Reading free and top correctly.
WHAT CONFUSES EVERYONE
A server reporting almost no free memory while behaving perfectly.
WHY IT HAPPENS
Linux uses unallocated memory as disk cache, and releases it instantly when needed.
WHAT THAT MEANS
Free memory is wasted memory, by design.
WHAT FIGURE TO READ INSTEAD
Available, which accounts for reclaimable cache.
WHAT BUFFERS AND CACHE ARE
Memory holding recently used disk data.
WHY THEY HELP
Repeated reads come from memory rather than disk.
WHAT TO WORRY ABOUT
Available memory approaching zero Sustained swap activity Processes being terminated by the kernel
WHAT RESIDENT MEMORY MEANS IN A PROCESS LIST
Physical memory the process currently holds.
WHAT VIRTUAL MEMORY MEANS THERE
Address space reserved, much of which is never used.
WHY VIRTUAL FIGURES ALARM PEOPLE UNNECESSARILY
They are frequently enormous and largely meaningless.
WHAT TO COMPARE ACROSS PROCESSES
Resident memory.
WHAT SHARED MEMORY COMPLICATES
Several processes showing the same memory, which cannot simply be added.
WHERE THAT MATTERS
Web server worker processes, which share a great deal.
WHAT TO ESTIMATE INSTEAD
Total usage from the system summary, not by adding processes.
WHAT TO DO ABOUT GENUINE PRESSURE
Reduce worker counts, or find the leak.
