[ Pobierz całość w formacie PDF ]

operate in various states of reduced power consumption, applications in user space that request
unnecessary work from system hardware prevent the hardware from entering those states. During the
development of Red Hat Enterprise Linux 6, audits were undertaken in the following areas to reduce
unnecessary demands on hardware:
Reduced wakeups
Red Hat Enterprise Linux 6 uses a tickless kernel (refer to Section 3.6,  Tickless Kernel ), which allows
the CPUs to remain in deeper idle states longer. However, the timer tick is not the only source of
excessive CPU wakeups, and function calls from applications can also prevent the CPU from entering or
remaining in idle states. Unnecessary function calls were reduced in over 50 applications.
Reduced storage and network IO
Input or output (IO) to storage devices and network interfaces forces devices to consume power. In
storage and network devices that feature reduced power states when idle (for example, ALPM or ASPM),
this traffic can prevent the device from entering or remaining in an idle state, and can prevent hard drives
from spinning down when not in use. Excessive and unnecessary demands on storage have been
minimized in several applications. In particular, those demands that prevented hard drives from spinning
down.
Initscript audit
Services that start automatically whether required or not have great potential to waste system
resources. Services instead should default to "off" or "on demand" wherever possible. For example, the
BlueZ service that enables Bluetooth support previously ran automatically when the system started,
whether Bluetooth hardware was present or not. The BlueZ initscript now checks that Bluetooth
hardware is present on the system before starting the service.
37
Red Hat Enterprise Linux 6 Power Management Guide
Chapter 4. Use Cases
This chapter describes two types of use case to illustrate the analysis and configuration methods
described elsewhere in this guide. The first example considers typical servers and the second is a
typical laptop.
4.1. Example  Server
A typical standard server nowadays comes with basically all of the necessary hardware features
supported in Red Hat Enterprise Linux 6. The first thing to take into consideration is the kinds of
workloads for which the server will mainly be used. Based on this information you can decide which
components can be optimized for power savings.
Regardless of the type of server, graphics performance is generally not required. Therefore, GPU power
savings can be left turned on.
Webserver
A webserver needs network and disk I/O. Depending on the external connection speed 100 Mbit/s might
be enough. If the machine serves mostly static pages, CPU performance might not be very important.
Power-management choices might therefore include:
no disk or network plugins for tuned.
ALPM turned on.
ondemand governor turned on.
network card limited to 100 Mbit/s.
Compute server
A compute server mainly needs CPU. Power management choices might include:
depending on the jobs and where data storage happens, disk or network plugins for tuned; or for
batch-mode systems, fully active tuned.
depending on utilization, perhaps the performance governor.
Mailserver
A mailserver needs mostly disk I/O and CPU. Power management choices might include:
ondemand governor turned on, because the last few percent of CPU performance are not important.
no disk or network plugins for tuned.
network speed should not be limited, because mail is often internal and can therefore benefit from a
1 Gbit/s or 10 Gbit/s link.
Fileserver
Fileserver requirements are similar to those of a mailserver, but depending on the protocol used, might
require more CPU performance. Typically, Samba-based servers require more CPU than NFS, and NFS
typically requires more than iSCSI. Even so, you should be able to use the ondem and governor.
Directory server
A directory server typically has lower requirements for disk I/O, especially if equipped with enough RAM.
Network latency is important although network I/O less so. You might consider latency network tuning
with a lower link speed, but you should test this carefully for your particular network.
38
Chapter 4. Use Cases
4.2. Example  Laptop
One other very common place where power management and savings can really make a difference are
laptops. As laptops by design normally already use drastically less energy than workstations or servers
the potential for absolute savings are less than for other machines. When in battery mode, though, any
saving can help to get a few more minutes of battery life out of a laptop. Although this section focuses on
laptops in battery mode, but you certainly can still use some or all of those tunings while running on AC
power as well.
Savings for single components usually make a bigger relative difference on laptops than they do on
workstations. For example, a 1 Gbit/s network interface running at 100 Mbits/s saves around 3 4 watts.
For a typical server with a total power consumption of around 400 watts, this saving is approximately
1 %. On a laptop with a total power consumption of around 40 watts, the power saving on just this one
component amounts to 10 % of the total.
Specific power-saving optimizations on a typical laptop include:
Configure the system BIOS to disable all hardware that you do not use. For example, parallel or serial
ports, card readers, webcams, WiFi, and Bluetooth just to name a few possible candidates.
Dim the display in darker environments where you do not need full illumination to read the screen
comfortably. Use System+Preferences ’! Power Management on the GNOME desktop, Kickoff
Application Launcher+Computer+System Settings+Advanced ’! Power Management on
the KDE desktop; or gnome-power-manager or xbacklight at the command line; or the function
keys on your laptop.
Use the laptop-battery-powersave profile of tuned-adm to enable a whole set of power-
saving mechanisms. Note that performance and latency for the hard drive and network interface are
impacted.
Additionally (or alternatively) you can perform many small adjustments to various system settings:
use the ondemand governor (enabled by default in Red Hat Enterprise Linux 6)
enable laptop mode (part of the laptop-battery-powersave profile): [ Pobierz całość w formacie PDF ]