Re: [RFC PATCH 0/4] Add support for LZ4-compressed kernels

[Nicolas Pitre]

On Mon, 28 Jan 2013, Andrew Morton wrote:

> On Sat, 26 Jan 2013 14:50:43 +0900
> Kyungsik Lee <kyungsik.lee@xxxxxxx> wrote:
> 
> > This patchset is for supporting LZ4 compressed kernel and initial ramdisk on
> > the x86 and ARM architectures.
> > 
> > According to http://code.google.com/p/lz4/, LZ4 is a very fast lossless
> > compression algorithm and also features an extremely fast decoder.
> > 
> > Kernel Decompression APIs are based on implementation by Yann Collet
> > (http://code.google.com/p/lz4/source/checkout).
> > De/compression Tools are also provided from the site above.
> > 
> > The initial test result on ARM(v7) based board shows that the size of kernel
> > with LZ4 compressed is 8% bigger than LZO compressed  but the decompressing
> > speed is faster(especially under the enabled unaligned memory access).
> > 
> > Test: 3.4 based kernel built with many modules
> > Uncompressed kernel size: 13MB
> > lzo: 6.3MB, 301ms
> > lz4: 6.8MB, 251ms(167ms, with enabled unaligned memory access)
> > 
> > It seems that it___s worth trying LZ4 compressed kernel image or ramdisk 
> > for making the kernel boot more faster.
> >
> > ...
> >
> >  20 files changed, 663 insertions(+), 3 deletions(-)
> >
> > ...
> >
> 
> What's this "with enabled unaligned memory access" thing?  You mean "if
> the arch supports CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS"?  If so,
> that's only x86, which isn't really in the target market for this
> patch, yes?

I'm guessing this is referring to commit 5010192d5a.

> It's a lot of code for a 50ms boot-time improvement.  Does anyone have
> any opinions on whether or not the benefits are worth the cost?

Well, we used to have only one compressed format.  Now we have nearly 
half a dozen, with the same worthiness issue between themselves.  
Either we keep it very simple, or we make it very flexible.  The former 
would argue in favor of removing some of the existing formats, the later 
would let this new format in.


Nicolas
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