Week 6 - Swapping, OOM, Memory Pressure (PSI)¶

6.1 Conceptual Core¶

Swap is the kernel's overflow valve when anon memory pressure exceeds available RAM. Modern systems often run swapless or with a small swap (e.g., zswap or zram).
The OOM killer is the kernel's last-resort mechanism: when all reclaim has failed and an allocation cannot be satisfied, it kills the process with the highest oom_score (heuristic of memory usage and adjusted by oom_score_adj).
PSI (Pressure Stall Information)-/proc/pressure/{cpu,memory,io} and per-cgroup `pressure.{cpu,memory,io} - reports time the system or a cgroup spent stalled on each resource. The modern signal for "this system is sad."

swapon, swapoff, /proc/swaps, vm.swappiness.
zram (compressed RAM-backed swap) configuration via systemd-zram-generator or manually with zramctl.
OOM tuning:
oom_score_adj per-process (/proc/<pid>/oom_score_adj, range -1000 to 1000).
systemd OOMScoreAdjust= directive.
vm.overcommit_memory (0/1/2): allow / always-allow / strict accounting.
PSI semantics:
`some - at least one task stalled.
`full - all runnable tasks stalled (system-wide can't reach this for CPU).
Numbers are 10s/60s/300s averages of stall percentage.

Write a memory-eater program. Run inside a memory.high=512M cgroup. Observe pressure.memory rise.
Push past memory.max; watch the OOM killer. Check dmesg and journalctl -k | grep -i 'killed process'.
Set oom_score_adj=-500 on a critical process; verify it survives an OOM event triggered by another, lower-priority hog.
Measure PSI under realistic load: capture pressure.memory every second for 5 minutes during a workload spike. Plot.

Add MemoryHigh= and MemoryMax= to every long-running service. Use MemoryHigh as a soft target (slows allocations) and MemoryMax as the hard cliff.

Hook bpftrace -e 'kprobe:oom_kill_process { printf("%s killed %s\n", comm, str(arg0->comm)) }' to observe OOM events live.