At 2013-10-30 00:10:39 -0700, da...@fetter.org wrote: > > How about documenting that 2MB is the quantum (OK, we'll say > "indivisible unit" or "smallest division" or something) and failing > with a message to that effect if someone tries to set it otherwise?
I don't think you understand the problem. We're not discussing a user setting here. The size that is passed to PGSharedMemoryCreate is based on shared_buffers and our estimates of how much memory we need for other things like ProcArray (see ipci.c:CreateSharedMemoryAndSemaphores). If this calculated size is not a multiple of a page size supported by the hardware (usually 2/4/16MB etc.), the allocation will fail under some commonly-used kernels. We can either ignore the problem and let the allocation fail, or try to discover the smallest supported huge page size (what the patch does now), or assume that 2MB pages can be used if any huge pages can be used and align accordingly. We could use a larger size, e.g. if we aligned to 16MB then it would work on hardware that supported 2/4/8/16MB pages, but we'd waste the extra memory unless we also increased NBuffers after the rounding up (which is also something Andres suggested earlier). I don't have a strong opinion on the available options, other than not liking the "do nothing" approach. -- Abhijit -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
