Use a long lived WaitEventSet for WaitLatch().

Create LatchWaitSet at backend startup time, and use it to implement
WaitLatch().  This avoids repeated epoll/kqueue setup and teardown
system calls.

Reorder SubPostmasterMain() slightly so that we restore the postmaster
pipe and Windows signal emulation before we reach InitPostmasterChild(),
to make this work in EXEC_BACKEND builds.

Reviewed-by: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Discussion: https://postgr.es/m/CA%2BhUKGJAC4Oqao%3DqforhNey20J8CiG2R%3DoBPqvfR0vOJrFysGw%40mail.gmail.com

src/backend/postmaster/postmaster.c

@@ -4896,9 +4896,6 @@ SubPostmasterMain(int argc, char *argv[])
 	IsPostmasterEnvironment = true;
 	whereToSendOutput = DestNone;
 
-	/* Setup as postmaster child */
-	InitPostmasterChild();
-
 	/* Setup essential subsystems (to ensure elog() behaves sanely) */
 	InitializeGUCOptions();
 
@@ -4913,6 +4910,18 @@ SubPostmasterMain(int argc, char *argv[])
 	/* Close the postmaster's sockets (as soon as we know them) */
 	ClosePostmasterPorts(strcmp(argv[1], "--forklog") == 0);
 
+	/*
+	 * Start our win32 signal implementation. This has to be done after we
+	 * read the backend variables, because we need to pick up the signal pipe
+	 * from the parent process.
+	 */
+#ifdef WIN32
+	pgwin32_signal_initialize();
+#endif
+
+	/* Setup as postmaster child */
+	InitPostmasterChild();
+
 	/*
 	 * Set up memory area for GSS information. Mirrors the code in ConnCreate
 	 * for the non-exec case.
@@ -4956,15 +4965,6 @@ SubPostmasterMain(int argc, char *argv[])
 	if (strcmp(argv[1], "--forkavworker") == 0)
 		AutovacuumWorkerIAm();
 
-	/*
-	 * Start our win32 signal implementation. This has to be done after we
-	 * read the backend variables, because we need to pick up the signal pipe
-	 * from the parent process.
-	 */
-#ifdef WIN32
-	pgwin32_signal_initialize();
-#endif
-
 	/* In EXEC_BACKEND case we will not have inherited these settings */
 	pqinitmask();
 	PG_SETMASK(&BlockSig);

src/backend/storage/ipc/latch.c

@@ -56,6 +56,7 @@
 #include "storage/latch.h"
 #include "storage/pmsignal.h"
 #include "storage/shmem.h"
+#include "utils/memutils.h"
 
 /*
  * Select the fd readiness primitive to use. Normally the "most modern"
@@ -129,6 +130,12 @@ struct WaitEventSet
 #endif
 };
 
+/* A common WaitEventSet used to implement WatchLatch() */
+static WaitEventSet *LatchWaitSet;
+
+/* The position of the latch in LatchWaitSet. */
+#define LatchWaitSetLatchPos 0
+
 #ifndef WIN32
 /* Are we currently in WaitLatch? The signal handler would like to know. */
 static volatile sig_atomic_t waiting = false;
@@ -242,6 +249,24 @@ InitializeLatchSupport(void)
 #endif
 }
 
+void
+InitializeLatchWaitSet(void)
+{
+	int			latch_pos PG_USED_FOR_ASSERTS_ONLY;
+
+	Assert(LatchWaitSet == NULL);
+
+	/* Set up the WaitEventSet used by WaitLatch(). */
+	LatchWaitSet = CreateWaitEventSet(TopMemoryContext, 2);
+	latch_pos = AddWaitEventToSet(LatchWaitSet, WL_LATCH_SET, PGINVALID_SOCKET,
+								  MyLatch, NULL);
+	if (IsUnderPostmaster)
+		AddWaitEventToSet(LatchWaitSet, WL_EXIT_ON_PM_DEATH,
+						  PGINVALID_SOCKET, NULL, NULL);
+
+	Assert(latch_pos == LatchWaitSetLatchPos);
+}
+
 /*
  * Initialize a process-local latch.
  */
@@ -365,8 +390,31 @@ int
 WaitLatch(Latch *latch, int wakeEvents, long timeout,
 		  uint32 wait_event_info)
 {
-	return WaitLatchOrSocket(latch, wakeEvents, PGINVALID_SOCKET, timeout,
-							 wait_event_info);
+	WaitEvent	event;
+
+	/* Postmaster-managed callers must handle postmaster death somehow. */
+	Assert(!IsUnderPostmaster ||
+		   (wakeEvents & WL_EXIT_ON_PM_DEATH) ||
+		   (wakeEvents & WL_POSTMASTER_DEATH));
+
+	/*
+	 * Some callers may have a latch other than MyLatch, or no latch at all,
+	 * or want to handle postmaster death differently.  It's cheap to assign
+	 * those, so just do it every time.
+	 */
+	if (!(wakeEvents & WL_LATCH_SET))
+		latch = NULL;
+	ModifyWaitEvent(LatchWaitSet, LatchWaitSetLatchPos, WL_LATCH_SET, latch);
+	LatchWaitSet->exit_on_postmaster_death =
+		((wakeEvents & WL_EXIT_ON_PM_DEATH) != 0);
+
+	if (WaitEventSetWait(LatchWaitSet,
+						 (wakeEvents & WL_TIMEOUT) ? timeout : -1,
+						 &event, 1,
+						 wait_event_info) == 0)
+		return WL_TIMEOUT;
+	else
+		return event.events;
 }
 
 /*
@@ -830,7 +878,8 @@ AddWaitEventToSet(WaitEventSet *set, uint32 events, pgsocket fd, Latch *latch,
 
 /*
  * Change the event mask and, in the WL_LATCH_SET case, the latch associated
- * with the WaitEvent.
+ * with the WaitEvent.  The latch may be changed to NULL to disable the latch
+ * temporarily, and then set back to a latch later.
  *
  * 'pos' is the id returned by AddWaitEventToSet.
  */
@@ -862,7 +911,6 @@ ModifyWaitEvent(WaitEventSet *set, int pos, uint32 events, Latch *latch)
 	if (event->events & WL_LATCH_SET &&
 		events != event->events)
 	{
-		/* we could allow to disable latch events for a while */
 		elog(ERROR, "cannot modify latch event");
 	}
 

src/backend/utils/init/miscinit.c

@@ -120,6 +120,7 @@ InitPostmasterChild(void)
 	InitializeLatchSupport();
 	MyLatch = &LocalLatchData;
 	InitLatch(MyLatch);
+	InitializeLatchWaitSet();
 
 	/*
 	 * If possible, make this process a group leader, so that the postmaster
@@ -152,6 +153,7 @@ InitStandaloneProcess(const char *argv0)
 	InitializeLatchSupport();
 	MyLatch = &LocalLatchData;
 	InitLatch(MyLatch);
+	InitializeLatchWaitSet();
 
 	/* Compute paths, no postmaster to inherit from */
 	if (my_exec_path[0] == '\0')

src/include/storage/latch.h

@@ -176,6 +176,7 @@ extern int	WaitLatch(Latch *latch, int wakeEvents, long timeout,
 					  uint32 wait_event_info);
 extern int	WaitLatchOrSocket(Latch *latch, int wakeEvents,
 							  pgsocket sock, long timeout, uint32 wait_event_info);
+extern void InitializeLatchWaitSet(void);
 
 /*
  * Unix implementation uses SIGUSR1 for inter-process signaling.