Allow maximum number of backends to be set at configure time

(--with-maxbackends).  Add a postmaster switch (-N backends) that allows
the limit to be reduced at postmaster start time.  (You can't increase it,
sorry to say, because there are still some fixed-size arrays.)
Grab the number of semaphores indicated by min(MAXBACKENDS, -N) at
postmaster startup, so that this particular form of bogus configuration
is exposed immediately rather than under heavy load.

4 files changed, 74 insertions, 33 deletions

diff --git a/src/backend/storage/ipc/ipci.c b/src/backend/storage/ipc/ipci.c
index 0a75af6c3e..fb27b92d22 100644
--- a/src/backend/storage/ipc/ipci.c
+++ b/src/backend/storage/ipc/ipci.c
@@ -7,7 +7,7 @@
  *
  *
  * IDENTIFICATION
- *	  $Header: /cvsroot/pgsql/src/backend/storage/ipc/ipci.c,v 1.18 1999/02/13 23:18:11 momjian Exp $
+ *	  $Header: /cvsroot/pgsql/src/backend/storage/ipc/ipci.c,v 1.19 1999/02/19 06:06:04 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -44,17 +44,19 @@ SystemPortAddressCreateIPCKey(SystemPortAddress address)
 
   CreateSharedMemoryAndSemaphores
   is called exactly *ONCE* by the postmaster.
-  It is *NEVER* called by the postgres backend
+  It is *NEVER* called by the postgres backend,
+  except in the case of a standalone backend.
 
   0) destroy any existing semaphores for both buffer
   and lock managers.
   1) create the appropriate *SHARED* memory segments
   for the two resource managers.
+  2) create shared semaphores as needed.
 
   **************************************************/
 
 void
-CreateSharedMemoryAndSemaphores(IPCKey key)
+CreateSharedMemoryAndSemaphores(IPCKey key, int maxBackends)
 {
 	int			size;
 
@@ -98,7 +100,7 @@ CreateSharedMemoryAndSemaphores(IPCKey key)
 	 *	do process table stuff
 	 * ----------------
 	 */
-	InitProcGlobal(key);
+	InitProcGlobal(key, maxBackends);
 	on_shmem_exit(ProcFreeAllSemaphores, NULL);
 
 	CreateSharedInvalidationState(key);
@@ -120,7 +122,7 @@ AttachSharedMemoryAndSemaphores(IPCKey key)
 	 */
 	if (key == PrivateIPCKey)
 	{
-		CreateSharedMemoryAndSemaphores(key);
+		CreateSharedMemoryAndSemaphores(key, 1);
 		return;
 	}
 
diff --git a/src/backend/storage/ipc/sinvaladt.c b/src/backend/storage/ipc/sinvaladt.c
index 0eee0c8022..c5dcea61ae 100644
--- a/src/backend/storage/ipc/sinvaladt.c
+++ b/src/backend/storage/ipc/sinvaladt.c
@@ -7,7 +7,7 @@
  *
  *
  * IDENTIFICATION
- *	  $Header: /cvsroot/pgsql/src/backend/storage/ipc/sinvaladt.c,v 1.16 1999/02/13 23:18:16 momjian Exp $
+ *	  $Header: /cvsroot/pgsql/src/backend/storage/ipc/sinvaladt.c,v 1.17 1999/02/19 06:06:03 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -119,7 +119,7 @@ SIAssignBackendId(SISeg *segInOutP, BackendTag backendTag)
 
 	stateP = NULL;
 
-	for (index = 0; index < MaxBackendId; index += 1)
+	for (index = 0; index < MAXBACKENDS; index++)
 	{
 		if (segInOutP->procState[index].tag == InvalidBackendTag ||
 			segInOutP->procState[index].tag == backendTag)
@@ -141,7 +141,7 @@ SIAssignBackendId(SISeg *segInOutP, BackendTag backendTag)
 
 	/* verify that all "procState" entries checked for matching tags */
 
-	for (index += 1; index < MaxBackendId; index += 1)
+	for (index++; index < MAXBACKENDS; index++)
 	{
 		if (segInOutP->procState[index].tag == backendTag)
 		{
@@ -565,7 +565,7 @@ SIDecProcLimit(SISeg *segP, int num)
 {
 	int			i;
 
-	for (i = 0; i < MaxBackendId; i++)
+	for (i = 0; i < MAXBACKENDS; i++)
 	{
 		/* decrement only, if there is a limit > 0	*/
 		if (segP->procState[i].limit > 0)
@@ -622,7 +622,7 @@ SISetProcStateInvalid(SISeg *segP)
 {
 	int			i;
 
-	for (i = 0; i < MaxBackendId; i++)
+	for (i = 0; i < MAXBACKENDS; i++)
 	{
 		if (segP->procState[i].limit == 0)
 		{
@@ -696,7 +696,7 @@ SIDelExpiredDataEntries(SISeg *segP)
 				h;
 
 	min = 9999999;
-	for (i = 0; i < MaxBackendId; i++)
+	for (i = 0; i < MAXBACKENDS; i++)
 	{
 		h = SIGetProcStateLimit(segP, i);
 		if (h >= 0)
@@ -740,7 +740,7 @@ SISegInit(SISeg *segP)
 	SISetEndEntryChain(segP, InvalidOffset);
 	SISetNumEntries(segP, 0);
 	SISetMaxNumEntries(segP, MAXNUMMESSAGES);
-	for (i = 0; i < MaxBackendId; i++)
+	for (i = 0; i < MAXBACKENDS; i++)
 	{
 		segP->procState[i].limit = -1;	/* no backend active  !! */
 		segP->procState[i].resetState = false;
diff --git a/src/backend/storage/lmgr/lock.c b/src/backend/storage/lmgr/lock.c
index 39fac457a1..06f6fe7b4d 100644
--- a/src/backend/storage/lmgr/lock.c
+++ b/src/backend/storage/lmgr/lock.c
@@ -7,7 +7,7 @@
  *
  *
  * IDENTIFICATION
- *	  $Header: /cvsroot/pgsql/src/backend/storage/lmgr/lock.c,v 1.41 1999/02/13 23:18:25 momjian Exp $
+ *	  $Header: /cvsroot/pgsql/src/backend/storage/lmgr/lock.c,v 1.42 1999/02/19 06:06:06 tgl Exp $
  *
  * NOTES
  *	  Outside modules can create a lock table and acquire/release
@@ -1492,8 +1492,8 @@ LockShmemSize()
 	nXidBuckets = 1 << (int) my_log2((NLOCKS_PER_XACT - 1) / DEF_FFACTOR + 1);
 	nXidSegs = 1 << (int) my_log2((nLockBuckets - 1) / DEF_SEGSIZE + 1);
 
-	size += MAXALIGN(NBACKENDS * sizeof(PROC)); /* each MyProc */
-	size += MAXALIGN(NBACKENDS * sizeof(LOCKMETHODCTL));		/* each
+	size += MAXALIGN(MAXBACKENDS * sizeof(PROC)); /* each MyProc */
+	size += MAXALIGN(MAXBACKENDS * sizeof(LOCKMETHODCTL));		/* each
 																 * lockMethodTable->ctl */
 	size += MAXALIGN(sizeof(PROC_HDR)); /* ProcGlobal */
 
@@ -1504,10 +1504,10 @@ LockShmemSize()
 		(MAXALIGN(sizeof(BUCKET_INDEX)) +
 		 MAXALIGN(sizeof(LOCK)));		/* contains hash key */
 
-	size += MAXALIGN(my_log2(NBACKENDS) * sizeof(void *));
+	size += MAXALIGN(my_log2(MAXBACKENDS) * sizeof(void *));
 	size += MAXALIGN(sizeof(HHDR));
 	size += nXidSegs * MAXALIGN(DEF_SEGSIZE * sizeof(SEGMENT));
-	size += NBACKENDS *			/* XXX not multiple of BUCKET_ALLOC_INCR? */
+	size += MAXBACKENDS *		/* XXX not multiple of BUCKET_ALLOC_INCR? */
 		(MAXALIGN(sizeof(BUCKET_INDEX)) +
 		 MAXALIGN(sizeof(XIDLookupEnt)));		/* contains hash key */
 
@@ -1552,7 +1552,7 @@ DeadLockCheck(SHM_QUEUE *lockQueue, LOCK *findlock, bool skip_check)
 	HTAB	   *xidTable;
 	bool		found;
 
-	static PROC *checked_procs[MaxBackendId];
+	static PROC *checked_procs[MAXBACKENDS];
 	static int	nprocs;
 	static bool MyNHolding;
 
@@ -1674,7 +1674,7 @@ DeadLockCheck(SHM_QUEUE *lockQueue, LOCK *findlock, bool skip_check)
 					if (j >= nprocs && lock != findlock)
 					{
 						checked_procs[nprocs++] = proc;
-						Assert(nprocs <= MaxBackendId);
+						Assert(nprocs <= MAXBACKENDS);
 
 						/*
 						 * For non-MyProc entries, we are looking only
diff --git a/src/backend/storage/lmgr/proc.c b/src/backend/storage/lmgr/proc.c
index de09b4a5b0..ae8fb17fca 100644
--- a/src/backend/storage/lmgr/proc.c
+++ b/src/backend/storage/lmgr/proc.c
@@ -7,7 +7,7 @@
  *
  *
  * IDENTIFICATION
- *	  $Header: /cvsroot/pgsql/src/backend/storage/lmgr/proc.c,v 1.48 1999/02/13 23:18:28 momjian Exp $
+ *	  $Header: /cvsroot/pgsql/src/backend/storage/lmgr/proc.c,v 1.49 1999/02/19 06:06:08 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -46,7 +46,7 @@
  *		This is so that we can support more backends. (system-wide semaphore
  *		sets run out pretty fast.)				  -ay 4/95
  *
- * $Header: /cvsroot/pgsql/src/backend/storage/lmgr/proc.c,v 1.48 1999/02/13 23:18:28 momjian Exp $
+ * $Header: /cvsroot/pgsql/src/backend/storage/lmgr/proc.c,v 1.49 1999/02/19 06:06:08 tgl Exp $
  */
 #include <sys/time.h>
 #include <unistd.h>
@@ -108,13 +108,24 @@ static void ProcFreeSem(IpcSemaphoreKey semKey, int semNum);
 /*
  * InitProcGlobal -
  *	  initializes the global process table. We put it here so that
- *	  the postmaster can do this initialization. (ProcFreeAllSem needs
+ *	  the postmaster can do this initialization. (ProcFreeAllSemaphores needs
  *	  to read this table on exiting the postmaster. If we have the first
  *	  backend do this, starting up and killing the postmaster without
  *	  starting any backends will be a problem.)
+ *
+ *	  We also allocate all the per-process semaphores we will need to support
+ *	  the requested number of backends.  We used to allocate semaphores
+ *	  only when backends were actually started up, but that is bad because
+ *	  it lets Postgres fail under load --- a lot of Unix systems are
+ *	  (mis)configured with small limits on the number of semaphores, and
+ *	  running out when trying to start another backend is a common failure.
+ *	  So, now we grab enough semaphores to support the desired max number
+ *	  of backends immediately at initialization --- if the sysadmin has set
+ *	  MaxBackends higher than his kernel will support, he'll find out sooner
+ *	  rather than later.
  */
 void
-InitProcGlobal(IPCKey key)
+InitProcGlobal(IPCKey key, int maxBackends)
 {
 	bool		found = false;
 
@@ -134,6 +145,24 @@ InitProcGlobal(IPCKey key)
 		ProcGlobal->currKey = IPCGetProcessSemaphoreInitKey(key);
 		for (i = 0; i < MAX_PROC_SEMS / PROC_NSEMS_PER_SET; i++)
 			ProcGlobal->freeSemMap[i] = 0;
+		/* Pre-create the semaphores for the first maxBackends processes */
+		for (i = 0;
+			 i < (maxBackends+PROC_NSEMS_PER_SET-1) / PROC_NSEMS_PER_SET;
+			 i++)
+		{
+			IPCKey		semKey = ProcGlobal->currKey + i;
+			int			semId;
+			int			semstat;
+
+			semId = IpcSemaphoreCreate(semKey,
+									   PROC_NSEMS_PER_SET,
+									   IPCProtection,
+									   IpcSemaphoreDefaultStartValue,
+									   0,
+									   &semstat);
+			/* mark this sema set allocated */
+			ProcGlobal->freeSemMap[i] = (1 << PROC_NSEMS_PER_SET);
+		}
 	}
 }
 
@@ -222,6 +251,11 @@ InitProcess(IPCKey key)
 
 		ProcGetNewSemKeyAndNum(&semKey, &semNum);
 
+		/* Note: because of the pre-allocation done in InitProcGlobal,
+		 * this call should always attach to an existing semaphore.
+		 * It will (try to) create a new group of semaphores only if
+		 * the postmaster tries to start more backends than it said it would.
+		 */
 		semId = IpcSemaphoreCreate(semKey,
 								   PROC_NSEMS_PER_SET,
 								   IPCProtection,
@@ -823,20 +857,20 @@ ProcGetNewSemKeyAndNum(IPCKey *key, int *semNum)
 {
 	int			i;
 	int32	   *freeSemMap = ProcGlobal->freeSemMap;
-	unsigned int fullmask;
+	int32		fullmask = (1 << (PROC_NSEMS_PER_SET+1)) - 1;
 
 	/*
 	 * we hold ProcStructLock when entering this routine. We scan through
 	 * the bitmap to look for a free semaphore.
 	 */
-	fullmask = ~0 >> (32 - PROC_NSEMS_PER_SET);
+
 	for (i = 0; i < MAX_PROC_SEMS / PROC_NSEMS_PER_SET; i++)
 	{
 		int			mask = 1;
 		int			j;
 
 		if (freeSemMap[i] == fullmask)
-			continue;			/* none free for this set */
+			continue;			/* this set is fully allocated */
 
 		for (j = 0; j < PROC_NSEMS_PER_SET; j++)
 		{
@@ -845,8 +879,9 @@ ProcGetNewSemKeyAndNum(IPCKey *key, int *semNum)
 
 				/*
 				 * a free semaphore found. Mark it as allocated.
+				 * Also set the bit indicating whole set is allocated.
 				 */
-				freeSemMap[i] |= mask;
+				freeSemMap[i] |= mask + (1 << PROC_NSEMS_PER_SET);
 
 				*key = ProcGlobal->currKey + i;
 				*semNum = j;
@@ -862,8 +897,7 @@ ProcGetNewSemKeyAndNum(IPCKey *key, int *semNum)
 
 /*
  * ProcFreeSem -
- *	  free up our semaphore in the semaphore set. If we're the last one
- *	  in the set, also remove the semaphore set.
+ *	  free up our semaphore in the semaphore set.
  */
 static void
 ProcFreeSem(IpcSemaphoreKey semKey, int semNum)
@@ -876,14 +910,19 @@ ProcFreeSem(IpcSemaphoreKey semKey, int semNum)
 	mask = ~(1 << semNum);
 	freeSemMap[i] &= mask;
 
-	if (freeSemMap[i] == 0)
-		IpcSemaphoreKill(semKey);
+	/* Formerly we'd release a semaphore set if it was now completely unused,
+	 * but now we keep the semaphores to ensure we won't run out when
+	 * starting new backends --- cf. InitProcGlobal.  Note that the
+	 * PROC_NSEMS_PER_SET+1'st bit of the freeSemMap entry remains set to
+	 * indicate it is still allocated; ProcFreeAllSemaphores() needs that.
+	 */
 }
 
 /*
  * ProcFreeAllSemaphores -
- *	  on exiting the postmaster, we free up all the semaphores allocated
- *	  to the lmgrs of the backends.
+ *	  called at shmem_exit time, ie when exiting the postmaster or
+ *	  destroying shared state for a failed set of backends.
+ *	  Free up all the semaphores allocated to the lmgrs of the backends.
  */
 void
 ProcFreeAllSemaphores()