Difference between revisions of "Disable Console"

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[[Category:Boot Time]]
 
[[Category:Boot Time]]
[[Category:Disable Console]]
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[[Category:Kernel]]

Latest revision as of 12:40, 16 November 2008

Description

You can save time during kernel bootup by disabling the console output. The easiest way to do this is to use the "quiet" option on the kernel command line (described below).

Printk output is usually directed to a serial port or a VGA console during bootup. By disabling console output, the time taken to output the characters (and perform things like software scrolling of the display buffer) is eliminated.

How to implement or use

To disable console output during kernel bootup, use the "quiet" option on the kernel command line.

To do this, just put the word "quiet" (without the quotes) in the kernel command line, with no other arguments. This will suppress printk output during booting. Note that printk messages are still buffered in the kernel and can be retrieved after booting using the "dmesg" command.

Expected Improvement

This saves time during kernel bootup by suppressing printk output. Printk output delays depend on a number of factors, but in the use cases cited below, the savings were in the range of a few hundred milliseconds.

With a serial console, the time to output characters is dependent on the serial port speed. However, with a VGA console, the time to output the characters is dependent on the speed of the processor. Therefore, the slower your processor, the more savings you will gain from this technique.

Resources

Projects

None

Specifications

None

Patches

None

Case Studies

Case 1

Hardware KMC SH board, using VGA console
Kernel Version CELF-1 (040126)
Configuration relatively small configuration (details not available)
Time without "quiet" option 637878 usec
Time with "quiet" option 461893 usec
Time savings 176 milliseconds

From data submitted by Maruyama Kiyoyasu of Mitsubishi.

Case 2

Hardware TI OMAP board, using serial console
Kernel Version CELF-1 (040126)
Configuration Kernel booted with XIP, CRAMFS root file system, with preset-LPJ
Time without "quiet" option 551735 usec
Time with "quiet" option 280676 usec
Time savings 271 milliseconds

From data submitted by Noboru Wakabayashi of Hitachi.