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Commit b09ff53

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macdicemacdice
committed
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 558a916 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
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‎src/backend/executor/nodeBitmapHeapscan.c

Copy file name to clipboardExpand all lines: src/backend/executor/nodeBitmapHeapscan.c
+3-15Lines changed: 3 additions & 15 deletions
Original file line numberDiff line numberDiff line change
@@ -707,7 +707,6 @@ ExecInitBitmapHeapScan(BitmapHeapScan *node, EState *estate, int eflags)
707707
{
708708
BitmapHeapScanState *scanstate;
709709
Relation currentRelation;
710-
int io_concurrency;
711710

712711
/* check for unsupported flags */
713712
Assert(!(eflags & (EXEC_FLAG_BACKWARD | EXEC_FLAG_MARK)));
@@ -737,8 +736,6 @@ ExecInitBitmapHeapScan(BitmapHeapScan *node, EState *estate, int eflags)
737736
scanstate->prefetch_iterator = NULL;
738737
scanstate->prefetch_pages = 0;
739738
scanstate->prefetch_target = 0;
740-
/* may be updated below */
741-
scanstate->prefetch_maximum = target_prefetch_pages;
742739
scanstate->pscan_len = 0;
743740
scanstate->initialized = false;
744741
scanstate->shared_tbmiterator = NULL;
@@ -794,20 +791,11 @@ ExecInitBitmapHeapScan(BitmapHeapScan *node, EState *estate, int eflags)
794791
ExecInitQual(node->bitmapqualorig, (PlanState *) scanstate);
795792

796793
/*
797-
* Determine the maximum for prefetch_target. If the tablespace has a
798-
* specific IO concurrency set, use that to compute the corresponding
799-
* maximum value; otherwise, we already initialized to the value computed
800-
* by the GUC machinery.
794+
* Maximum number of prefetches for the tablespace if configured, otherwise
795+
* the current value of the effective_io_concurrency GUC.
801796
*/
802-
io_concurrency =
797+
scanstate->prefetch_maximum =
803798
get_tablespace_io_concurrency(currentRelation->rd_rel->reltablespace);
804-
if (io_concurrency != effective_io_concurrency)
805-
{
806-
double maximum;
807-
808-
if (ComputeIoConcurrency(io_concurrency, &maximum))
809-
scanstate->prefetch_maximum = rint(maximum);
810-
}
811799

812800
scanstate->ss.ss_currentRelation = currentRelation;
813801

‎src/backend/storage/buffer/bufmgr.c

Copy file name to clipboardExpand all lines: src/backend/storage/buffer/bufmgr.c
+7-67Lines changed: 7 additions & 67 deletions
Original file line numberDiff line numberDiff line change
@@ -110,6 +110,13 @@ bool zero_damaged_pages = false;
110110
int bgwriter_lru_maxpages = 100;
111111
double bgwriter_lru_multiplier = 2.0;
112112
bool track_io_timing = false;
113+
114+
/*
115+
* How many buffers PrefetchBuffer callers should try to stay ahead of their
116+
* ReadBuffer calls by. Zero means "never prefetch". This value is only used
117+
* for buffers not belonging to tablespaces that have their
118+
* effective_io_concurrency parameter set.
119+
*/
113120
int effective_io_concurrency = 0;
114121

115122
/*
@@ -120,15 +127,6 @@ int checkpoint_flush_after = 0;
120127
int bgwriter_flush_after = 0;
121128
int backend_flush_after = 0;
122129

123-
/*
124-
* How many buffers PrefetchBuffer callers should try to stay ahead of their
125-
* ReadBuffer calls by. This is maintained by the assign hook for
126-
* effective_io_concurrency. Zero means "never prefetch". This value is
127-
* only used for buffers not belonging to tablespaces that have their
128-
* effective_io_concurrency parameter set.
129-
*/
130-
int target_prefetch_pages = 0;
131-
132130
/* local state for StartBufferIO and related functions */
133131
static BufferDesc *InProgressBuf = NULL;
134132
static bool IsForInput;
@@ -461,64 +459,6 @@ static int ckpt_buforder_comparator(const void *pa, const void *pb);
461459
static int ts_ckpt_progress_comparator(Datum a, Datum b, void *arg);
462460

463461

464-
/*
465-
* ComputeIoConcurrency -- get the number of pages to prefetch for a given
466-
* number of spindles.
467-
*/
468-
bool
469-
ComputeIoConcurrency(int io_concurrency, double *target)
470-
{
471-
double new_prefetch_pages = 0.0;
472-
int i;
473-
474-
/*
475-
* Make sure the io_concurrency value is within valid range; it may have
476-
* been forced with a manual pg_tablespace update.
477-
*/
478-
io_concurrency = Min(Max(io_concurrency, 0), MAX_IO_CONCURRENCY);
479-
480-
/*----------
481-
* The user-visible GUC parameter is the number of drives (spindles),
482-
* which we need to translate to a number-of-pages-to-prefetch target.
483-
* The target value is stashed in *extra and then assigned to the actual
484-
* variable by assign_effective_io_concurrency.
485-
*
486-
* The expected number of prefetch pages needed to keep N drives busy is:
487-
*
488-
* drives | I/O requests
489-
* -------+----------------
490-
* 1 | 1
491-
* 2 | 2/1 + 2/2 = 3
492-
* 3 | 3/1 + 3/2 + 3/3 = 5 1/2
493-
* 4 | 4/1 + 4/2 + 4/3 + 4/4 = 8 1/3
494-
* n | n * H(n)
495-
*
496-
* This is called the "coupon collector problem" and H(n) is called the
497-
* harmonic series. This could be approximated by n * ln(n), but for
498-
* reasonable numbers of drives we might as well just compute the series.
499-
*
500-
* Alternatively we could set the target to the number of pages necessary
501-
* so that the expected number of active spindles is some arbitrary
502-
* percentage of the total. This sounds the same but is actually slightly
503-
* different. The result ends up being ln(1-P)/ln((n-1)/n) where P is
504-
* that desired fraction.
505-
*
506-
* Experimental results show that both of these formulas aren't aggressive
507-
* enough, but we don't really have any better proposals.
508-
*
509-
* Note that if io_concurrency = 0 (disabled), we must set target = 0.
510-
*----------
511-
*/
512-
513-
for (i = 1; i <= io_concurrency; i++)
514-
new_prefetch_pages += (double) io_concurrency / (double) i;
515-
516-
*target = new_prefetch_pages;
517-
518-
/* This range check shouldn't fail, but let's be paranoid */
519-
return (new_prefetch_pages >= 0.0 && new_prefetch_pages < (double) INT_MAX);
520-
}
521-
522462
/*
523463
* PrefetchBuffer -- initiate asynchronous read of a block of a relation
524464
*

‎src/backend/utils/misc/guc.c

Copy file name to clipboardExpand all lines: src/backend/utils/misc/guc.c
+3-26Lines changed: 3 additions & 26 deletions
Original file line numberDiff line numberDiff line change
@@ -196,7 +196,6 @@ static bool check_autovacuum_max_workers(int *newval, void **extra, GucSource so
196196
static bool check_max_wal_senders(int *newval, void **extra, GucSource source);
197197
static bool check_autovacuum_work_mem(int *newval, void **extra, GucSource source);
198198
static bool check_effective_io_concurrency(int *newval, void **extra, GucSource source);
199-
static void assign_effective_io_concurrency(int newval, void *extra);
200199
static void assign_pgstat_temp_directory(const char *newval, void *extra);
201200
static bool check_application_name(char **newval, void **extra, GucSource source);
202201
static void assign_application_name(const char *newval, void *extra);
@@ -2882,7 +2881,7 @@ static struct config_int ConfigureNamesInt[] =
28822881
0,
28832882
#endif
28842883
0, MAX_IO_CONCURRENCY,
2885-
check_effective_io_concurrency, assign_effective_io_concurrency, NULL
2884+
check_effective_io_concurrency, NULL, NULL
28862885
},
28872886

28882887
{
@@ -11457,36 +11456,14 @@ check_max_worker_processes(int *newval, void **extra, GucSource source)
1145711456
static bool
1145811457
check_effective_io_concurrency(int *newval, void **extra, GucSource source)
1145911458
{
11460-
#ifdef USE_PREFETCH
11461-
double new_prefetch_pages;
11462-
11463-
if (ComputeIoConcurrency(*newval, &new_prefetch_pages))
11464-
{
11465-
int *myextra = (int *) guc_malloc(ERROR, sizeof(int));
11466-
11467-
*myextra = (int) rint(new_prefetch_pages);
11468-
*extra = (void *) myextra;
11469-
11470-
return true;
11471-
}
11472-
else
11473-
return false;
11474-
#else
11459+
#ifndef USE_PREFETCH
1147511460
if (*newval != 0)
1147611461
{
1147711462
GUC_check_errdetail("effective_io_concurrency must be set to 0 on platforms that lack posix_fadvise().");
1147811463
return false;
1147911464
}
11480-
return true;
11481-
#endif /* USE_PREFETCH */
11482-
}
11483-
11484-
static void
11485-
assign_effective_io_concurrency(int newval, void *extra)
11486-
{
11487-
#ifdef USE_PREFETCH
11488-
target_prefetch_pages = *((int *) extra);
1148911465
#endif /* USE_PREFETCH */
11466+
return true;
1149011467
}
1149111468

1149211469
static void

‎src/include/storage/bufmgr.h

Copy file name to clipboardExpand all lines: src/include/storage/bufmgr.h
+1-5Lines changed: 1 addition & 5 deletions
Original file line numberDiff line numberDiff line change
@@ -57,7 +57,7 @@ extern bool zero_damaged_pages;
5757
extern int bgwriter_lru_maxpages;
5858
extern double bgwriter_lru_multiplier;
5959
extern bool track_io_timing;
60-
extern int target_prefetch_pages;
60+
extern int effective_io_concurrency;
6161

6262
extern int checkpoint_flush_after;
6363
extern int backend_flush_after;
@@ -66,9 +66,6 @@ extern int bgwriter_flush_after;
6666
/* in buf_init.c */
6767
extern PGDLLIMPORT char *BufferBlocks;
6868

69-
/* in guc.c */
70-
extern int effective_io_concurrency;
71-
7269
/* in localbuf.c */
7370
extern PGDLLIMPORT int NLocBuffer;
7471
extern PGDLLIMPORT Block *LocalBufferBlockPointers;
@@ -161,7 +158,6 @@ extern PGDLLIMPORT int32 *LocalRefCount;
161158
/*
162159
* prototypes for functions in bufmgr.c
163160
*/
164-
extern bool ComputeIoConcurrency(int io_concurrency, double *target);
165161
extern void PrefetchBuffer(Relation reln, ForkNumber forkNum,
166162
BlockNumber blockNum);
167163
extern Buffer ReadBuffer(Relation reln, BlockNumber blockNum);

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