-----Original Message-----
From: David Sterba [mailto:dsterba at suse.cz]
Sent: Wednesday, January 30, 2013 7:55 AM
To: Kyungsik Lee
Cc: Russell King; Thomas Gleixner; Ingo Molnar; H. Peter Anvin; Michal
Marek; linux-arm-kernel at lists.infradead.org; linux-kernel at vger.kernel.org;
linux-kbuild at vger.kernel.org; x86 at kernel.org; Nitin Gupta; Richard Purdie;
Josh Triplett; Joe Millenbach; Andrew Morton; Albin Tonnerre;
hyojun.im at lge.com; chan.jeong at lge.com; gunho.lee at lge.com;
minchan.kim at lge.com; namhyung.kim at lge.com; raphael.andy.lee at gmail.com
Subject: Re: [RFC PATCH 0/4] Add support for LZ4-compressed kernels

On Sat, Jan 26, 2013 at 02:50:43PM +0900, Kyungsik Lee wrote:

quoted

This patchset is for supporting LZ4 compressed kernel and initial
ramdisk on the x86 and ARM architectures.

Have you considered the 'high compression' mode of lz4?
http://code.google.com/p/lz4/source/browse/trunk/lz4hc.c

Yes, The high compression mode of lz4 is set as default
in the submitted patch. The figure shows:

Uncompressed size: 13MB
lz4: Normal compression mode
output size: 8.1MB (62.7%)
decompression time: 232ms(148ms, with enabled unaligned memory access)

lz4: High compression mode
output size: 6.8MB (52.3%)
decompression time: 251ms(167ms)

The size of output in NC(Normal Compression) is bigger than HC(High Compression)
But decompression in NC is a bit faster than HC in ARM based board.


I assume that 

The compression format remains the same, the compressor tries harder (but
is slower), resulting compression ratio is better.

an examle compression for vmlinux.bin of x86_64 build:

input size: 16509520 bytes

lz4 (svn 88):
output size:        6393684    (38.7%)
compression time:      41.7 ms (395 MB/s)
decompression time:    13.7 ms (1204 MB/s)

lz4hc (svn 88):
output size:        5319137    (32.2%)
compression time:       683 ms (24 MB/s)
decompression time:    13.1 ms (1259 MB/s)

compressed file delta: 6393684 - 5319137 = 1074547 ~ 1MB

tested on a Nehalem box; same test on my slow desktop gives

lz4:
compression time:      97   ms (169 MB/s)
decompression time:    25.7 ms (643 MB/s)

lz4hc:
compression time:    1386 ms (11 MB/s)
decompression time:    26 ms (619 MB/s)

While the decompression time is almost the same, image size is smaller.
The kernel image compression is run in userspace and the low speed is not
much of concern for a one-time operation.

For the reference, lzo (current kernel version) run on the destktop:

output size:         6026256 (36.5%)
decompression time:     79.6 ms (207 MB/s)

quoted

It seems that it?s worth trying LZ4 compressed kernel image or ramdisk
for making the kernel boot more faster.

There's another potential user of lz4: btrfs. I've submitted a feature
preview integrating lz4 compression
http://thread.gmane.org/gmane.comp.file-systems.btrfs/15744
and we have tried to integrate the HC mode as well
http://thread.gmane.org/gmane.comp.file-systems.btrfs/18165
.
So far it's on a slow track, conceptually it works, but I the code needs
some work so it could live under lib/* (we've used the svn sources with
minor changes, no kernel coding style). It would be easier for me to
enhance the existing lib/lz4/* codebase.

Also zram could consider lz4, I'm not sure if there are other potential
users.


david