Re: dynamically allocating chunks from shared memory

On Mon, Aug 9, 2010 at 12:03 PM, Bruce Momjian <bruce(at)momjian(dot)us> wrote:
> 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.

The point isn't what happens when you resize individual chunks; it's
what happens when you need to expand the arena.

--
Robert Haas
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