Poll postmaster less frequently in recovery.

Since commits 9f09529 and f98b847 we don't poll the postmaster
pipe at all during crash recovery on Linux and FreeBSD, but on other
operating systems we were still doing it for every WAL record.  Do it
less frequently on operating systems where system calls are required, at
the cost of delaying exit a bit after postmaster death.  This avoids
expensive system calls reported to slow down CPU-bound recovery by as
much as 10-30%.

Reviewed-by: Heikki Linnakangas <hlinnaka@iki.fi>
Reviewed-by: Fujii Masao <masao.fujii@oss.nttdata.com>
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Discussion: https://postgr.es/m/CA%2BhUKGK1607VmtrDUHQXrsooU%3Dap4g4R2yaoByWOOA3m8xevUQ%40mail.gmail.com
Discussion: https://postgr.es/m/7261eb39-0369-f2f4-1bb5-62f3b6083b5e@iki.fi

Author: macdice

src/backend/postmaster/startup.c

@@ -34,6 +34,17 @@
 #include "utils/timeout.h"
 
 
+#ifndef USE_POSTMASTER_DEATH_SIGNAL
+/*
+ * On systems that need to make a system call to find out if the postmaster has
+ * gone away, we'll do so only every Nth call to HandleStartupProcInterrupts().
+ * This only affects how long it takes us to detect the condition while we're
+ * busy replaying WAL.  Latch waits and similar which should react immediately
+ * through the usual techniques.
+ */
+#define POSTMASTER_POLL_RATE_LIMIT 1024
+#endif
+
 /*
  * Flags set by interrupt handlers for later service in the redo loop.
  */
@@ -134,6 +145,10 @@ StartupRereadConfig(void)
 void
 HandleStartupProcInterrupts(void)
 {
+#ifdef POSTMASTER_POLL_RATE_LIMIT
+	static uint32 postmaster_poll_count = 0;
+#endif
+
 	/*
 	 * Process any requests or signals received recently.
 	 */
@@ -151,9 +166,15 @@ HandleStartupProcInterrupts(void)
 
 	/*
 	 * Emergency bailout if postmaster has died.  This is to avoid the
-	 * necessity for manual cleanup of all postmaster children.
+	 * necessity for manual cleanup of all postmaster children.  Do this less
+	 * frequently on systems for which we don't have signals to make that
+	 * cheap.
 	 */
-	if (IsUnderPostmaster && !PostmasterIsAlive())
+	if (IsUnderPostmaster &&
+#ifdef POSTMASTER_POLL_RATE_LIMIT
+		postmaster_poll_count++ % POSTMASTER_POLL_RATE_LIMIT == 0 &&
+#endif
+		!PostmasterIsAlive())
 		exit(1);
 
 	/* Process barrier events */