Re: dynamically allocating chunks from shared memory

Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> On Mon, Aug 9, 2010 at 11:41 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> ... and on some platforms, it'll be flat out impossible. We looked at
>> this years ago and concluded that changing the size of the shmem segment
>> after postmaster start was impractical from a portability standpoint.
>> I have not seen anything to change that conclusion.

> I haven't done extensive research into this, but I did take a look at
> it briefly. It looked to me like the style of shared memory we're
> using now (I guess it's System V) has no way to resize a shared memory
> segment at all, and certainly no way that's portable. However it also
> looked as though POSIX shm (shm_open, etc.) can be resized using
> ftruncate(). Whether this is portable to all the platforms we run on,
> or whether the behavior of ftruncate() in combination with shm_open()
> is in the standard, I'm not sure.

It's not portable. That's exactly what we were looking into back when.

> I believe I went back and reread
> the old threads on this topic and it seems like the sticking point as
> far as POSIX shm goes it that it lacks a readable equivalent of
> shm_nattch.

Yeah, that was another little problem. In principle though we only need
one SysV-style shmem segment to get the required interlock, and there
could be add-on shmem segments using POSIX or other APIs. But that
doesn't get you out from under the portability issue or the memory space
management issue (it's unlikely you can enlarge a segment without
remapping it).

regards, tom lane