Allow per-tablespace effective_io_concurrency

Per discussion, nowadays it is possible to have tablespaces that have
wildly different I/O characteristics from others.  Setting different
effective_io_concurrency parameters for those has been measured to
improve performance.

Author: Julien Rouhaud
Reviewed by: Andres Freund

2 files changed, 15 insertions, 42 deletions

diff --git a/src/backend/utils/cache/spccache.c b/src/backend/utils/cache/spccache.c
index 1a0c884b248..1c78dfe76a1 100644
--- a/src/backend/utils/cache/spccache.c
+++ b/src/backend/utils/cache/spccache.c
@@ -23,6 +23,7 @@
 #include "commands/tablespace.h"
 #include "miscadmin.h"
 #include "optimizer/cost.h"
+#include "storage/bufmgr.h"
 #include "utils/catcache.h"
 #include "utils/hsearch.h"
 #include "utils/inval.h"
@@ -198,3 +199,14 @@ get_tablespace_page_costs(Oid spcid,
 			*spc_seq_page_cost = spc->opts->seq_page_cost;
 	}
 }
+
+int
+get_tablespace_io_concurrency(Oid spcid)
+{
+	TableSpaceCacheEntry *spc = get_tablespace(spcid);
+
+	if (!spc->opts || spc->opts->effective_io_concurrency < 0)
+		return effective_io_concurrency;
+	else
+		return spc->opts->effective_io_concurrency;
+}
diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index b3dac51b779..8ebf4246b8e 100644
--- a/src/backend/utils/misc/guc.c
+++ b/src/backend/utils/misc/guc.c
@@ -490,7 +490,6 @@ static int	wal_block_size;
 static bool data_checksums;
 static int	wal_segment_size;
 static bool integer_datetimes;
-static int	effective_io_concurrency;
 static bool assert_enabled;
 
 /* should be static, but commands/variable.c needs to get at this */
@@ -2352,7 +2351,7 @@ static struct config_int ConfigureNamesInt[] =
 		},
 		&effective_io_concurrency,
 #ifdef USE_PREFETCH
-		1, 0, 1000,
+		1, 0, MAX_IO_CONCURRENCY,
 #else
 		0, 0, 0,
 #endif
@@ -9986,47 +9985,9 @@ static bool
 check_effective_io_concurrency(int *newval, void **extra, GucSource source)
 {
 #ifdef USE_PREFETCH
-	double		new_prefetch_pages = 0.0;
-	int			i;
-
-	/*----------
-	 * 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 *newval = 0 (disabled), we must set target = 0.
-	 *----------
-	 */
-
-	for (i = 1; i <= *newval; i++)
-		new_prefetch_pages += (double) *newval / (double) i;
+	double		new_prefetch_pages;
 
-	/* This range check shouldn't fail, but let's be paranoid */
-	if (new_prefetch_pages >= 0.0 && new_prefetch_pages < (double) INT_MAX)
+	if (ComputeIoConcurrency(*newval, &new_prefetch_pages))
 	{
 		int		   *myextra = (int *) guc_malloc(ERROR, sizeof(int));