Friday, 27 March 2009

Memory Usage and OS differences

The work that we have done so far on reducing memory usage has concentrated on the windows platform, but we also run our products on UNIX (AIX & HP) and z/OS.

Each platform is different with respect to when memory is allocated, so an understanding of this is important when trying to reduce memory usage, especially if your code will operate on different platforms.

The examples that we are using relate to generated C or COBOL code and not other platforms like Java or .Net.

For C code, the storage for local views is allocated on initial load of the executable. For static modules this will be when the executable is loaded, and for DLLs, this will also be when the executable is invoked. However, once it has been invoked, the memory will remain allocated until the executable terminates. For an executable loaded by the Transaction Enabler (TE), this might be some time!

You can avoid loading a DLL that might not be invoked for each execution with the /DELAYLOAD linker option on Windows and equivalent options for UNIX (e.g. -blazy on AIX). We are experimenting with this and will report back on findings in a later posting, but so far we have seen a benefit of Windows without a noticeable overhead. The main benefit on Windows is that we can now keep many more executables loaded in the TE and thus do not have the overhead of these being swapped in and out.

On z/OS, the COBOL working storage is allocated from the heap for re-entrant code. (Normally Gen programs are compiled with the RENT option). This means that the working storage is allocated on first execution of the action block and then not freed until the executable terminates. If you have an executable with a lot of action blocks that are rarely invoked, then they will not present a memory overhead compared with C code where the storage for all action blocks will be allocated on initial load of the executable.

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