Improve control logic for bgwriter hibernation mode.

Commit 6d90eaaa89a007e0d365f49d6436f35d2392cfeb added a hibernation mode
to the bgwriter to reduce the server's idle-power consumption.  However,
its interaction with the detailed behavior of BgBufferSync's feedback
control loop wasn't very well thought out.  That control loop depends
primarily on the rate of buffer allocation, not the rate of buffer
dirtying, so the hibernation mode has to be designed to operate only when
no new buffer allocations are happening.  Also, the check for whether the
system is effectively idle was not quite right and would fail to detect
a constant low level of activity, thus allowing the bgwriter to go into
hibernation mode in a way that would let the cycle time vary quite a bit,
possibly further confusing the feedback loop.  To fix, move the wakeup
support from MarkBufferDirty and SetBufferCommitInfoNeedsSave into
StrategyGetBuffer, and prevent the bgwriter from entering hibernation mode
unless no buffer allocations have happened recently.

In addition, fix the delaying logic to remove the problem of possibly not
responding to signals promptly, which was basically caused by trying to use
the process latch's is_set flag for multiple purposes.  I can't prove it
but I'm suspicious that that hack was responsible for the intermittent
"postmaster does not shut down" failures we've been seeing in the buildfarm
lately.  In any case it did nothing to improve the readability or
robustness of the code.

In passing, express the hibernation sleep time as a multiplier on
BgWriterDelay, not a constant.  I'm not sure whether there's any value in
exposing the longer sleep time as an independently configurable setting,
but we can at least make it act like this for little extra code.

3 files changed, 7 insertions, 4 deletions

diff --git a/src/include/postmaster/bgwriter.h b/src/include/postmaster/bgwriter.h
index 540c4e9cc0..d17dd17487 100644
--- a/src/include/postmaster/bgwriter.h
+++ b/src/include/postmaster/bgwriter.h
@@ -1,7 +1,10 @@
 /*-------------------------------------------------------------------------
  *
  * bgwriter.h
- *	  Exports from postmaster/bgwriter.c.
+ *	  Exports from postmaster/bgwriter.c and postmaster/checkpointer.c.
+ *
+ * The bgwriter process used to handle checkpointing duties too.  Now
+ * there is a separate process, but we did not bother to split this header.
  *
  * Portions Copyright (c) 1996-2012, PostgreSQL Global Development Group
  *
@@ -32,7 +35,6 @@ extern bool ForwardFsyncRequest(RelFileNodeBackend rnode, ForkNumber forknum,
 					BlockNumber segno);
 extern void AbsorbFsyncRequests(void);
 
-/* These were previously called BgWriterShmem... */
 extern Size CheckpointerShmemSize(void);
 extern void CheckpointerShmemInit(void);
 
diff --git a/src/include/storage/buf_internals.h b/src/include/storage/buf_internals.h
index e43719ed73..4129ce5244 100644
--- a/src/include/storage/buf_internals.h
+++ b/src/include/storage/buf_internals.h
@@ -16,6 +16,7 @@
 #define BUFMGR_INTERNALS_H
 
 #include "storage/buf.h"
+#include "storage/latch.h"
 #include "storage/lwlock.h"
 #include "storage/shmem.h"
 #include "storage/smgr.h"
@@ -188,6 +189,8 @@ extern bool StrategyRejectBuffer(BufferAccessStrategy strategy,
 					 volatile BufferDesc *buf);
 
 extern int	StrategySyncStart(uint32 *complete_passes, uint32 *num_buf_alloc);
+extern void StrategyNotifyBgWriter(Latch *bgwriterLatch);
+
 extern Size StrategyShmemSize(void);
 extern void StrategyInitialize(bool init);
 
diff --git a/src/include/storage/proc.h b/src/include/storage/proc.h
index c4808f44a0..7552e188f3 100644
--- a/src/include/storage/proc.h
+++ b/src/include/storage/proc.h
@@ -188,8 +188,6 @@ typedef struct PROC_HDR
 	PGPROC	   *freeProcs;
 	/* Head of list of autovacuum's free PGPROC structures */
 	PGPROC	   *autovacFreeProcs;
-	/* BGWriter process's latch */
-	Latch	   *bgwriterLatch;
 	/* WALWriter process's latch */
 	Latch	   *walwriterLatch;
 	/* Checkpointer process's latch */