Heuristic for "hung" Perl Code

Software “hangs,” where the software appears wedged or otherwise unresponsive, are remarkably easy to produce, though the hang may stem from any number of different problems.

$ perl -e '$i='

Describing the problem as “my code hangs” or the equally bad “it doesn’t work” only leads to new questions (hung like laundry?) and various areas of additional research:

Add warn statements (or use the debugger, or Devel::ptkdb, or perhaps DDD; I simply use warn statements) if the cause of the hang is unknown. Narrow down the problem as much as possible. Once isolated, try to reproduce the problem in a minimal test script.
The hang might be a buffering problem. Peruse the “Suffering from Buffering” article to understand how, if at all, the program buffers output. For standard output, try setting $|=1.
Check—carefully, if on an unknown system—for NFS or other remote filesystems, as these may wedge a process in the kernel. The degree of care required depends on the ease with which a new shell can be opened, or a hung df(1) command worked around. If possible, work under screen or dtach.
Otherwise, is the hang due to the process waiting for terminal input? Or is it buffering for output? Waiting for input from a local filesystem or remote network resource? Waiting on a child process? These questions can be answered with lsof, or a system tracing utility such as ktrace(1) or strace. These tools, while complex, are fairly easy to learn. Does the process hang after a specific system call, such as read(2)? Or does it loop on a particular system call or sequence of system calls? Read the man page if unfamiliar with the call.
If possible, trace perl -e '$i=' or grep something to see how a process hangs on input. Also find and trace a process that hangs on output, or one that is wedged in a loop on a particular system call, so that identifying hung processes becomes easier in “the wild” of production.

If these steps do not resolve the problem, consult Getting Answers prior to seeking help from others.

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Jeremy Mates’s Blog

Heuristic for "hung" Perl Code