Hi, On 08/09/2010 06:31 PM, Bruce Momjian 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?
There's absolutely no need to pre-allocate 20 MB in advance in a threaded environment. You just allocate memory in small chunks. For a threaded-model, that memory is shared by default, so the total amount of shared memory can grow and shrink very easily. (And even makes usused memory available to other processes, not just other threads).
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.
Okay, I think I now understand the requirement of continuity you mentioned earlier already. I agree that with the current approach, we cannot simply use such a dynamic allocator to solve all of our problems.
Every subsystem would need to be converted to something that allocates shared memory in smaller chunks for such a dynamic allocator to be of any use. Robert already pointed out that this may be troublesome for shared_buffers, which is by far the largest consumer of shared memory. I didn't look into this, yet. And I'd like to hear more about the feasibility of that approach for other subsystems.
Another issue to be discussed would be the limits of sharing free memory between subsystems. Maybe we even reach the conclusion that we absolutely *want* fixed maximum sizes for every single subsystem so as to be able to guarantee a certain amount of multi-xact or SLRU entries at any point in time (otherwise one memory hungry subsystem could possibly eat it all up with another subsystem getting the OOM error when trying to allocate for its very first entry).
Thanks for bringing this discussion to live again. Regards Markus Wanner -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
