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Allow per-tablespace effective_io_concurrency

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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
This commit is contained in:
Alvaro Herrera
2015-09-08 12:51:42 -03:00
parent 665a00c9e2
commit 1aba62ec63
12 changed files with 145 additions and 63 deletions
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45
src/backend/utils/misc/guc.c
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@@ -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;
double new_prefetch_pages;
/*----------
* 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;
/* 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));