[linuxsh-shmedia-dev] Compressed kernels

[Stuart M. <stu...@st...>]

Folks

As you may have spotted from a previous posting I've been playing with
compressed kernels, with the aim of improving the download speed.
After a number of unexpected problems, it is now working fine, so I
thought I might post some figures.

First some background. The Linux compressed kernels work by taking the
normal ELF kernel image, removing some sections (primarily empty zero page,
and those to do with debugging), and the compressing the resulting file
using gzip. This is then built as binary data into a new program, which
when booted decompresses the kernel image, and jumps into it.

So after benchmarking this, we get the following results (figures are
approx. - stopwatch and visual inspection - so don't extrapolate too far!):

gdb from BSF 20020329
---------------------

Compressed kernel (size 632K, 616K data + 16K loader)
	download	 90 seconds = 7K/sec
	decompress	 15 seconds
	total		105 seconds = 16K/sec

Uncompressed kernel (size 1702K)
	download	103 seconds = 16K/sec


gdb from Madrid 0.6
-------------------

Compressed kernel (size 664K, 648K data + 16K loader)
	download	 17 seconds = 39K/sec
	decompress	  2 seconds
	total		 19 seconds = 94K/sec

Uncompressed kernel (size 1789K)
	download	 44 seconds = 40K/sec


Note that this is not a fair comparison, because as well as changing the
version of gdb being used, I also changed the decompression code to enable
the cache, hence the big fall in decompression time.

This testing did turn up one anomily. The download speed is very dependant on
the alignment of the data being downloaded. If it is 8 byte aligned speeds
are in the order of 40K/sec, while 4 byte aligned data falls to under 9K/sec.
This is almost certainly a result of how the debug hardware is being driven,
so needs looking at,

I'll check this in later this week.

Stuart

-- 
Stuart Menefy                                        stu...@st...
STMicroelectronics Ltd                         ST Intranet: mo.bri.st.com
Bristol, UK                         Rest of the World: www.linuxsh.st.com