Simplify the effective_io_concurrency setting.

The effective_io_concurrency GUC and equivalent tablespace option were
previously passed through a formula based on a theory about RAID
spindles and probabilities, to arrive at the number of pages to prefetch
in bitmap heap scans.  Tomas Vondra, Andres Freund and others argued
that it was anachronistic and hard to justify, and commit 558a9165
already started down the path of bypassing it in new code.  We agreed to
drop that logic and use the value directly.

For the default setting of 1, there is no change in effect.  Higher
settings can be converted from the old meaning to the new with:

  select round(sum(OLD / n::float)) from generate_series(1, OLD) s(n);

We might want to consider renaming the GUC before the next release given
the change in meaning, but it's not clear that many users had set it
very carefully anyway.  That decision is deferred for now.

Discussion: https://postgr.es/m/CA%2BhUKGJUw08dPs_3EUcdO6M90GnjofPYrWp4YSLaBkgYwS-AqA%40mail.gmail.com

src/backend/executor/nodeBitmapHeapscan.c

@@ -707,7 +707,6 @@ ExecInitBitmapHeapScan(BitmapHeapScan *node, EState *estate, int eflags)
 {
 	BitmapHeapScanState *scanstate;
 	Relation	currentRelation;
-	int			io_concurrency;
 
 	/* check for unsupported flags */
 	Assert(!(eflags & (EXEC_FLAG_BACKWARD | EXEC_FLAG_MARK)));
@@ -737,8 +736,6 @@ ExecInitBitmapHeapScan(BitmapHeapScan *node, EState *estate, int eflags)
 	scanstate->prefetch_iterator = NULL;
 	scanstate->prefetch_pages = 0;
 	scanstate->prefetch_target = 0;
-	/* may be updated below */
-	scanstate->prefetch_maximum = target_prefetch_pages;
 	scanstate->pscan_len = 0;
 	scanstate->initialized = false;
 	scanstate->shared_tbmiterator = NULL;
@@ -794,20 +791,11 @@ ExecInitBitmapHeapScan(BitmapHeapScan *node, EState *estate, int eflags)
 		ExecInitQual(node->bitmapqualorig, (PlanState *) scanstate);
 
 	/*
-	 * Determine the maximum for prefetch_target.  If the tablespace has a
-	 * specific IO concurrency set, use that to compute the corresponding
-	 * maximum value; otherwise, we already initialized to the value computed
-	 * by the GUC machinery.
+	 * Maximum number of prefetches for the tablespace if configured, otherwise
+	 * the current value of the effective_io_concurrency GUC.
 	 */
-	io_concurrency =
+	scanstate->prefetch_maximum =
 		get_tablespace_io_concurrency(currentRelation->rd_rel->reltablespace);
-	if (io_concurrency != effective_io_concurrency)
-	{
-		double		maximum;
-
-		if (ComputeIoConcurrency(io_concurrency, &maximum))
-			scanstate->prefetch_maximum = rint(maximum);
-	}
 
 	scanstate->ss.ss_currentRelation = currentRelation;
 

src/backend/storage/buffer/bufmgr.c

@@ -110,6 +110,13 @@ bool		zero_damaged_pages = false;
 int			bgwriter_lru_maxpages = 100;
 double		bgwriter_lru_multiplier = 2.0;
 bool		track_io_timing = false;
+
+/*
+ * How many buffers PrefetchBuffer callers should try to stay ahead of their
+ * ReadBuffer calls by.  Zero means "never prefetch".  This value is only used
+ * for buffers not belonging to tablespaces that have their
+ * effective_io_concurrency parameter set.
+ */
 int			effective_io_concurrency = 0;
 
 /*
@@ -120,15 +127,6 @@ int			checkpoint_flush_after = 0;
 int			bgwriter_flush_after = 0;
 int			backend_flush_after = 0;
 
-/*
- * How many buffers PrefetchBuffer callers should try to stay ahead of their
- * ReadBuffer calls by.  This is maintained by the assign hook for
- * effective_io_concurrency.  Zero means "never prefetch".  This value is
- * only used for buffers not belonging to tablespaces that have their
- * effective_io_concurrency parameter set.
- */
-int			target_prefetch_pages = 0;
-
 /* local state for StartBufferIO and related functions */
 static BufferDesc *InProgressBuf = NULL;
 static bool IsForInput;
@@ -461,64 +459,6 @@ static int	ckpt_buforder_comparator(const void *pa, const void *pb);
 static int	ts_ckpt_progress_comparator(Datum a, Datum b, void *arg);
 
 
-/*
- * ComputeIoConcurrency -- get the number of pages to prefetch for a given
- *		number of spindles.
- */
-bool
-ComputeIoConcurrency(int io_concurrency, double *target)
-{
-	double		new_prefetch_pages = 0.0;
-	int			i;
-
-	/*
-	 * Make sure the io_concurrency value is within valid range; it may have
-	 * been forced with a manual pg_tablespace update.
-	 */
-	io_concurrency = Min(Max(io_concurrency, 0), MAX_IO_CONCURRENCY);
-
-	/*----------
-	 * The user-visible GUC parameter is the number of drives (spindles),
-	 * which we need to translate to a number-of-pages-to-prefetch target.
-	 * The target value is stashed in *extra and then assigned to the actual
-	 * variable by assign_effective_io_concurrency.
-	 *
-	 * The expected number of prefetch pages needed to keep N drives busy is:
-	 *
-	 * drives |   I/O requests
-	 * -------+----------------
-	 *		1 |   1
-	 *		2 |   2/1 + 2/2 = 3
-	 *		3 |   3/1 + 3/2 + 3/3 = 5 1/2
-	 *		4 |   4/1 + 4/2 + 4/3 + 4/4 = 8 1/3
-	 *		n |   n * H(n)
-	 *
-	 * This is called the "coupon collector problem" and H(n) is called the
-	 * harmonic series.  This could be approximated by n * ln(n), but for
-	 * reasonable numbers of drives we might as well just compute the series.
-	 *
-	 * Alternatively we could set the target to the number of pages necessary
-	 * so that the expected number of active spindles is some arbitrary
-	 * percentage of the total.  This sounds the same but is actually slightly
-	 * different.  The result ends up being ln(1-P)/ln((n-1)/n) where P is
-	 * that desired fraction.
-	 *
-	 * Experimental results show that both of these formulas aren't aggressive
-	 * enough, but we don't really have any better proposals.
-	 *
-	 * Note that if io_concurrency = 0 (disabled), we must set target = 0.
-	 *----------
-	 */
-
-	for (i = 1; i <= io_concurrency; i++)
-		new_prefetch_pages += (double) io_concurrency / (double) i;
-
-	*target = new_prefetch_pages;
-
-	/* This range check shouldn't fail, but let's be paranoid */
-	return (new_prefetch_pages >= 0.0 && new_prefetch_pages < (double) INT_MAX);
-}
-
 /*
  * PrefetchBuffer -- initiate asynchronous read of a block of a relation
  *

src/backend/utils/misc/guc.c

@@ -196,7 +196,6 @@ static bool check_autovacuum_max_workers(int *newval, void **extra, GucSource so
 static bool check_max_wal_senders(int *newval, void **extra, GucSource source);
 static bool check_autovacuum_work_mem(int *newval, void **extra, GucSource source);
 static bool check_effective_io_concurrency(int *newval, void **extra, GucSource source);
-static void assign_effective_io_concurrency(int newval, void *extra);
 static void assign_pgstat_temp_directory(const char *newval, void *extra);
 static bool check_application_name(char **newval, void **extra, GucSource source);
 static void assign_application_name(const char *newval, void *extra);
@@ -2882,7 +2881,7 @@ static struct config_int ConfigureNamesInt[] =
 		0,
 #endif
 		0, MAX_IO_CONCURRENCY,
-		check_effective_io_concurrency, assign_effective_io_concurrency, NULL
+		check_effective_io_concurrency, NULL, NULL
 	},
 
 	{
@@ -11457,36 +11456,14 @@ check_max_worker_processes(int *newval, void **extra, GucSource source)
 static bool
 check_effective_io_concurrency(int *newval, void **extra, GucSource source)
 {
-#ifdef USE_PREFETCH
-	double		new_prefetch_pages;
-
-	if (ComputeIoConcurrency(*newval, &new_prefetch_pages))
-	{
-		int		   *myextra = (int *) guc_malloc(ERROR, sizeof(int));
-
-		*myextra = (int) rint(new_prefetch_pages);
-		*extra = (void *) myextra;
-
-		return true;
-	}
-	else
-		return false;
-#else
+#ifndef USE_PREFETCH
 	if (*newval != 0)
 	{
 		GUC_check_errdetail("effective_io_concurrency must be set to 0 on platforms that lack posix_fadvise().");
 		return false;
 	}
-	return true;
-#endif							/* USE_PREFETCH */
-}
-
-static void
-assign_effective_io_concurrency(int newval, void *extra)
-{
-#ifdef USE_PREFETCH
-	target_prefetch_pages = *((int *) extra);
 #endif							/* USE_PREFETCH */
+	return true;
 }
 
 static void

src/include/storage/bufmgr.h

@@ -57,7 +57,7 @@ extern bool zero_damaged_pages;
 extern int	bgwriter_lru_maxpages;
 extern double bgwriter_lru_multiplier;
 extern bool track_io_timing;
-extern int	target_prefetch_pages;
+extern int	effective_io_concurrency;
 
 extern int	checkpoint_flush_after;
 extern int	backend_flush_after;
@@ -66,9 +66,6 @@ extern int	bgwriter_flush_after;
 /* in buf_init.c */
 extern PGDLLIMPORT char *BufferBlocks;
 
-/* in guc.c */
-extern int	effective_io_concurrency;
-
 /* in localbuf.c */
 extern PGDLLIMPORT int NLocBuffer;
 extern PGDLLIMPORT Block *LocalBufferBlockPointers;
@@ -161,7 +158,6 @@ extern PGDLLIMPORT int32 *LocalRefCount;
 /*
  * prototypes for functions in bufmgr.c
  */
-extern bool ComputeIoConcurrency(int io_concurrency, double *target);
 extern void PrefetchBuffer(Relation reln, ForkNumber forkNum,
 						   BlockNumber blockNum);
 extern Buffer ReadBuffer(Relation reln, BlockNumber blockNum);