Linux

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. ...

systemd-tmpfiles describes filesystem objects and housekeeping rules in configuration files. A rule can create a directory with a chosen mode, while an age rule can make old entries eligible for cleanup. The command must actually run the relevant operation for either effect to occur. This is useful lifecycle policy, but it is not a continuous watcher or a hard storage quota. ...

A Type=oneshot service represents finite work whose startup completes when its start commands finish successfully. That makes it useful for setup steps and scheduled jobs whose completion matters to a dependent unit. The word oneshot describes one activation’s process model. It does not create a permanent record that prevents the task from running again after a restart, another activation, or a reboot. ...

A veth pair consists of two interconnected virtual Ethernet devices. A frame transmitted through one is delivered to the other, where the receiving network stack or bridge handles it. The two ends can reside in separate Linux network namespaces, making the pair useful for container networking. Creating the pair supplies a link; addresses, routes, bridge membership, forwarding, and access policy are additional configuration. ...

Treating all faults as slow conflates the cheap and the expensive, which is why the split between minor and major matters when you tune memory-heavy services. Minor faults map a page A minor page fault happens when the page is already resident in RAM but the process does not yet have it mapped. The kernel only has to wire up a page-table entry — pointing the virtual address at physical memory that is already there. Common cases: first touching a freshly allocated zeroed region, mapping a shared library that another process already loaded, or a page that was swapped in once and sits in memory. Minor faults are cheap: a few hundred nanoseconds, no disk, no IO. They happen constantly in normal-running processes and are not, by themselves, a performance problem. ...

Socket activation lets systemd open a listening endpoint before the application process starts. Activity on that endpoint can then trigger a compatible service, which receives descriptors for sockets the manager already owns. The useful boundary is listener ownership versus application lifetime. An open endpoint is not yet proof that the application has initialized, accepted a request, or produced a correct response. ...

A restart policy decides whether a service manager should try another process after the current one ends. It can recover from a transient failure, but it cannot correct a missing configuration or a deterministic bug. The useful contract has three parts: which endings qualify, how long to wait, and how many starts are permitted within a time window. Those settings answer different questions. ...

Linux procfs exposes process and kernel information through filesystem interfaces. Many entries generate their contents when read instead of storing ordinary disk-file bytes. Each entry has a documented meaning, permission model, and format. Reading several entries does not freeze the process or create a transactionally consistent snapshot, so useful observations must be tied to the right process instance and interpretation. ...