Understanding what the machine is doing.
WHAT TO MEASURE
Processor use Memory use Disk activity and space Network throughput Load average
WHAT top AND htop SHOW
Processes and their consumption, live.
WHAT vmstat SHOWS
System-wide activity over intervals.
WHAT iostat SHOWS
Disk throughput and wait times.
WHAT iotop SHOWS
Which process is causing disk activity.
WHY THAT MATTERS
High load with idle processors is almost always disk waiting.
WHAT THE WAIT FIGURE IN top MEANS
Time spent waiting for input or output rather than computing.
WHAT A HIGH VALUE INDICATES
The disk is the constraint.
WHAT free SHOWS
Memory use, including cache.
WHY MEMORY LOOKS FULL WHEN IT IS NOT
Linux uses free memory as cache and releases it on demand.
WHAT TO LOOK AT INSTEAD
The available figure.
WHAT SWAP ACTIVITY INDICATES
Genuine memory pressure, and it is far slower than memory.
WHAT TO DO ABOUT SUSTAINED SWAPPING
Find what is consuming memory, before adding more.
WHAT TO ESTABLISH
A baseline of normal, so abnormal is recognisable.
