From 10f6646847515b1ab02735c24b04abaf1996f65f Mon Sep 17 00:00:00 2001 From: Thomas Munro Date: Wed, 19 Mar 2025 11:40:56 +1300 Subject: [PATCH] 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 (earlier version) Discussion: https://postgr.es/m/CA%2BhUKG%2B2T9p-%2BzM6Eeou-RAJjTML6eit1qn26f9twznX59qtCA%40mail.gmail.com --- doc/src/sgml/config.sgml | 23 ++++++++++++++++++- src/backend/commands/variable.c | 18 +++++++++++++++ src/backend/storage/aio/aio_init.c | 17 +++++--------- src/backend/storage/buffer/bufmgr.c | 5 +++- src/backend/utils/misc/guc_tables.c | 16 ++++++++++++- src/backend/utils/misc/postgresql.conf.sample | 2 ++ src/include/storage/bufmgr.h | 4 +++- src/include/utils/guc_hooks.h | 2 ++ 8 files changed, 72 insertions(+), 15 deletions(-) diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml index 9e9c02cde83..2988865b116 100644 --- a/doc/src/sgml/config.sgml +++ b/doc/src/sgml/config.sgml @@ -2625,6 +2625,24 @@ include_dir 'conf.d' + + io_max_combine_limit (integer) + + io_max_combine_limit configuration parameter + + + + + Controls the largest I/O size in operations that combine I/O, and silently + limits the user-settable parameter io_combine_limit. + This parameter can only be set in + the postgresql.conf file or on the server + command line. + The default is 128kB. + + + + io_combine_limit (integer) @@ -2633,7 +2651,10 @@ include_dir 'conf.d' - Controls the largest I/O size in operations that combine I/O. + Controls the largest I/O size in operations that combine I/O. If set + higher than the io_max_combine_limit parameter, the + lower value will silently be used instead, so both may need to be raised + to increase the I/O size. The default is 128kB. diff --git a/src/backend/commands/variable.c b/src/backend/commands/variable.c index 4ad6e236d69..f550a3c0c63 100644 --- a/src/backend/commands/variable.c +++ b/src/backend/commands/variable.c @@ -1156,6 +1156,24 @@ assign_maintenance_io_concurrency(int newval, void *extra) #endif } +/* + * GUC assign hooks that recompute io_combine_limit whenever + * io_combine_limit_guc and io_max_combine_limit are changed. These are needed + * because the GUC subsystem doesn't support dependencies between GUCs, and + * they may be assigned in either order. + */ +void +assign_io_max_combine_limit(int newval, void *extra) +{ + io_max_combine_limit = newval; + io_combine_limit = Min(io_max_combine_limit, io_combine_limit_guc); +} +void +assign_io_combine_limit(int newval, void *extra) +{ + io_combine_limit_guc = newval; + io_combine_limit = Min(io_max_combine_limit, io_combine_limit_guc); +} /* * These show hooks just exist because we want to show the values in octal. diff --git a/src/backend/storage/aio/aio_init.c b/src/backend/storage/aio/aio_init.c index 4e405ce7ca8..2ede7e80b65 100644 --- a/src/backend/storage/aio/aio_init.c +++ b/src/backend/storage/aio/aio_init.c @@ -18,6 +18,7 @@ #include "storage/aio.h" #include "storage/aio_internal.h" #include "storage/aio_subsys.h" +#include "storage/bufmgr.h" #include "storage/io_worker.h" #include "storage/ipc.h" #include "storage/proc.h" @@ -72,15 +73,9 @@ AioHandleShmemSize(void) static Size AioHandleIOVShmemSize(void) { - /* - * Each IO handle can have an PG_IOV_MAX long iovec. - * - * XXX: Right now the amount of space available for each IO is PG_IOV_MAX. - * While it's tempting to use the io_combine_limit GUC, that's - * PGC_USERSET, so we can't allocate shared memory based on that. - */ + /* each IO handle can have up to io_max_combine_limit iovec objects */ return mul_size(sizeof(struct iovec), - mul_size(mul_size(PG_IOV_MAX, AioProcs()), + mul_size(mul_size(io_max_combine_limit, AioProcs()), io_max_concurrency)); } @@ -89,7 +84,7 @@ AioHandleDataShmemSize(void) { /* each buffer referenced by an iovec can have associated data */ return mul_size(sizeof(uint64), - mul_size(mul_size(PG_IOV_MAX, AioProcs()), + mul_size(mul_size(io_max_combine_limit, AioProcs()), io_max_concurrency)); } @@ -160,7 +155,7 @@ AioShmemInit(void) bool found; uint32 io_handle_off = 0; uint32 iovec_off = 0; - uint32 per_backend_iovecs = io_max_concurrency * PG_IOV_MAX; + uint32 per_backend_iovecs = io_max_concurrency * io_max_combine_limit; pgaio_ctl = (PgAioCtl *) ShmemInitStruct("AioCtl", AioCtlShmemSize(), &found); @@ -213,7 +208,7 @@ AioShmemInit(void) ConditionVariableInit(&ioh->cv); dclist_push_tail(&bs->idle_ios, &ioh->node); - iovec_off += PG_IOV_MAX; + iovec_off += io_max_combine_limit; } } diff --git a/src/backend/storage/buffer/bufmgr.c b/src/backend/storage/buffer/bufmgr.c index 79ca9d18d07..d04afa5ab9c 100644 --- a/src/backend/storage/buffer/bufmgr.c +++ b/src/backend/storage/buffer/bufmgr.c @@ -160,9 +160,12 @@ int maintenance_io_concurrency = DEFAULT_MAINTENANCE_IO_CONCURRENCY; /* * Limit on how many blocks should be handled in single I/O operations. * StartReadBuffers() callers should respect it, as should other operations - * that call smgr APIs directly. + * that call smgr APIs directly. It is computed as the minimum of underlying + * GUCs io_combine_limit_guc and io_max_combine_limit. */ int io_combine_limit = DEFAULT_IO_COMBINE_LIMIT; +int io_combine_limit_guc = DEFAULT_IO_COMBINE_LIMIT; +int io_max_combine_limit = DEFAULT_IO_COMBINE_LIMIT; /* * GUC variables about triggering kernel writeback for buffers written; OS diff --git a/src/backend/utils/misc/guc_tables.c b/src/backend/utils/misc/guc_tables.c index 60a40ed445a..ead80257192 100644 --- a/src/backend/utils/misc/guc_tables.c +++ b/src/backend/utils/misc/guc_tables.c @@ -3252,6 +3252,20 @@ struct config_int ConfigureNamesInt[] = NULL }, + { + {"io_max_combine_limit", + PGC_POSTMASTER, + RESOURCES_IO, + gettext_noop("Server-wide limit that clamps io_combine_limit."), + NULL, + GUC_UNIT_BLOCKS + }, + &io_max_combine_limit, + DEFAULT_IO_COMBINE_LIMIT, + 1, MAX_IO_COMBINE_LIMIT, + NULL, assign_io_max_combine_limit, NULL + }, + { {"io_combine_limit", PGC_USERSET, @@ -3263,7 +3277,7 @@ struct config_int ConfigureNamesInt[] = &io_combine_limit, DEFAULT_IO_COMBINE_LIMIT, 1, MAX_IO_COMBINE_LIMIT, - NULL, NULL, NULL + NULL, assign_io_combine_limit, NULL }, { diff --git a/src/backend/utils/misc/postgresql.conf.sample b/src/backend/utils/misc/postgresql.conf.sample index beb05a89501..66bda60f4ca 100644 --- a/src/backend/utils/misc/postgresql.conf.sample +++ b/src/backend/utils/misc/postgresql.conf.sample @@ -200,6 +200,8 @@ #backend_flush_after = 0 # measured in pages, 0 disables #effective_io_concurrency = 16 # 1-1000; 0 disables prefetching #maintenance_io_concurrency = 16 # 1-1000; 0 disables prefetching +#io_max_combine_limit = 128kB # usually 1-32 blocks (depends on OS) + # (change requires restart) #io_combine_limit = 128kB # usually 1-32 blocks (depends on OS) #io_method = worker # worker, sync (change requires restart) diff --git a/src/include/storage/bufmgr.h b/src/include/storage/bufmgr.h index 7f5def6bada..ecc1c3a909b 100644 --- a/src/include/storage/bufmgr.h +++ b/src/include/storage/bufmgr.h @@ -163,7 +163,9 @@ extern PGDLLIMPORT int maintenance_io_concurrency; #define MAX_IO_COMBINE_LIMIT PG_IOV_MAX #define DEFAULT_IO_COMBINE_LIMIT Min(MAX_IO_COMBINE_LIMIT, (128 * 1024) / BLCKSZ) -extern PGDLLIMPORT int io_combine_limit; +extern PGDLLIMPORT int io_combine_limit; /* min of the two GUCs below */ +extern PGDLLIMPORT int io_combine_limit_guc; +extern PGDLLIMPORT int io_max_combine_limit; extern PGDLLIMPORT int checkpoint_flush_after; extern PGDLLIMPORT int backend_flush_after; diff --git a/src/include/utils/guc_hooks.h b/src/include/utils/guc_hooks.h index a3eba8fbe21..0f1e74f96c9 100644 --- a/src/include/utils/guc_hooks.h +++ b/src/include/utils/guc_hooks.h @@ -86,6 +86,8 @@ extern const char *show_log_timezone(void); extern bool check_maintenance_io_concurrency(int *newval, void **extra, GucSource source); extern void assign_maintenance_io_concurrency(int newval, void *extra); +extern void assign_io_max_combine_limit(int newval, void *extra); +extern void assign_io_combine_limit(int newval, void *extra); extern bool check_max_slot_wal_keep_size(int *newval, void **extra, GucSource source); extern void assign_max_wal_size(int newval, void *extra);