Re: dynamically allocating chunks from shared memory

Robert Haas wrote:
> On Mon, Aug 9, 2010 at 2:50 PM, Bruce Momjian <bruce(at)momjian(dot)us> wrote:
> > Robert Haas wrote:
> >> On Mon, Aug 9, 2010 at 2:33 PM, Bruce Momjian <bruce(at)momjian(dot)us> wrote:
> >> >> > Let me be more concrete. ?Suppose you are using threads, and you want to
> >> >> > increase your shared memory from 20MB to 30MB. ?How do you do that? ?If
> >> >> > you want it contiguous, you have to use realloc, which might move the
> >> >> > pointer. ?If you allocate another 10MB chunk, you then have shared
> >> >> > memory fragments, which is the same as adding another shared memory
> >> >> > segment.
> >> >>
> >> >> You probably wouldn't do either of those things. ?You'd just allocate
> >> >> small chunks here and there for whatever you need them for.
> >> >
> >> > Well, then we do that with shared memory then --- my point is that it is
> >> > the same problem with threads or processes.
> >>
> >> Well, I think your point is wrong, then. ?:-)
> >>
> >> It's not the same at all. ?If you have a bunch of threads in one
> >> address space, "shared" memory is really just process-local. ?You can
> >> grow the total amount of allocated space just by calling malloc().
> >> With our architecture, you can't.
> >
> > You effectively have to add infrastructure to add/remove shared memory
> > segments to match memory requests. ?It is another step, but it is the
> > same behavior.
>
> That would be one way to tackle the problem, but there are
> difficulties. If we just created new shared memory segments at need,
> we might end up with a lot of shared memory segments. I suspect that
> would get complicated and present many management difficulties - which
> is why I'm so far of the opinion that we should try to architect the
> system to avoid the need for this functionality. I don't think it's
> going to be too easy to provide, short of (as Tom says) moving to the
> MySQL model of many threads working in a single process.

You could allocate shared memory in chunks and then pass that out to
requestors, the same way sbrk() does it.

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

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