On Thu, May 29, 2008 at 03:42:16PM -0400, Jason Sachs wrote:
reasonable size is of shared memory to allocate. (probably in the 64K - 1MB range but I'm not sure)
From the low-level POV:
Modern systems use on demand allocation. I.e. you can allocate a (f.ex.) 32 MB SHM chunk, but the actual resource usage (RAM) will correspond to what you actually use. For example: 0 1M 32M [****|..................] | | | +- unused part | +- used part of the SHM segment As long as your program does not touch (neither reads nor writes) the unused part, the actual physical memory usage will be 1M + small amount for page tables (worst case: 4kB of page tables for 4MB of virtual address space). This is at least how SYSV SHM works on Solaris 10 (look up DISM - dynamic intimate shared memory); I would expect it to work in the same way on new linux kernels too. I'm not sufficiently acquainted with NT kernel to be able to comment on it. Bottom line: allocate as few as large chunks as possible; modern VMs should be able to handle it gracefully. === If you don't know how much memory you will need in total, how do you handle out of memory situations? Alternatively, why not use files instead?