Markus Wanner wrote: > 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).
Yep, you would have to use chunks in threads/malloc, and you have to do the same thing with shared memory. -- Bruce Momjian <br...@momjian.us> http://momjian.us EnterpriseDB http://enterprisedb.com + It's impossible for everything to be true. + -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
