Re: dynamically allocating chunks from shared memory

On Mon, Aug 9, 2010 at 2:33 PM, Bruce Momjian <bruce@momjian.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.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise Postgres Company