t *ShmemLock; VariableCache ShmemVariableCache; Backend *ShmemBackendArray; +#ifndef HAVE_SPINLOCKS + PGSemaphore SpinlockSemaArray; +#endif LWLock *LWLockArray; slock_t *ProcStructLock; PROC_HDR *ProcGlobal; @@ -5626,6 +5629,9 @@ save_backend_variables(BackendParameters *param, Port *port, param->ShmemVariableCache = ShmemVariableCache; param->ShmemBackendArray = ShmemBackendArray; +#ifndef HAVE_SPINLOCKS + param->SpinlockSemaArray = SpinlockSemaArray; +#endif param->LWLockArray = LWLockArray; param->ProcStructLock = ProcStructLock; param->ProcGlobal = ProcGlobal; @@ -5854,6 +5860,9 @@ restore_backend_variables(BackendParameters *param, Port *port) ShmemVariableCache = param->ShmemVariableCache; ShmemBackendArray = param->ShmemBackendArray; +#ifndef HAVE_SPINLOCKS + SpinlockSemaArray = param->SpinlockSemaArray; +#endif LWLockArray = param->LWLockArray; ProcStructLock = param->ProcStructLock; ProcGlobal = param->ProcGlobal; diff --git a/src/backend/storage/ipc/ipci.c b/src/backend/storage/ipc/ipci.c index 1c8783f2c0..3c04fc31ba 100644 --- a/src/backend/storage/ipc/ipci.c +++ b/src/backend/storage/ipc/ipci.c @@ -105,6 +105,7 @@ CreateSharedMemoryAndSemaphores(bool makePrivate, int port) * need to be so careful during the actual allocation phase. */ size = 100000; + size = add_size(size, SpinlockSemaSize()); size = add_size(size, hash_estimate_size(SHMEM_INDEX_SIZE, sizeof(ShmemIndexEnt))); size = add_size(size, BufferShmemSize()); diff --git a/src/backend/storage/ipc/shmem.c b/src/backend/storage/ipc/shmem.c index e93b988ad7..70b02ca838 100644 --- a/src/backend/storage/ipc/shmem.c +++ b/src/backend/storage/ipc/shmem.c @@ -116,9 +116,24 @@ InitShmemAllocation(void) Assert(shmhdr != NULL); /* - * Initialize the spinlock used by ShmemAlloc. We have to do the space - * allocation the hard way, since obviously ShmemAlloc can't be called - * yet. + * If spinlocks are disabled, initialize emulation layer. We have to do + * the space allocation the hard way, since obviously ShmemAlloc can't be + * called yet. + */ +#ifndef HAVE_SPINLOCKS + { + PGSemaphore spinsemas; + + spinsemas = (PGSemaphore) (((char *) shmhdr) + shmhdr->freeoffset); + shmhdr->freeoffset += MAXALIGN(SpinlockSemaSize()); + SpinlockSemaInit(spinsemas); + Assert(shmhdr->freeoffset <= shmhdr->totalsize); + } +#endif + + /* + * Initialize the spinlock used by ShmemAlloc; we have to do this the hard + * way, too, for the same reasons as above. */ ShmemLock = (slock_t *) (((char *) shmhdr) + shmhdr->freeoffset); shmhdr->freeoffset += MAXALIGN(sizeof(slock_t)); diff --git a/src/backend/storage/lmgr/spin.c b/src/backend/storage/lmgr/spin.c index 08782178de..3d116bc7a2 100644 --- a/src/backend/storage/lmgr/spin.c +++ b/src/backend/storage/lmgr/spin.c @@ -29,6 +29,18 @@ #include "storage/spin.h" +PGSemaphore SpinlockSemaArray; + +/* + * Report the amount of shared memory needed to store semaphores for spinlock + * support. + */ +Size +SpinlockSemaSize(void) +{ + return SpinlockSemas() * sizeof(PGSemaphoreData); +} + #ifdef HAVE_SPINLOCKS /* @@ -52,22 +64,20 @@ SpinlockSemas(void) int SpinlockSemas(void) { - int nsemas; - - /* - * It would be cleaner to distribute this logic into the affected modules, - * similar to the way shmem space estimation is handled. - * - * For now, though, there are few enough users of spinlocks that we just - * keep the knowledge here. - */ - nsemas = NumLWLocks(); /* one for each lwlock */ - nsemas += NBuffers; /* one for each buffer header */ - nsemas += max_wal_senders; /* one for each wal sender process */ - nsemas += num_xloginsert_slots; /* one for each WAL insertion slot */ - nsemas += 30; /* plus a bunch for other small-scale use */ - - return nsemas; + return NUM_SPINLOCK_SEMAPHORES; +} + +/* + * Initialize semaphores. + */ +extern void +SpinlockSemaInit(PGSemaphore spinsemas) +{ + int i; + + for (i = 0; i < NUM_SPINLOCK_SEMAPHORES; ++i) + PGSemaphoreCreate(&spinsemas[i]); + SpinlockSemaArray = spinsemas; } /* @@ -77,13 +87,15 @@ SpinlockSemas(void) void s_init_lock_sema(volatile slock_t *lock) { - PGSemaphoreCreate((PGSemaphore) lock); + static int counter = 0; + + *lock = (++counter) % NUM_SPINLOCK_SEMAPHORES; } void s_unlock_sema(volatile slock_t *lock) { - PGSemaphoreUnlock((PGSemaphore) lock); + PGSemaphoreUnlock(&SpinlockSemaArray[*lock]); } bool @@ -98,7 +110,7 @@ int tas_sema(volatile slock_t *lock) { /* Note that TAS macros return 0 if *success* */ - return !PGSemaphoreTryLock((PGSemaphore) lock); + return !PGSemaphoreTryLock(&SpinlockSemaArray[*lock]); } #endif /* !HAVE_SPINLOCKS */ diff --git a/src/include/pg_config_manual.h b/src/include/pg_config_manual.h index 2387a434aa..20c5ff0e90 100644 --- a/src/include/pg_config_manual.h +++ b/src/include/pg_config_manual.h @@ -56,6 +56,14 @@ */ #define NUM_USER_DEFINED_LWLOCKS 4 +/* + * When we don't have native spinlocks, we use semaphores to simulate them. + * Decreasing this value reduces consumption of OS resources; increasing it + * may improve performance, but supplying a real spinlock implementation is + * probably far better. + */ +#define NUM_SPINLOCK_SEMAPHORES 1024 + /* * Define this if you want to allow the lo_import and lo_export SQL * functions to be executed by ordinary users. By default these diff --git a/src/include/storage/s_lock.h b/src/include/storage/s_lock.h index b52f0e7f85..2297f77280 100644 --- a/src/include/storage/s_lock.h +++ b/src/include/storage/s_lock.h @@ -915,7 +915,7 @@ spin_delay(void) * to fall foul of kernel limits on number of semaphores, so don't use this * unless you must! The subroutines appear in spin.c. */ -typedef PGSemaphoreData slock_t; +typedef int slock_t; extern bool s_lock_free_sema(volatile slock_t *lock); extern void s_unlock_sema(volatile slock_t *lock); diff --git a/src/include/storage/spin.h b/src/include/storage/spin.h index e720165249..2ac510db7a 100644 --- a/src/include/storage/spin.h +++ b/src/include/storage/spin.h @@ -69,5 +69,11 @@ extern int SpinlockSemas(void); +extern Size SpinlockSemaSize(void); + +#ifndef HAVE_SPINLOCKS +extern void SpinlockSemaInit(PGSemaphore); +extern PGSemaphore SpinlockSemaArray; +#endif #endif /* SPIN_H */ -- 2.40.0