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 558a9165e08
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

1 files changed, 3 insertions, 26 deletions

diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index 4c6d6486623..326e773b25f 100644
--- a/src/backend/utils/misc/guc.c
+++ b/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