Re: [HACKERS] Reducing sema usage (was Postmaster dies with many child processes)

> A considerably more straightforward approach is just to forget about
> incremental allocation of semaphores and grab all we could need at
> postmaster startup. ("OK, Mac, you told me to allow up to N backends?
> Fine, I'm going to grab N semaphores at startup, and if I can't get them
> I won't play.") This would force the DB admin to either reconfigure the
> kernel or reduce MaxBackendId to something the kernel can support right
> off the bat, rather than allowing the problem to lurk undetected until
> too many clients are started simultaneously. (Note there are still
> potential gotchas with running out of processes, swap space, or file
> table slots, so we wouldn't have really guaranteed that N backends can
> be started safely.)
>
> If we make MaxBackendId settable from a postmaster command-line switch
> then this second approach is probably not too inconvenient, though it
> surely isn't pretty.
>
> Any thoughts about which way to jump? I'm sort of inclined to take
> the simpler approach myself...

Semaphore are hard enough without overloading them. I say just gram
them on startup. They are cheap. Many databases use semaphores for
every row/page they lock, and boy that can be a lot of semaphores. We
are only getting a few.

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