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Simplify the effective_io_concurrency setting.

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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 558a9165e0
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
This commit is contained in:
Thomas Munro
2020-03-16 11:43:18 +13:00
parent f207bb0b8f
commit b09ff53667
4 changed files with 14 additions and 113 deletions
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29
src/backend/utils/misc/guc.c
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@ -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;
}
#endif /* USE_PREFETCH */
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 */
}
static void