Stacks, Stack Sizes, and Parallelization

The executing program maintains a main memory stack for the parent program and distinct stacks for each thread. Stacks are temporary memory address spaces used to hold arguments and AUTOMATIC variables over subprogram invocations.

The default size of the main stack is about 8 megabytes. The Fortran compilers normally allocate local variables and arrays as STATIC (not on the stack). However, the -stackvar option forces allocation of all local variables and arrays on the stack (as if they were AUTOMATIC variables). Use of -stackvar is recommended with parallelization because it improves the optimizer's ability to parallelize CALLs in loops. -stackvar is required with explicitly parallelized loops containing subprogram calls. (See the discussion of -stackvar in the Fortran User's Guide.)

The limit command displays the current main stack size as well as setting it:

demo% limit             C shell example
cputime       unlimited
filesize       unlimited
datasize       2097148 kbytes
stacksize       8192 kbytes            <- current main stack size
coredumpsize       0 kbytes
descriptors       64 
memorysize       unlimited
demo% limit stacksize 65536       <- set main stack to 64Mb
demo% limit stacksize
stacksize       65536 kbytes

demo$ >ulimit -a         Korn Shell example
time(seconds)        unlimited
file(blocks)         unlimited
data(kbytes)         2097148
stack(kbytes)        8192
coredump(blocks)     0
nofiles(descriptors) 64
vmemory(kbytes)      unlimited
demo$ ulimit -s 65536
demo$ ulimit -s
65536

Each thread of a multithreaded program has its own thread stack. This stack mimics the main program stack but is unique to the thread. The thread's PRIVATE arrays and variables (local to the thread) are allocated on the thread stack. The default size is 256 kilobytes. The size is set with the STACKSIZE environment variable:

demo% setenv STACKSIZE 8192    <- Set thread stack size to 8 Mb    C shell
                          -or-
demo$ STACKSIZE=8192           Bourne/Korn Shell
demo$ export STACKSIZE

Setting the thread stack size to a value larger than the default may be necessary for most parallelized Fortran codes. However, it may not be possible to know just how large to set it, except by trial and error, especially if private/local arrays are involved. If the stack size is too small for a thread to run, the program will abort with a segmentation fault.