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Introduce num_os_semaphores GUC.

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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
This commit is contained in:
Nathan Bossart
2024-07-26 15:28:55 -05:00
parent 8a53539bd6
commit 0dcaea5690
4 changed files with 54 additions and 13 deletions
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6
src/backend/storage/ipc/ipci.c
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@@ -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);
}