Avoid unnecessary process wakeups in the log collector.

syslogger was coded to wake up once per second whether there was anything
useful to do or not.  As part of our campaign to reduce the server's idle
power consumption, change it to use a latch for waiting.  Now, in the
absence of any data to log or any signals to service, it will only wake up
at the programmed logfile rotation times (if any).

src/backend/postmaster/syslogger.c

@@ -9,8 +9,7 @@
  * in postgresql.conf. If these limits are reached or passed, the
  * current logfile is closed and a new one is created (rotated).
  * The logfiles are stored in a subdirectory (configurable in
- * postgresql.conf), using an internal naming scheme that mangles
- * creation time and current postmaster pid.
+ * postgresql.conf), using a user-selectable naming scheme.
  *
  * Author: Andreas Pflug <pgadmin@pse-consulting.de>
  *
@@ -40,6 +39,7 @@
 #include "postmaster/postmaster.h"
 #include "postmaster/syslogger.h"
 #include "storage/ipc.h"
+#include "storage/latch.h"
 #include "storage/pg_shmem.h"
 #include "utils/guc.h"
 #include "utils/ps_status.h"
@@ -93,6 +93,7 @@ static FILE *syslogFile = NULL;
 static FILE *csvlogFile = NULL;
 static char *last_file_name = NULL;
 static char *last_csv_file_name = NULL;
+static Latch sysLoggerLatch;
 
 /*
  * Buffers for saving partial messages from different backends.
@@ -168,12 +169,14 @@ SysLoggerMain(int argc, char *argv[])
 	char	   *currentLogDir;
 	char	   *currentLogFilename;
 	int			currentLogRotationAge;
+	pg_time_t	now;
 
 	IsUnderPostmaster = true;	/* we are a postmaster subprocess now */
 
 	MyProcPid = getpid();		/* reset MyProcPid */
 
 	MyStartTime = time(NULL);	/* set our start time in case we call elog */
+	now = MyStartTime;
 
 #ifdef EXEC_BACKEND
 	syslogger_parseArgs(argc, argv);
@@ -246,6 +249,9 @@ SysLoggerMain(int argc, char *argv[])
 		elog(FATAL, "setsid() failed: %m");
 #endif
 
+	/* Initialize private latch for use by signal handlers */
+	InitLatch(&sysLoggerLatch);
+
 	/*
 	 * Properly accept or ignore signals the postmaster might send us
 	 *
@@ -296,14 +302,19 @@ SysLoggerMain(int argc, char *argv[])
 	{
 		bool		time_based_rotation = false;
 		int			size_rotation_for = 0;
+		long		cur_timeout;
+		int			cur_flags;
 
 #ifndef WIN32
-		int			bytesRead;
 		int			rc;
-		fd_set		rfds;
-		struct timeval timeout;
 #endif
 
+		/* Clear any already-pending wakeups */
+		ResetLatch(&sysLoggerLatch);
+
+		/*
+		 * Process any requests or signals received recently.
+		 */
 		if (got_SIGHUP)
 		{
 			got_SIGHUP = false;
@@ -353,11 +364,10 @@ SysLoggerMain(int argc, char *argv[])
 			}
 		}
 
-		if (!rotation_requested && Log_RotationAge > 0 && !rotation_disabled)
+		if (Log_RotationAge > 0 && !rotation_disabled)
 		{
 			/* Do a logfile rotation if it's time */
-			pg_time_t	now = (pg_time_t) time(NULL);
-
+			now = (pg_time_t) time(NULL);
 			if (now >= next_rotation_time)
 				rotation_requested = time_based_rotation = true;
 		}
@@ -389,28 +399,40 @@ SysLoggerMain(int argc, char *argv[])
 			logfile_rotate(time_based_rotation, size_rotation_for);
 		}
 
-#ifndef WIN32
-
 		/*
-		 * Wait for some data, timing out after 1 second
+		 * Calculate time till next time-based rotation, so that we don't
+		 * sleep longer than that.  We assume the value of "now" obtained
+		 * above is still close enough.  Note we can't make this calculation
+		 * until after calling logfile_rotate(), since it will advance
+		 * next_rotation_time.
 		 */
-		FD_ZERO(&rfds);
-		FD_SET(syslogPipe[0], &rfds);
-
-		timeout.tv_sec = 1;
-		timeout.tv_usec = 0;
-
-		rc = select(syslogPipe[0] + 1, &rfds, NULL, NULL, &timeout);
-
-		if (rc < 0)
+		if (Log_RotationAge > 0 && !rotation_disabled)
 		{
-			if (errno != EINTR)
-				ereport(LOG,
-						(errcode_for_socket_access(),
-						 errmsg("select() failed in logger process: %m")));
+			if (now < next_rotation_time)
+				cur_timeout = (next_rotation_time - now) * 1000L; /* msec */
+			else
+				cur_timeout = 0;
+			cur_flags = WL_TIMEOUT;
+		}
+		else
+		{
+			cur_timeout = -1L;
+			cur_flags = 0;
 		}
-		else if (rc > 0 && FD_ISSET(syslogPipe[0], &rfds))
+
+		/*
+		 * Sleep until there's something to do
+		 */
+#ifndef WIN32
+		rc = WaitLatchOrSocket(&sysLoggerLatch,
+							   WL_LATCH_SET | WL_SOCKET_READABLE | cur_flags,
+							   syslogPipe[0],
+							   cur_timeout);
+
+		if (rc & WL_SOCKET_READABLE)
 		{
+			int			bytesRead;
+
 			bytesRead = read(syslogPipe[0],
 							 logbuffer + bytes_in_logbuffer,
 							 sizeof(logbuffer) - bytes_in_logbuffer);
@@ -445,8 +467,8 @@ SysLoggerMain(int argc, char *argv[])
 
 		/*
 		 * On Windows we leave it to a separate thread to transfer data and
-		 * detect pipe EOF.  The main thread just wakes up once a second to
-		 * check for SIGHUP and rotation conditions.
+		 * detect pipe EOF.  The main thread just wakes up to handle SIGHUP
+		 * and rotation conditions.
 		 *
 		 * Server code isn't generally thread-safe, so we ensure that only one
 		 * of the threads is active at a time by entering the critical section
@@ -454,7 +476,9 @@ SysLoggerMain(int argc, char *argv[])
 		 */
 		LeaveCriticalSection(&sysloggerSection);
 
-		pg_usleep(1000000L);
+		(void) WaitLatch(&sysLoggerLatch,
+						 WL_LATCH_SET | cur_flags,
+						 cur_timeout);
 
 		EnterCriticalSection(&sysloggerSection);
 #endif   /* WIN32 */
@@ -957,7 +981,7 @@ write_syslogger_file(const char *buffer, int count, int destination)
 /*
  * Worker thread to transfer data from the pipe to the current logfile.
  *
- * We need this because on Windows, WaitForSingleObject does not work on
+ * We need this because on Windows, WaitforMultipleObjects does not work on
  * unnamed pipes: it always reports "signaled", so the blocking ReadFile won't
  * allow for SIGHUP; and select is for sockets only.
  */
@@ -1010,6 +1034,9 @@ pipeThread(void *arg)
 	/* if there's any data left then force it out now */
 	flush_pipe_input(logbuffer, &bytes_in_logbuffer);
 
+	/* set the latch to waken the main thread, which will quit */
+	SetLatch(&sysLoggerLatch);
+
 	LeaveCriticalSection(&sysloggerSection);
 	_endthread();
 	return 0;
@@ -1285,12 +1312,22 @@ set_next_rotation_time(void)
 static void
 sigHupHandler(SIGNAL_ARGS)
 {
+	int			save_errno = errno;
+
 	got_SIGHUP = true;
+	SetLatch(&sysLoggerLatch);
+
+	errno = save_errno;
 }
 
 /* SIGUSR1: set flag to rotate logfile */
 static void
 sigUsr1Handler(SIGNAL_ARGS)
 {
+	int			save_errno = errno;
+
 	rotation_requested = true;
+	SetLatch(&sysLoggerLatch);
+
+	errno = save_errno;
 }