From: eLinux.org

Fast Kernel Decompression

This page has notes about faster kernel decompression.

Contents

Description

Currently, the method used to compress the kernel is gzip. However,
other compression and decompression methods exist which may allow
improvements in kernel decompression (and hence startup) performance.

This page documents Sony’s investigation of UCL
compression/decompression performance, for possible use in speeding up
bootup time on an embedded device. In our testing UCL decompressed a
sample file system image 43% faster than gunzip, and a sample kernel
image 28% faster than gunzip.

From the UCL web page, it states:

  • UCL is written in ANSI C. Both the source code and the compressed
    data format are designed to be portable across platforms.

  • UCL implements a number of algorithms with the following features:

    • Decompression is simple and *very* fast.
    • Requires no memory for decompression.
    • The decompressors can be squeezed into less than 200 bytes of
      code.
    • Focuses on compression levels for generating pre-compressed data
      which achieve a quite competitive compression ratio.
    • Allows you to dial up extra compression at a speed cost in the
      compressor. The speed of the decompressor is not reduced.
    • Algorithm is thread safe.
    • Algorithm is lossless.

  • UCL supports in-place decompression.

  • UCL and the UCL algorithms and implementations are distributed under
    the terms of the GNU General Public License (GPL) { auf Deutsch }.
    Special licenses for commercial and other applications are available
    by contacting the author.

Another method of speeding up the kernel load phase of bootup is to use
DMA Copy Of Kernel On
Startup

How to implement or use

Get UCL from following URL and use sample command “uclpack”

  1.    http://www.oberhumer.com/opensource/ucl/download/ucl-1.03.tar.gz

untar the file, build , and use the sample command “uclpack”, located
at: ucl-1.03/examples/uclpack in the untar’ed source tree.

Expected Improvement

The case study below is intended to show a performance improvement in
decompressing a sample file system and sample kernel.

Resources

Projects

[Are there other compressors with better decompression performance than
gzip??

Case Studies

Case 1

For this use case, we compiled both uclpack and gzip for the
PowerPC platform. Then we ran the programs on the
target platform, compressing and decompressing two different file images

  • an initrd filesystem image, and a linux kernel image (originally
    uncompressed).

The size and performance results from running these commands are in the
tables below.



























































































































Image file:
 initrd-2.6.5-1.358
method
 UCL
 GZIP
 improved %
parameter
 -b4194304
 -8
 .
source file size
 819200
 819200
 .
compressed size
 187853
 189447
 .
compression rate
 77.1%
 76.9%
 0.3%
compression time: user (sec)
 5.13
 2.03
 -152.5%
sys (sec)
 0.09
 0.06
 -36.5%
total (sec)
 5.22
 2.09
 -149.0%
decompression time: user (sec)
 0.12
 0.3
 59.7%
sys (sec)
 0.1
 0.08
 -16.9%
total (sec)
 0.22
 0.39
 43.0%
....
Image file:
 vmlinux-2.4.20 for ibm-440gp
method
 UCL
 GZIP
 improved %
parameter
 -b4194304
 -8
 .
source file size
 1810351
 1810351
 .
compressed size
 790250
 776807
 .
compression rate
 56.3%
 57.1%
 -1.3%
compression time: user (sec)
 17.29
 6.07
 -185.0%
sys (sec)
 0.04
 0.02
 -92.4%
total (sec)
 17.33
 6.09
 -184.6%
decompression time: user (sec)
 0.12
 0.16
 26.1%
sys (sec)
 0.03
 0.04
 35.8%
total (sec)
 0.15
 0.2
 28.2%
Hardware 

PPC440GP - 300 MHZ

Kernel Version 

Linux kernel running on target was 2.6.11, kernel which was compressed with Linux 2.4.20

Configuration 

See above tables for parameters to gzip and ucl

Time without change 

[put that here]

Time with change 

[put that here]

Case 2

Case 3

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