% improvement
Posted: Wed Apr 02, 2014 12:03 am
if I reduce the runtime from 100s to 80s, what's my % improvement?
"Improvement" implies you are measuring performance. Performance is proportional to the inverse of wallclock time.
Therefore, if you want to quote a percentage improvement, you would calculate performance like so:
new = 1/80s = 0.0125 units of work per second
old = 1/100s = 0.0100 units of work per second
improvement = new/old = 0.0125/0.0100 = 1.25x or +25%
The 20% figure is accurate if you are quoting a reduction in wallclock time, but you would typically phrase that as -20%, because wallclock time decreased.