  Although cloud computing has been transformational to the IT industry, it is
built on large data centres that often consume massive amounts of electrical
power. Efforts have been made to reduce the energy clouds consume, with certain
data centres now approaching a Power Usage Effectiveness (PUE) factor of 1.08.
While this is an incredible mark, it also means that the IT infrastructure
accounts for a large part of the power consumed by a data centre. Hence, means
to monitor and analyse how energy is spent have never been so crucial. Such
monitoring is required not only for understanding how power is consumed, but
also for assessing the impact of energy management policies. In this article,
we draw lessons from experience on monitoring large-scale systems and introduce
an energy monitoring software framework called KiloWatt API (KWAPI), able to
handle OpenStack clouds. The framework --- whose architecture is scalable,
extensible, and completely integrated into OpenStack --- supports several
wattmeter devices, multiple measurement formats, and minimises communication
overhead.
