Red Hat Enterprise Linux 7 性能调优指南

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"《Red Hat Enterprise Linux 7 性能调优指南》是一份由 Red Hat 官方发布的重要文档，针对 Red Hat Enterprise Linux 7 系统提供了详细的性能优化策略和技巧。这份指南于 2018 年 5 月 21 日更新，作者包括 Milan Navrátil、Laura Bailey、Charlie Boyle 等来自 Red Hat Customer Content Services 的专家团队。该指南旨在帮助用户提升系统的整体吞吐量，确保在企业级环境中获得最佳性能。内容覆盖了广泛的系统优化领域，如内存管理、CPU调度、磁盘 I/O、网络性能、进程优化、数据库调优、虚拟化性能等关键环节。每个章节深入浅出地介绍了如何通过调整参数、使用工具和技术来识别和解决性能瓶颈。例如，指南可能会指导用户如何通过分析 sysbench、vmstat、iostat 等命令来监控和评估系统性能，或者如何调整内核参数以优化特定工作负载。在版权方面，此文档遵循 Creative Commons Attribution-ShareAlike 3.0 Unported License，允许用户在保留原作者Red Hat署名及链接的前提下进行分享和修改，但必须去除所有 Red Hat 商标，并确保在修改版本中尊重版权要求。作为 Red Hat 的官方出版物，该指南体现了 Red Hat 对性能优化的深度理解和实践经验，为 Red Hat Enterprise Linux 7 的运维人员和系统管理员提供了一手的优化指南，是提高系统效率和响应速度的重要参考资料。无论是对于日常维护还是应对性能问题的应急处理，这份文档都具有很高的实用价值。"

CHAPTER 3. TUNED

3.1. TUNED OVERVIEW

Tuned is a daemon that uses udev to monitor connected devices and statically and dynamically tunes

system settings according to a selected profile. Tuned is distributed with a number of predefined profiles

for common use cases like high throughput, low latency, or powersave. It is possible to modify the rules

defined for each profile and customize how to tune a particular device. To revert all changes made to the

system settings by a certain profile, you can either switch to another profile or deactivate the tuned

service.

NOTE

Starting with Red Hat Enterprise Linux 7.2, you can run Tuned in no-daemon mode,

which does not require any resident memory. In this mode, tuned applies the settings and

exits. The no-daemon mode is disabled by default because a lot of tuned functionality is

missing in this mode, including D-Bus support, hot-plug support, or rollback support for

settings. To enable no-daemon mode, set the following in the /etc/tuned/tuned-

main.conf file: daemon = 0.

Static tuning mainly consists of the application of predefined sysctl and sysfs settings and one-shot

activation of several configuration tools like ethtool. Tuned also monitors the use of system components

and tunes system settings dynamically based on that monitoring information.

Dynamic tuning accounts for the way that various system components are used differently throughout

the uptime for any given system. For example, the hard drive is used heavily during startup and login,

but is barely used later when the user might mainly work with applications such as web browsers or

email clients. Similarly, the CPU and network devices are used differently at different times. Tuned

monitors the activity of these components and reacts to the changes in their use.

As a practical example, consider a typical office workstation. Most of the time, the Ethernet network

interface is very inactive. Only a few emails go in and out every once in a while or some web pages

might be loaded. For those kinds of loads, the network interface does not have to run at full speed all the

time, as it does by default. Tuned has a monitoring and tuning plug-in for network devices that can

detect this low activity and then automatically lower the speed of that interface, typically resulting in a

lower power usage. If the activity on the interface increases for a longer period of time, for example

because a DVD image is being downloaded or an email with a large attachment is opened, tuned

detects this and sets the interface speed to maximum to offer the best performance while the activity

level is so high. This principle is used for other plug-ins for CPU and hard disks as well.

Dynamic tuning is globally disabled in Red Hat Enterprise Linux and can be enabled by editing the

/etc/tuned/tuned-main.conf file and changing the dynamic_tuning flag to 1.

3.1.1. Plug-ins

Tuned uses two types of plugins: monitoring plugins and tuning plugins. Monitoring plugins are used to

get information from a running system. Currently, the following monitoring plugins are implemented:

disk

Gets disk load (number of IO operations) per device and measurement interval.

net

Performance Tuning Guide

Gets network load (number of transferred packets) per network card and measurement interval.

load

Gets CPU load per CPU and measurement interval.

The output of the monitoring plugins can be used by tuning plugins for dynamic tuning. Currently

implemented dynamic tuning algorithms try to balance the performance and powersave and are

therefore disabled in the performance profiles (dynamic tuning for individual plugins can be enabled or

disabled in the tuned profiles). Monitoring plugins are automatically instantiated whenever their metrics

are needed by any of the enabled tuning plugins. If two tuning plugins require the same data, only one

instance of the monitoring plugin is created and the data is shared.

Each tuning plugin tunes an individual subsystem and takes several parameters that are populated from

the tuned profiles. Each subsystem can have multiple devices (for example, multiple CPUs or network

cards) that are handled by individual instances of the tuning plugins. Specific settings for individual

devices are also supported. The supplied profiles use wildcards to match all devices of individual

subsystems (for details on how to change this, refer to Section 3.1.3, “Custom Profiles”), which allows

the plugins to tune these subsystems according to the required goal (selected profile) and the only thing

that the user needs to do is to select the correct tuned profile.

Currently, the following tuning plugins are implemented (only some of these plugins implement dynamic

tuning, parameters supported by plugins are also listed):

cpu

Sets the CPU governor to the value specified by the governor parameter and dynamically changes

the PM QoS CPU DMA latency according to the CPU load. If the CPU load is lower than the value

specified by the load_threshold parameter, the latency is set to the value specified by the

latency_high parameter, otherwise it is set to value specified by latency_low. Also the latency

can be forced to a specific value without being dynamically changed further. This can be

accomplished by setting the force_latency parameter to the required latency value.

eeepc_she

Dynamically sets the FSB speed according to the CPU load; this feature can be found on some

netbooks and is also known as the Asus Super Hybrid Engine. If the CPU load is lower or equal to the

value specified by the load_threshold_powersave parameter, the plugin sets the FSB speed to

the value specified by the she_powersave parameter (for details about the FSB frequencies and

corresponding values, see the kernel documentation, the provided defaults should work for most

users). If the CPU load is higher or equal to the value specified by the load_threshold_normal

parameter, it sets the FSB speed to the value specified by the she_normal parameter. Static tuning

is not supported and the plugin is transparently disabled if the hardware support for this feature is not

detected.

net

Configures wake-on-lan to the values specified by the wake_on_lan parameter (it uses same syntax

as the ethtool utility). It also dynamically changes the interface speed according to the interface

utilization.

sysctl

Sets various sysctl settings specified by the plugin parameters. The syntax is name=value, where

name is the same as the name provided by the sysctl tool. Use this plugin if you need to change

settings that are not covered by other plugins (but prefer specific plugins if the settings are covered

CHAPTER 3. TUNED

by them).

usb

Sets autosuspend timeout of USB devices to the value specified by the autosuspend parameter.

The value 0 means that autosuspend is disabled.

Enables or disables transparent huge pages depending on the Boolean value of the

transparent_hugepages parameter.

audio

Sets the autosuspend timeout for audio codecs to the value specified by the timeout parameter.

Currently snd_hda_intel and snd_ac97_codec are supported. The value 0 means that the

autosuspend is disabled. You can also enforce the controller reset by setting the Boolean parameter

reset_controller to true.

disk

Sets the elevator to the value specified by the elevator parameter. It also sets ALPM to the value

specified by the alpm parameter, ASPM to the value specified by the aspm parameter, scheduler

quantum to the value specified by the scheduler_quantum parameter, disk spindown timeout to the

value specified by the spindown parameter, disk readahead to the value specified by the

readahead parameter, and can multiply the current disk readahead value by the constant specified

by the readahead_multiply parameter. In addition, this plugin dynamically changes the advanced

power management and spindown timeout setting for the drive according to the current drive

utilization. The dynamic tuning can be controlled by the Boolean parameter dynamic and is enabled

by default.

mounts

Enables or disables barriers for mounts according to the Boolean value of the disable_barriers

parameter.

script

This plugin can be used for the execution of an external script that is run when the profile is loaded or

unloaded. The script is called by one argument which can be start or stop (it depends on whether

the script is called during the profile load or unload). The script file name can be specified by the

script parameter. Note that you need to correctly implement the stop action in your script and revert

all setting you changed during the start action, otherwise the roll-back will not work. For your

convenience, the functions Bash helper script is installed by default and allows you to import and

use various functions defined in it. Note that this functionality is provided mainly for backwards

compatibility and it is recommended that you use it as the last resort and prefer other plugins if they

cover the required settings.

sysfs

Sets various sysfs settings specified by the plugin parameters. The syntax is name=value, where

name is the sysfs path to use. Use this plugin in case you need to change some settings that are not

covered by other plugins (please prefer specific plugins if they cover the required settings).

video

Sets various powersave levels on video cards (currently only the Radeon cards are supported). The

Performance Tuning Guide

This command requires the tuned service to be running.

To view the available installed profiles, run:

tuned-adm list

To view the currently activated profile, run:

tuned-adm active

To select or activate a profile, run:

tuned-adm profile profile

For example:

tuned-adm profile powersave

As an experimental feature it is possible to select more profiles at once. The tuned application will try to

merge them during the load. If there are conflicts the settings from the last specified profile will take

precedence. This is done automatically and there is no checking whether the resulting combination of

parameters makes sense. If used without thinking, the feature may tune some parameters the opposite

way which may be counterproductive. An example of such situation would be setting the disk for the

high throughput by using the throughput-performance profile and concurrently setting the disk

spindown to the low value by the spindown-disk profile. The following example optimizes the system

for run in a virtual machine for the best performance and concurrently tune it for the low power

consumption while the low power consumption is the priority:

tuned-adm profile virtual-guest powersave

To let tuned recommend you the best suitable profile for your system without changing any existing

profiles and using the same logic as used during the installation, run the following command:

tuned-adm recommend

Tuned itself has additional options that you can use when you run it manually. However, this is not

recommended and is mostly intended for debugging purposes. The available options can be viewing

using the following command:

tuned --help

3.1.3. Custom Profiles

Distribution-specific profiles are stored in the /usr/lib/tuned/ directory. Each profile has its own

directory. The profile consists of the main configuration file called tuned.conf, and optionally other

files, for example helper scripts.

If you need to customize a profile, copy the profile directory into the /etc/tuned/ directory, which is

used for custom profiles. If there are two profiles of the same name, the profile included in

/etc/tuned/ is used.

Performance Tuning Guide

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