Re: [HACKERS] mmap and MAP_ANON

>
> "G�ran Thyni" <goran@bildbasen.se> writes:
> > Linux can only MAP_SHARED if the file is a *real* file,
> > devices or trick like MAP_ANON does only work with MAP_PRIVATE.
>
> Well, this makes some sense: MAP_SHARED implies that the shared memory
> will also be accessible to independently started processes, and
> to do that you have to have an openable filename to refer to the
> data segment by.
>
> MAP_PRIVATE will *not* work for our purposes: according to my copy
> of mmap(2):

Right.
> so privately mapped segments are useless for interprocess communication,
> even after we get rid of exec().

Yep.

>
> mmaping /dev/zero, as has been suggested earlier in this thread,
> seems like a really bad idea to me.  Would that not imply that
> any process anywhere in the system that also decides to mmap /dev/zero
> would get its hands on the Postgres shared memory segment?  You
> can't restrict permissions on /dev/zero to prevent it.

Good point.

>
> Am I right in thinking that the contents of the shared memory segment
> do not need to outlive a particular postmaster run?  (If they do, then
> we have to mmap a real file anyway.)  If so, then MAP_ANON(YMOUS) is
> a reasonable solution on systems that support it.  On those that
> don't support it, we will have to mmap a real file owned by (and only
> readable/writable by) the postgres user.  Time for another configure
> test.

MAP_ANON is the best, because it can be restricted to only postmaster
children.

The problem with using a real file is that the filesystem is going to be
flushing those dirty pages to disk, and that could really hurt
performance.

Actually, when I install Informix, I always have to modify the kernel to
allow a larger amount of SYSV shared memory.  Maybe we just need to give
people per-OS instructions on how to do that.  Under BSD/OS, I now have
32MB of shared memory, or 3900 8k shared buffers.


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