a SPECpower result, the energy efficiency values are entitled as
performance to power ratio.
(4) Server overall energy efficiency. Server overall energy
efficiency is overall performance to power ratio of a server in a
SPECpower result, i.e., the ratio of sum of ssj_ops over sum of
power for 10 utilization levels (from 10% to 100%) and active
idle. The server overall energy efficiency is also referred as a
server’s SPECpower score.
(5) Peak energy efficiency. Peak energy efficiency is
defined as the greatest (peak) energy efficiency of a server
among all utilization levels.
(6) Energy proportionality (EP). In this paper we use
the energy proportionality metric in [14]. Take a server from
SPECpower dataset in 2016 with overall score 12212 as an
example. We plot its power-utilization curve in Fig.1. Note that
the power in Fig.1 is normalized to its power at 100%
utilization. The solid line is the energy proportionality curve of
the sample server and the dotted line is of an ideally energy
proportional server. With the power-utilization curve in Fig.1,
we can compute the energy proportionality of the server as
following [14]:
ൌ ͳ െ
ೝೌ
ି
ೌ
ೌ
(1)
Thus the power-utilization curve in Fig.1 is also called the
energy proportionality curve. From Eq.1 we can see that EP is a
value equal to or greater than zero but less than 2.0. For an
ideally energy proportional server, its EP value is 1.0. For the
server in Fig.1, we approximate its EP by summarizing the
areas of ten trapezoids corresponding to ten utilization intervals
and then get its EP value 1.02 according to Eq.1
III. E
VOLUTIONARY TREND ON ENERGY PROPORTIONALITY
A.
Energy Proportionality Evolution
During the last decade, server energy efficiency improves
significantly thanks to technical breakthroughs including low
power electronics and designs, dynamic voltage frequency
scaling, and cross level coordinated power savings. We
compute the energy proportionality value of each server
according to Eq.1 and present the trend curve of all the servers’
EP and EE values in Fig.2 and their statistics in Fig.3 and Fig.4.
Not surprisingly, both energy proportionality and energy
efficiency improve with times. The average energy
proportionality increased significantly from 0.30 to 0.82 from
2005 to 2012 and seems stagnant at 0.84 in 2016. Similarly, the
minimal EP in each year also improves significantly. For
example, newest servers made in 2016 have minimal EP of
0.73, which is the greatest EP value in 2009. Among 477
servers, the least energy proportionality value is 0.18 (in 2008)
and the highest energy proportionality value is 1.05 (in 2012).
From Fig.2 and Fig.3 we observe that there are two significant
improvement steps in EP, one is from 2008 to 2009, and
another is from 2011 to 2012. From 2008 to 2009, the average
and median EP increase from 0.37 to 0.55 (+48.65%), and 0.37
to 0.56 (+51.35%), respectively.
Similarly, from 2011 to 2012, the average and median EP
increase from 0.66 to 0.82 (+24.24%), and 0.67 to 0.85
(+26.87%), respectively. From 2008 to 2009, the majority of
the servers switch their processor microarchitecture from Core
(Penryn) to Nehalem. From 2011 to 2012, the majority of the
servers switch their processor microarchitecture from Nehalem
(Westmere) to Sandy Bridge. These two switches are called
tock in Intel’s tick-tock chip iteration model which designates a
new microarchitecture.
Although energy proportionality fluctuates with times in
Fig.3, their energy efficiency increases monotonically with
times in Fig.4, including its average value, maximal value and
median value of each year in all three metrics. Only the
minimal values of the three metrics in 2014 decrease because
one server in 2014 has low EE as 1469 and low EP as 0.32.
This server has a tower form factor and Intel Core i5-4570
processor usually not designed for server purpose use, which is
quite different from other servers in 2014 or other years. Since
there are few results for year 2004, 2005, 2006, and 2014, the
statistical results in these years pose outliners and greater
deviations. For example, EP decreases significantly from 2004
to 2005 and from 2012 to 2014. But the median EP in 2014 still
increases compared with 2013.
We plot the CDF chart of energy proportionality in Fig. 5.
Among 477 servers, 25.21% servers have EP between 0.6 and
0.7, and 17.44% servers have EP between 0.8 and 0.9. These
two intervals have the most EP values. Moreover, 99.58%
servers have EP less than 1.0.
Fig. 1. Energy proportionality curve.
Fig.2. EP and EE evolution.
Fig.3. Stats. trend of EP.
0
0.5
1
0% 20% 40% 60% 80% 100%
Power
Utilization
2016 EP=1.02
Ideal
0.0
0.2
0.4
0.6
0.8
1.0
1.2
0
2000
4000
6000
8000
10000
12000
14000
2004 2010 2012 2015
EP
EE
EE
EP
0.0
0.2
0.4
0.6
0.8
1.0
1.2
2004 2007 2010 2013 201
EP
Max
Median
Average
Min
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