Instrument
The Operations Score.
The book's signature four-band 0-100 score, live. Feed it a measured signal and read it against the service-level indicators: contention is proof of slowness, and the worst signal decides.
The dial
Scores the highlighted signal. 100 means no contention at all; the SLI sits at the top of green (75); 0 is four times the SLI or worse.
88 /100
green · healthy
CPU ready time of 1.2% is within the 2.5% indicator, scoring 88 in the green band.
The four signals
The service-level indicators. Move a slider (or type) and the dial follows whichever signal you touched. Zero (green) on the left, four times the SLI (red) on the right; the tick marks the SLI at the top of green.
CPUCPU ready timeSLI 2.5%
%88
Memorymemory latencySLI 1%
%90
Diskdisk latencySLI 10ms
ms85
Networktransmit-dropped packetsSLI 0
pkts100
Worst signal: disk latency at 6ms — all four signals are inside their SLIs.85
Same maths iO uses when you give it a measured value. Thresholds and the four bands come from the guide: Performance: VM →