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Hack 91. Tweak Your Kernel Without Recompiling
You don't need to compile a hot rodversion
of a Linux kernel or even recompile your existing kernel to improve
Linux performance on the desktop. If you're running
one of the later 2.6 kernels, this pack of hacks includes a few
tricks you can use to modify the performance of the kernel you
already have installed and running.One of the most hotly debated performance topics is how Linux should
determine what memory to swap to disk, how much it should swap, and
when to do it. The answer is simple. Use one or more of the following
tweaks to control this process yourself.
10.5.1. Setting Swappiness via /proc
A kernel parameter is available that is represented as a pseudofile
called /proc/sys/vm/swappiness. The default
value for the parameter is 70. You can log in and view the default
value with this command:
# cat /proc/sys/vm/swappinessThe number is a rough gauge for how likely it is that Linux will swap
70
to disk whatever it considers to be swappable. If you set it to 100,
it will swap most aggressively. If you set it to 20, it will tend to
swap a lot less. Here is how you can set it to either value (logged
in as root):
# echo "100" > /proc/sys/vm/swappinessThe argument for either side goes like this. Set the swappiness value
# cat /proc/sys/vm/swappiness
100
# echo "20" > /proc/sys/vm/swappiness
# cat /proc/sys/vm/swappiness
20
to a high number, even to 100, if you run several bloated
applications and do not often switch between them. The end result
will be that when you do switch to an application you left unused for
a while, it might take several seconds to respond, because it must be
swapped back into memory from disk. However, any application you use
at any given time should run faster, because it is not competing with
seldom-used applications for memory, as those applications are likely
to be swapped out to disk.On the other hand, if you set the swappiness value to a low number,
your applications are likely to respond instantly when you switch
between them, even if you leave one or more of them unused for long
periods of time. The possible downside to this approach is that if
one or more of your applications needs a lot of memory to store data
(such as a database application), it will run slower than if you set
the swappiness value higher.So, here's what you should do. Set the value to a
high number, use your computer for desktop operation as you normally
would, and see how things behave for a few hours. Then set it to a
low number, continue to use your computer for desktop operation as
you normally would, and see for yourself what the difference feels
like. Then pick the performance characteristics you like best.
Perhaps you'll even be happiest with a default value
of 70.
10.5.2. Tuning Network Performance via /proc
Here are a couple more parameters you
might want to tweak, especially if you are using a home computer
hooked directly to a DSL or cable router:
# echo "0" > /proc/sys/net/ipv4/tcp_sackThese settings turn off some unnecessary network activity. You might
# echo "0" > /proc/sys/net/ipv4/tcp_timestamps
not perceive any change in performance, but it's
worth a try.
10.5.3. Tuning Disk Access via Your Bootloader
Each program you run
under Linux will want to access the disk
at some point or another. The Linux kernel determines how to
prioritize how each program gets access to the disk, using three
methods: Anticipatory Scheduling, Complete Fairness Queuing, and
Deadline Queuing. (Actually, the kernel uses a fourth method, called
noop, but noop is not
likely to be applicable as a default method for a desktop user.)Each method has its advantages and disadvantages. Only two of them
are likely to be appropriate for desktop performance: Anticipatory
Scheduling and Complete Fairness Queuing. Most people seem to favor
Complete Fairness Queuing for best desktop performance, but some
argue that Anticipatory Scheduling is actually superior for desktop
performance. Here's how the three methods work.Anticipatory Scheduling
The kernel tries to anticipate how the disk will be accessed. If a
program is reading data, the anticipatory scheduler assumes
it's likely that it will want to continue reading
data. So, while it pauses according to a schedule, it anticipates
picking up where it left off. If it guesses correctly, the disk heads
don't have to jump around much and you get better
performance.
Complete Fairness Queuing
The kernel queues the disk to be used equally by every running
program. No single program can hog disk access. If several programs
are using the disk at the same time, each one will be responsive. On
the other hand, this can also cause the disk head to jump around a
lot more than it has to, thus slowing the entire
system's overall responsiveness.
Deadline Queuing
This type of queuing allows an application to dominate disk access.
Other applications that want to use the disk are put on the queue and
must wait. If the application hogging the disk exceeds a certain
deadline, the kernel passes disk access to the next requesting
application on the queue. This is a good queuing system for a
database server, but it is least likely to be best for a desktop
system.
The choice really comes down to Anticipatory Scheduling and Complete
Fairness Queuing. There are advocates for both when it comes to
desktop performance, so you should try both and decide for yourself
how they suit your desktop use.You can specify your choice of scheduler by passing parameters to the
kernel in your bootloader.LILO lets you append instructions to the boot process by adding a
line such as the following for each boot entry in the
/etc/lilo.conf file:
append="<added instructions>"If you want to use the Anticipatory Scheduler, use this:
append="elevator=as <possibly more added instructions>"If you want to use Complete Fairness Queuing, use this instead:
append="elevator=cfq <possibly more added instructions>"A complete example might look like this:
image=/boot/vmlinuz-2.6.9Run the lilo command after you make your changes
label=Linux
root=/dev/hda1
append="elevator=as video=vesafb:ywrap,mtrr,1024x768-16@60"
to lilo.conf, and then reboot for the changes to
take effect.With the GRUB bootloader, you simply append the instructions to the
kernel line yourself. For example, if you want to use Anticipatory
Scheduling, your boot entry might look like this:
title My Default LinuxIf you want to try Complete Fairness Queuing, make your boot entry
root (hd1,0)
kernel /boot/vmlinuz-2.6.9 ro root=/dev/hda1 elevator=as
look more like this:
title My Default LinuxYou do not need to set up GRUB again. Simply reboot for the change to
root (hd1,0)
kernel /boot/vmlinuz-2.6.9 ro root=/dev/hda1 elevator=cfq
take effect.
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