Rethink prior patch to filter out dead backend entries from the pgstats

file.  The original code probed the PGPROC array separately for each PID,
which was not good for large numbers of backends: not only is the runtime
O(N^2) but most of it is spent holding ProcArrayLock.  Instead, take the
lock just once and copy the active PIDs into an array, then use qsort
and bsearch so that the lookup time is more like O(N log N).

1 files changed, 25 insertions, 5 deletions

diff --git a/src/backend/postmaster/pgstat.c b/src/backend/postmaster/pgstat.c
index 82cf1bc4ef1..ae20a3bb710 100644
--- a/src/backend/postmaster/pgstat.c
+++ b/src/backend/postmaster/pgstat.c
@@ -13,7 +13,7 @@
  *
  *	Copyright (c) 2001-2005, PostgreSQL Global Development Group
  *
- *	$PostgreSQL: pgsql/src/backend/postmaster/pgstat.c,v 1.112 2005/11/22 18:17:17 momjian Exp $
+ *	$PostgreSQL: pgsql/src/backend/postmaster/pgstat.c,v 1.113 2005/12/16 04:03:40 tgl Exp $
  * ----------
  */
 #include "postgres.h"
@@ -2453,6 +2453,16 @@ pgstat_write_statsfile(void)
 	}
 }
 
+/*
+ * qsort/bsearch comparison routine for PIDs
+ *
+ * We assume PIDs are nonnegative, so there's no overflow risk
+ */
+static int
+comparePids(const void *v1, const void *v2)
+{
+	return *((const int *) v1) - *((const int *) v2);
+}
 
 /* ----------
  * pgstat_read_statsfile() -
@@ -2478,7 +2488,7 @@ pgstat_read_statsfile(HTAB **dbhash, Oid onlydb,
 	int			maxbackends = 0;
 	int			havebackends = 0;
 	bool		found;
-	bool		check_pids;
+	int		   *live_pids;
 	MemoryContext use_mcxt;
 	int			mcxt_flags;
 
@@ -2497,13 +2507,17 @@ pgstat_read_statsfile(HTAB **dbhash, Oid onlydb,
 	{
 		use_mcxt = NULL;
 		mcxt_flags = 0;
-		check_pids = false;
+		live_pids = NULL;
 	}
 	else
 	{
 		use_mcxt = TopTransactionContext;
 		mcxt_flags = HASH_CONTEXT;
-		check_pids = true;
+		live_pids = GetAllBackendPids();
+		/* Sort the PID array so we can use bsearch */
+		if (live_pids[0] > 1)
+			qsort((void *) &live_pids[1], live_pids[0], sizeof(int),
+				  comparePids);
 	}
 
 	/*
@@ -2706,7 +2720,13 @@ pgstat_read_statsfile(HTAB **dbhash, Oid onlydb,
 				/*
 				 * If possible, check PID to verify still running
 				 */
-				if (check_pids && !IsBackendPid(beentry->procpid))
+				if (live_pids &&
+					(live_pids[0] == 0 ||
+					 bsearch((void *) &beentry->procpid,
+							 (void *) &live_pids[1],
+							 live_pids[0],
+							 sizeof(int),
+							 comparePids) == NULL))
 				{
 					/*
 					 * Note: we could send a BETERM message to tell the