Kernel

Tuning vm.overcommit_memory shifts the failure point from a blind heuristic back to your own budget, which changes exactly who eats an OOM kill. Why malloc rarely fails vm.overcommit_memory defaults to 0, the heuristic mode that says yes to almost every mmap and malloc while it can plausibly honor them, and only refuses when the system is genuinely short on memory. Committing virtual memory costs almost nothing up front because pages are handed out lazily, so most allocations succeed and most normal applications stay alive at near-zero cost. The price of the generosity is that the kernel can over-commit far beyond RAM, betting that you will not touch every page; when many bytes actually get written, someone has to pay with an OOM kill. ...

sysctl is not a configuration file format. It is a live view of the kernel’s parameter table, reachable by dotted name and mirrored under /proc/sys, so reading and writing a value is literally reading and writing a pseudo-file. That is why sysctl -w takes effect immediately and why nothing set that way survives a reboot. ...