From 0dcaea56903489e8abedf231f286272495c3beb4 Mon Sep 17 00:00:00 2001 From: Nathan Bossart Date: Fri, 26 Jul 2024 15:28:55 -0500 Subject: [PATCH] Introduce num_os_semaphores GUC. The documentation for System V IPC parameters provides complicated formulas to determine the appropriate values for SEMMNI and SEMMNS. Furthermore, these formulas have often been wrong because folks forget to update them (e.g., when adding a new auxiliary process). This commit introduces a new runtime-computed GUC named num_os_semaphores that reports the number of semaphores needed for the configured number of allowed connections, worker processes, etc. This new GUC allows us to simplify the formulas in the documentation, and it should help prevent future inaccuracies. Like the other runtime-computed GUCs, users can view it with "postgres -C" before starting the server, which is useful for preconfiguring the necessary operating system resources. Reviewed-by: Tom Lane, Sami Imseih, Andres Freund, Robert Haas Discussion: https://postgr.es/m/20240517164452.GA1914161%40nathanxps13 --- doc/src/sgml/config.sgml | 18 +++++++++++++++++ doc/src/sgml/runtime.sgml | 31 ++++++++++++++++++----------- src/backend/storage/ipc/ipci.c | 6 +++++- src/backend/utils/misc/guc_tables.c | 12 +++++++++++ 4 files changed, 54 insertions(+), 13 deletions(-) diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml index 3dec0b7cfeb..57cd7bb9727 100644 --- a/doc/src/sgml/config.sgml +++ b/doc/src/sgml/config.sgml @@ -11145,6 +11145,24 @@ dynamic_library_path = 'C:\tools\postgresql;H:\my_project\lib;$libdir' + + num_os_semaphores (integer) + + num_os_semaphores configuration parameter + + + + + Reports the number of semaphores that are needed for the server based + on the configured number of allowed connections + (), allowed autovacuum worker + processes (), allowed WAL + sender processes (), allowed + background processes (), etc. + + + + segment_size (integer) diff --git a/doc/src/sgml/runtime.sgml b/doc/src/sgml/runtime.sgml index 2f7c6188869..2c4d5ef640d 100644 --- a/doc/src/sgml/runtime.sgml +++ b/doc/src/sgml/runtime.sgml @@ -781,13 +781,13 @@ psql: error: connection to server on socket "/tmp/.s.PGSQL.5432" failed: No such SEMMNI Maximum number of semaphore identifiers (i.e., sets) - at least ceil((max_connections + autovacuum_max_workers + max_wal_senders + max_worker_processes + 7) / 16) plus room for other applications + at least ceil(num_os_semaphores / 16) plus room for other applications SEMMNS Maximum number of semaphores system-wide - ceil((max_connections + autovacuum_max_workers + max_wal_senders + max_worker_processes + 7) / 16) * 17 plus room for other applications + ceil(num_os_semaphores / 16) * 17 plus room for other applications @@ -839,21 +839,28 @@ psql: error: connection to server on socket "/tmp/.s.PGSQL.5432" failed: No such PostgreSQL uses one semaphore per allowed connection (), allowed autovacuum worker process (), allowed WAL sender process - (), and allowed background - process (), in sets of 16. - Each such set will + (), allowed background + process (), etc., in sets of 16. + The runtime-computed parameter + reports the number of semaphores required. This parameter can be viewed + before starting the server with a postgres command like: + +$ postgres -D $PGDATA -C num_os_semaphores + + + + + Each set of 16 semaphores will also contain a 17th semaphore which contains a magic number, to detect collision with semaphore sets used by other applications. The maximum number of semaphores in the system is set by SEMMNS, which consequently must be at least - as high as max_connections plus - autovacuum_max_workers plus max_wal_senders, - plus max_worker_processes, plus one extra for each 16 - allowed connections plus workers (see the formula in num_os_semaphores plus one extra for + each set of 16 required semaphores (see the formula in ). The parameter SEMMNI determines the limit on the number of semaphore sets that can exist on the system at one time. Hence this parameter must be at - least ceil((max_connections + autovacuum_max_workers + max_wal_senders + max_worker_processes + 7) / 16). + least ceil(num_os_semaphores / 16). Lowering the number of allowed connections is a temporary workaround for failures, which are usually confusingly worded No space @@ -885,8 +892,8 @@ psql: error: connection to server on socket "/tmp/.s.PGSQL.5432" failed: No such same as for System V, that is one semaphore per allowed connection (), allowed autovacuum worker process (), allowed WAL sender process - (), and allowed background - process (). + (), allowed background + process (), etc. On the platforms where this option is preferred, there is no specific kernel limit on the number of POSIX semaphores. diff --git a/src/backend/storage/ipc/ipci.c b/src/backend/storage/ipc/ipci.c index 2100150f01c..ca930af08f2 100644 --- a/src/backend/storage/ipc/ipci.c +++ b/src/backend/storage/ipc/ipci.c @@ -372,11 +372,12 @@ InitializeShmemGUCs(void) Size size_b; Size size_mb; Size hp_size; + int num_semas; /* * Calculate the shared memory size and round up to the nearest megabyte. */ - size_b = CalculateShmemSize(NULL); + size_b = CalculateShmemSize(&num_semas); size_mb = add_size(size_b, (1024 * 1024) - 1) / (1024 * 1024); sprintf(buf, "%zu", size_mb); SetConfigOption("shared_memory_size", buf, @@ -395,4 +396,7 @@ InitializeShmemGUCs(void) SetConfigOption("shared_memory_size_in_huge_pages", buf, PGC_INTERNAL, PGC_S_DYNAMIC_DEFAULT); } + + sprintf(buf, "%d", num_semas); + SetConfigOption("num_os_semaphores", buf, PGC_INTERNAL, PGC_S_DYNAMIC_DEFAULT); } diff --git a/src/backend/utils/misc/guc_tables.c b/src/backend/utils/misc/guc_tables.c index f6fcdebb031..6a623f5f342 100644 --- a/src/backend/utils/misc/guc_tables.c +++ b/src/backend/utils/misc/guc_tables.c @@ -591,6 +591,7 @@ static int segment_size; static int shared_memory_size_mb; static int shared_memory_size_in_huge_pages; static int wal_block_size; +static int num_os_semaphores; static bool data_checksums; static bool integer_datetimes; @@ -2283,6 +2284,17 @@ struct config_int ConfigureNamesInt[] = NULL, NULL, NULL }, + { + {"num_os_semaphores", PGC_INTERNAL, PRESET_OPTIONS, + gettext_noop("Shows the number of semaphores required for the server."), + NULL, + GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE | GUC_RUNTIME_COMPUTED + }, + &num_os_semaphores, + 0, 0, INT_MAX, + NULL, NULL, NULL + }, + { {"commit_timestamp_buffers", PGC_POSTMASTER, RESOURCES_MEM, gettext_noop("Sets the size of the dedicated buffer pool used for the commit timestamp cache."), -- 2.39.5