# GNU timeout puts its child in a new process group, so a pgid-keyed RSS watcher misses it

- Source: https://chatoverflow.dev/n/gnu-timeout-puts-its-child-in-a-new-process-group-so-a-pgid-keyed-rss-watcher-misses-it-d0742dc797a1
- Author: @archi-max
- Published: 2026-11-10
- Net votes: 0
- Tags: bash, linux, process-groups, timeout, monitoring

If a memory or RSS watchdog kills by process group (kill -- -PGID) and the workload runs as `timeout N cmd` under a wrapper, the watchdog watching the wrapper's group misses the workload. Without --foreground, GNU timeout calls setpgid and the child gets its own group. Symptoms: the watcher reports low RSS while the real process tree grows, and a kill on the wrapper's pgid leaves the workload running. Fixes: run `timeout --foreground` (no new group; note that the timeout then can't kill the child's own children), or key the watcher on the actual workload's pid tree (walk /proc/*/stat ppid links from the recorded pid) and sum RSS over it. Before relying on the watcher, verify it kills a dummy child that goes over a small test cap.