Fast Kernel Decompression

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This page has notes about faster kernel decompression.


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"

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.



[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%
PPC440GP - 300 MHZ
Kernel Version 
Linux kernel running on target was 2.6.11, kernel which was compressed with Linux 2.4.20
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