When memory runs short.
WHAT SWAP IS
Disk space used as an overflow for memory.
WHY IT IS SLOW
Disk is orders of magnitude slower than memory.
WHAT LIGHT SWAP USE MEANS
Normal: inactive pages moved out.
WHAT SUSTAINED SWAPPING MEANS
Genuine pressure, and severe slowness.
HOW TO SEE ACTIVITY RATHER THAN USAGE
The swap in and out columns in vmstat.
WHY THE DISTINCTION MATTERS
Swap being used is not a problem; swap being actively read and written is.
HOW MUCH TO CONFIGURE
Enough to absorb spikes, not enough to let a leaking process run indefinitely.
WHAT SWAPPINESS CONTROLS
How readily the system moves pages to swap.
WHAT TO SET IT LOWER FOR
Servers, where responsiveness matters.
WHAT THE OUT-OF-MEMORY KILLER IS
The kernel terminating a process when memory is exhausted.
HOW TO TELL IT ACTED
Messages in the system log naming the process.
WHY THAT MATTERS IN DIAGNOSIS
A database that vanished with no explanation was usually killed this way.
WHAT TO DO ABOUT IT
Find what consumed the memory, rather than only adding more.
WHAT COMMONLY CONSUMES IT ON WEB SERVERS
Too many worker processes configured A single application leaking
WHAT TO CHECK
Process counts against available memory.
