10.1.6 Stacks, Stack Sizes, and Parallelization

The executing program maintains a main memory stack for the initial thread executing the program, as well as distinct stacks for each helper 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 the 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 subprogram calls in loops. -stackvar is required with explicitly parallelized loops containing subprogram calls. (See the discussion of -stackvar in the Fortran User’s Guide.)

Using the C shell (csh), the limit command displays the current main stack size as well as sets 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

With Bourne or Korn shells, the corresponding command is ulimit:

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 helper thread of a multithreaded program has its own thread stack. This stack mimics the initial thread 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 8 megabytes on 64–bit SPARC and 64-bit x86 platforms, 4 megabytes otherwise. 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 some parallelized Fortran codes. However, it may not be possible to know just how large it should be, 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.