Reduce the number of semaphores used under --disable-spinlocks.

robertmhaas · robertmhaas · commit daa7527afc22 · 2014-01-08T18:58:00.000-05:00
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.
diff --git 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
@@ -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
@@ -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
@@ -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
@@ -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
@@ -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
@@ -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 */
