On Thu, May 10, 2007 at 12:01:34PM -0400, Robert G. Brown wrote: > I am (as you may well know) extremely fortran averse. However, a > researcher in our department has recently asked what the current limits > are on the size of an array in modern fortran(s) under linux. I suppose > he'd like an answer for both 32 and 64 bit systems. From what I have > been able to google out, it looks like the answer is 2^31 bytes in 32 > bit systems and as big as the hardware permits less maybe a GB in 64 bit > systems. Is this correct? Any fortran experts out there? I'd be happy > for answers for more than one compiler, of course -- I'd guess that e.g. > pathscale might have a greater capacity than e.g. gfortran than g77 > legacy...
In *theory*, the limits are as you described. However, that theory doesn't acknowledge memory layouts, shared libraries, memory-mapped space, &etc. Clearly, these details are mostly (only) significant on a 32-bit system. The gross 32-bit layout, from bottom to top is: program & data, shared libs & mmap, heap, and stack; with the space between the code/data and shared libs controlled by brk(2). Getting to Fortran, the next interesting bit is where the array is located. If in a common, the array is going to be in the break-controlled region between the program and the shared libraries & mmap. I don't think this varies among compilers, but could be wrong here; it's true for gcc. The kernel location TASK_UNMAPPED_BASE figures in this, as that's the base for the shared libraries & mmap(2) space. On i386, this is defined as TASK_SIZE/3, where TASK_SIZE is the top of the process address space, at 3GB, i.e., TASK_UNMAPPED_BASE=1GB. See the source at include/<arch>/processor.h for these values. Shared library positioning can be tweaked via prelink(8), so max common-resident array size may well depend on the specific system. Otherwise, the array's going to be in heap or stack, and that's somewhere between TASK_UNMAPPED_BASE and TASK_SIZE. I think I recall that malloc(3) will also return stuff below TASK_UNMAPPED_BASE if there's not enough space above. On a 64-bit system, the 32-bit limits apply to a 32-bit code, otherwise, well the limitation is not likely the system. -- David N. Lombard, Intel, Irvine, CA I do not speak for Intel Corporation; all comments are strictly my own. _______________________________________________ Beowulf mailing list, Beowulf@beowulf.org To change your subscription (digest mode or unsubscribe) visit http://www.beowulf.org/mailman/listinfo/beowulf
