Re: [PATCH 2/6] conf/bitbake: improve compression image handling

[Phil Blundell <philb@gnu.org>]

On Thu, 2011-04-07 at 16:48 -0700, Tom Rini wrote:
> On 04/07/2011 10:29 AM, Otavio Salvador wrote:
> > On Thu, Apr 7, 2011 at 14:23, Phil Blundell <philb@gnu.org> wrote:
> >> On Thu, 2011-04-07 at 13:19 +0000, Otavio Salvador wrote:
> >>>  XZ_COMPRESSION_LEVEL ?= "-e -9"
> >>>  LZMA_COMPRESSION_LEVEL ?= "-e -9"
> >>> +GZIP_COMPRESSION_LEVEL ?= "-9"
> >>> +BZIP2_COMPRESSION_LEVEL ?= "-9"
> >>
> >> It seems a bit sad to have to keep adding new vars to the global
> >> configuration set (and hence parse them for every single recipe) every
> >> time someone wants to tweak a new compression format.  Can we not just
> >> have a single IMAGE_COMPRESSION_LEVEL that does for everything?
> > 
> > For me it doesn't matter; I can add it if preferred.
> 
> Why are we even tweaking gzip/bzip2?  XZ/LZMA can have serious tradeoffs
> at higher levels but I thought for bzip2/gzip it was either do it or no
> compression, not "turn it down a little bit".

There is a bit of a tradeoff for gzip, though I'm not sure you would
notice it on modern hardware.  For bzip2 it's more significant; if
you're bzipping large files (several GB) then the choice between -9 and
-1 is quite noticeable even on a relatively powerful cpu.  I'm not sure
you'd notice much difference with either gzip or bzip on a 32MB jffs2
image.

And for unzipping, my recollection is that the decompressor stages of
both gzip and bzip run in more-or-less constant time irrespective of the
settings that you used when compressing.  

So yeah, I agree, it isn't obvious that there is going to be much
real-world benefit from tweaking these knobs for gzip and bzip2.  If
someone wants that ability then fine, but ideally in a way that doesn't
cause further metadata bloat for everyone else.  One obvious way to do
that would be to put the default variable settings in the
image-generating classes, rather than in bitbake.conf, so that they
don't get set at all for the majority of recipes if you are happy to
accept the defaults.

p.