Practically Correct, Just-in-Time Shell Script Parallelization

Recent shell-script parallelization systems enjoy mostly automated parallel speedups by compiling scripts ahead-of-time. Unfortunately, such static parallelization is hampered by the dynamic behaviors pervasive in shell scripts—e.g., variable expansion and command substitution—which often requires reasoning about the current state of the shell and filesystem.

We present a just-in-time (JIT) shell-script compiler, PaSh-JIT, that intermixes evaluation and parallelization during a script's run-time execution. JIT parallelization collects run-time information about the system’s state, but must not alter the behavior of the original script and must maintain minimal overhead. PaSh-JIT addresses these challenges by (1) using a dynamic interposition framework, guided by a static preprocessing pass, (2) developing runtime support for transparently pausing and resuming shell execution; and (3) operating as a stateful server, communicating with the current shell by passing messages—all without requiring modifications to the system's underlying shell interpreter.

When run on a wide variety of benchmarks, including the POSIX shell test suite, PaSh-JIT (1) does not break scripts, even in cases that are likely to break shells in widespread use; and (2) offers significant speedups, whenever parallelization is possible. These results show that PaSh-JIT can be used as a drop-in replacement for any non-interactive shell use, providing significant speedups without any risk of breakage.