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Introduce io_max_combine_limit.
The existing io_combine_limit can be changed by users. The new io_max_combine_limit is fixed at server startup time, and functions as a silent clamp on the user setting. That in itself is probably quite useful, but the primary motivation is: aio_init.c allocates shared memory for all asynchronous IOs including some per-block data, and we didn't want to waste memory you'd never used by assuming they could be up to PG_IOV_MAX. This commit already halves the size of 'AioHandleIov' and 'AioHandleData'. A follow-up commit can now expand PG_IOV_MAX without affecting that. Since our GUC system doesn't support dependencies or cross-checks between GUCs, the user-settable one now assigns a "raw" value to io_combine_limit_guc, and the lower of io_combine_limit_guc and io_max_combine_limit is maintained in io_combine_limit. Reviewed-by: Andres Freund <andres@anarazel.de> (earlier version) Discussion: https://postgr.es/m/CA%2BhUKG%2B2T9p-%2BzM6Eeou-RAJjTML6eit1qn26f9twznX59qtCA%40mail.gmail.com
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8 files changed

+72
-15
lines changed

8 files changed

+72
-15
lines changed

‎doc/src/sgml/config.sgml

Lines changed: 22 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -2625,6 +2625,24 @@ include_dir 'conf.d'
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</listitem>
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</varlistentry>
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2628+
<varlistentry id="guc-io-max-combine-limit" xreflabel="io_max_combine_limit">
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<term><varname>io_max_combine_limit</varname> (<type>integer</type>)
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<indexterm>
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<primary><varname>io_max_combine_limit</varname> configuration parameter</primary>
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</indexterm>
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</term>
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<listitem>
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<para>
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Controls the largest I/O size in operations that combine I/O, and silently
2637+
limits the user-settable parameter <varname>io_combine_limit</varname>.
2638+
This parameter can only be set in
2639+
the <filename>postgresql.conf</filename> file or on the server
2640+
command line.
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The default is 128kB.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry id="guc-io-combine-limit" xreflabel="io_combine_limit">
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<term><varname>io_combine_limit</varname> (<type>integer</type>)
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<indexterm>
@@ -2633,7 +2651,10 @@ include_dir 'conf.d'
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</term>
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<listitem>
26352653
<para>
2636-
Controls the largest I/O size in operations that combine I/O.
2654+
Controls the largest I/O size in operations that combine I/O. If set
2655+
higher than the <varname>io_max_combine_limit</varname> parameter, the
2656+
lower value will silently be used instead, so both may need to be raised
2657+
to increase the I/O size.
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The default is 128kB.
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</para>
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</listitem>

‎src/backend/commands/variable.c

Lines changed: 18 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -1156,6 +1156,24 @@ assign_maintenance_io_concurrency(int newval, void *extra)
11561156
#endif
11571157
}
11581158

1159+
/*
1160+
* GUC assign hooks that recompute io_combine_limit whenever
1161+
* io_combine_limit_guc and io_max_combine_limit are changed. These are needed
1162+
* because the GUC subsystem doesn't support dependencies between GUCs, and
1163+
* they may be assigned in either order.
1164+
*/
1165+
void
1166+
assign_io_max_combine_limit(int newval, void *extra)
1167+
{
1168+
io_max_combine_limit = newval;
1169+
io_combine_limit = Min(io_max_combine_limit, io_combine_limit_guc);
1170+
}
1171+
void
1172+
assign_io_combine_limit(int newval, void *extra)
1173+
{
1174+
io_combine_limit_guc = newval;
1175+
io_combine_limit = Min(io_max_combine_limit, io_combine_limit_guc);
1176+
}
11591177

11601178
/*
11611179
* These show hooks just exist because we want to show the values in octal.

‎src/backend/storage/aio/aio_init.c

Lines changed: 6 additions & 11 deletions
Original file line numberDiff line numberDiff line change
@@ -18,6 +18,7 @@
1818
#include "storage/aio.h"
1919
#include "storage/aio_internal.h"
2020
#include "storage/aio_subsys.h"
21+
#include "storage/bufmgr.h"
2122
#include "storage/io_worker.h"
2223
#include "storage/ipc.h"
2324
#include "storage/proc.h"
@@ -72,15 +73,9 @@ AioHandleShmemSize(void)
7273
static Size
7374
AioHandleIOVShmemSize(void)
7475
{
75-
/*
76-
* Each IO handle can have an PG_IOV_MAX long iovec.
77-
*
78-
* XXX: Right now the amount of space available for each IO is PG_IOV_MAX.
79-
* While it's tempting to use the io_combine_limit GUC, that's
80-
* PGC_USERSET, so we can't allocate shared memory based on that.
81-
*/
76+
/* each IO handle can have up to io_max_combine_limit iovec objects */
8277
return mul_size(sizeof(struct iovec),
83-
mul_size(mul_size(PG_IOV_MAX, AioProcs()),
78+
mul_size(mul_size(io_max_combine_limit, AioProcs()),
8479
io_max_concurrency));
8580
}
8681

@@ -89,7 +84,7 @@ AioHandleDataShmemSize(void)
8984
{
9085
/* each buffer referenced by an iovec can have associated data */
9186
return mul_size(sizeof(uint64),
92-
mul_size(mul_size(PG_IOV_MAX, AioProcs()),
87+
mul_size(mul_size(io_max_combine_limit, AioProcs()),
9388
io_max_concurrency));
9489
}
9590

@@ -160,7 +155,7 @@ AioShmemInit(void)
160155
bool found;
161156
uint32 io_handle_off = 0;
162157
uint32 iovec_off = 0;
163-
uint32 per_backend_iovecs = io_max_concurrency * PG_IOV_MAX;
158+
uint32 per_backend_iovecs = io_max_concurrency * io_max_combine_limit;
164159

165160
pgaio_ctl = (PgAioCtl *)
166161
ShmemInitStruct("AioCtl", AioCtlShmemSize(), &found);
@@ -213,7 +208,7 @@ AioShmemInit(void)
213208
ConditionVariableInit(&ioh->cv);
214209

215210
dclist_push_tail(&bs->idle_ios, &ioh->node);
216-
iovec_off += PG_IOV_MAX;
211+
iovec_off += io_max_combine_limit;
217212
}
218213
}
219214

‎src/backend/storage/buffer/bufmgr.c

Lines changed: 4 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -160,9 +160,12 @@ int maintenance_io_concurrency = DEFAULT_MAINTENANCE_IO_CONCURRENCY;
160160
/*
161161
* Limit on how many blocks should be handled in single I/O operations.
162162
* StartReadBuffers() callers should respect it, as should other operations
163-
* that call smgr APIs directly.
163+
* that call smgr APIs directly. It is computed as the minimum of underlying
164+
* GUCs io_combine_limit_guc and io_max_combine_limit.
164165
*/
165166
int io_combine_limit = DEFAULT_IO_COMBINE_LIMIT;
167+
int io_combine_limit_guc = DEFAULT_IO_COMBINE_LIMIT;
168+
int io_max_combine_limit = DEFAULT_IO_COMBINE_LIMIT;
166169

167170
/*
168171
* GUC variables about triggering kernel writeback for buffers written; OS

‎src/backend/utils/misc/guc_tables.c

Lines changed: 15 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -3252,6 +3252,20 @@ struct config_int ConfigureNamesInt[] =
32523252
NULL
32533253
},
32543254

3255+
{
3256+
{"io_max_combine_limit",
3257+
PGC_POSTMASTER,
3258+
RESOURCES_IO,
3259+
gettext_noop("Server-wide limit that clamps io_combine_limit."),
3260+
NULL,
3261+
GUC_UNIT_BLOCKS
3262+
},
3263+
&io_max_combine_limit,
3264+
DEFAULT_IO_COMBINE_LIMIT,
3265+
1, MAX_IO_COMBINE_LIMIT,
3266+
NULL, assign_io_max_combine_limit, NULL
3267+
},
3268+
32553269
{
32563270
{"io_combine_limit",
32573271
PGC_USERSET,
@@ -3263,7 +3277,7 @@ struct config_int ConfigureNamesInt[] =
32633277
&io_combine_limit,
32643278
DEFAULT_IO_COMBINE_LIMIT,
32653279
1, MAX_IO_COMBINE_LIMIT,
3266-
NULL, NULL, NULL
3280+
NULL, assign_io_combine_limit, NULL
32673281
},
32683282

32693283
{

‎src/backend/utils/misc/postgresql.conf.sample

Lines changed: 2 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -200,6 +200,8 @@
200200
#backend_flush_after = 0 # measured in pages, 0 disables
201201
#effective_io_concurrency = 16 # 1-1000; 0 disables prefetching
202202
#maintenance_io_concurrency = 16 # 1-1000; 0 disables prefetching
203+
#io_max_combine_limit = 128kB # usually 1-32 blocks (depends on OS)
204+
# (change requires restart)
203205
#io_combine_limit = 128kB # usually 1-32 blocks (depends on OS)
204206

205207
#io_method = worker # worker, sync (change requires restart)

‎src/include/storage/bufmgr.h

Lines changed: 3 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -163,7 +163,9 @@ extern PGDLLIMPORT int maintenance_io_concurrency;
163163

164164
#define MAX_IO_COMBINE_LIMIT PG_IOV_MAX
165165
#define DEFAULT_IO_COMBINE_LIMIT Min(MAX_IO_COMBINE_LIMIT, (128 * 1024) / BLCKSZ)
166-
extern PGDLLIMPORT int io_combine_limit;
166+
extern PGDLLIMPORT int io_combine_limit; /* min of the two GUCs below */
167+
extern PGDLLIMPORT int io_combine_limit_guc;
168+
extern PGDLLIMPORT int io_max_combine_limit;
167169

168170
extern PGDLLIMPORT int checkpoint_flush_after;
169171
extern PGDLLIMPORT int backend_flush_after;

‎src/include/utils/guc_hooks.h

Lines changed: 2 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -86,6 +86,8 @@ extern const char *show_log_timezone(void);
8686
extern bool check_maintenance_io_concurrency(int *newval, void **extra,
8787
GucSource source);
8888
extern void assign_maintenance_io_concurrency(int newval, void *extra);
89+
extern void assign_io_max_combine_limit(int newval, void *extra);
90+
extern void assign_io_combine_limit(int newval, void *extra);
8991
extern bool check_max_slot_wal_keep_size(int *newval, void **extra,
9092
GucSource source);
9193
extern void assign_max_wal_size(int newval, void *extra);

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