Re: dynamically allocating chunks from shared memory

Robert Haas wrote:
> On Mon, Aug 9, 2010 at 11:02 AM, Bruce Momjian <bruce(at)momjian(dot)us> wrote:
> > I am not sure threads would greatly help us. ?The major problem is that
> > all of our our structures are currently contiguous in memory for quick
> > access. ?I don't see how threading would help with that. ?We could use
> > realloc(), but we can do the same in shared memory if we had a chunk
> > infrastructure, though concurrent access to that memory would hurt us in
> > either threads or shared memory.
> >
> > Fundamentally, recreating the libc memory allocation routines is not
> > that hard. ?(Everyone has to detach from the shared memory segment, but
> > they have to stop using it too, so it doesn't seem that hard.)
>
> I actually don't think that's true. The advantage (and disadvantage)
> of using threads is that everything runs in one address space. So you
> just allocate more memory and everyone immediately sees it. In a
> process environment, that's not the case: to expand or shrink the size
> of the shared memory arena, everyone needs to explicitly change their
> own mapping.

You can't expand the size of malloc'ed memory --- you have to call
realloc(), and then you effectively get a new pointer. Shared memory
has a similar limitation. If you allocate shared memory in chunks so
you don't need to change the location, you are effectively doing another
malloc(), like you would in a threaded process.

--
Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://enterprisedb.com

+ It's impossible for everything to be true. +