Re: OpenBSD versus semaphores

Thomas Munro <thomas(dot)munro(at)enterprisedb(dot)com> writes:
> On Tue, Jan 8, 2019 at 7:14 PM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> So I looked around for an alternative, and found out that modern
>> OpenBSD releases support named POSIX semaphores (though not unnamed
>> ones, at least not shared unnamed ones). What's more, it appears that
>> in this implementation, named semaphores don't eat open file descriptors
>> as they do on macOS, removing our major objection to using them.

> No OpenBSD here, but I was curious enough to peek at their
> implementation. Like others, they create a tiny file under /tmp for
> each one, mmap() and close the fd straight away.

Oh, yeah, I can see a bunch of tiny mappings with procmap. I wonder
whether that scales any better than an open FD per semaphore, when
it comes to forking a bunch of child processes that will inherit
all those mappings or FDs. I've not tried to benchmark child process
launch as such --- as I said, I'm not running this on hardware that
would support serious benchmarking.

BTW, I just finished finding out that recent NetBSD (8.99.25) has
working code paths for *both* named and unnamed POSIX semaphores.
However, it appears that both code paths involve an open FD per
semaphore, so it's likely not something we want to recommend using.

regards, tom lane