Re: [RESEND PATCH v2 0/2] Add support for ZSTD-compressed kernel

[Adam Borowski]

On Fri, Aug 17, 2018 at 12:22:44PM -0700, Andi Kleen wrote:
> On Fri, Aug 17, 2018 at 07:57:46PM +0200, Adam Borowski wrote:
> > > The "favourite compressor" seems to roughly change every year, so if
> > > we keep adding new ones things will get more and more convoluted.
> > 
> > The above patchset drops just bzip2.  It is the only one that's strictly
> > beaten in every way (ratio, time, memory usage), there are also no other
> 
> Does time include build time? I've been reverting back to gzip recently
> because I care very much about that.

Too lazy to benchmark a kernel image (IIRC Nick Terrell posted that a while
ago), here's copypasta of a random 16824672 byte executable, in userspace,
with default level setting:

	comp	decomp	size
xz	8.038s	0.356s	4320292
bz2	2.265s	0.730s	5234516
zst	0.274s	0.102s	5657626
gz	0.880s	0.152s	6515505
Z	0.499s	0.133s	8932459
lzo	0.100s	0.095s	9198874

As you can see, zstd's compression time is drastically better than gzip,
while ratio is better.  The default level is very low (-3 on -1..-22 scale)
but you can crank it up for stronger compression.

The defaults fit your use case.

> > uses of bzip2 anywhere in the kernel so we'd get to drop its code
> > completely: 900 lines of Linus' happiness.
> 
> Great!
> 
> > Other candidates are lzo and bare lzma (you want lz4, zstd or xz instead),
> > but those are used elsewhere thus there's hardly any gain.  If you want them
> > gone, please say so -- I'll include their droppage.
> 
> Yes would be good to remove the kernel image support for those too
> just to simplify the config process, even if it doesn't save much code.

There's one caveat: fast choices are quite new:
* lz4 userspace tools are not even in current Debian stable (just unstable)
* uncompressed kernel got in only this merge window
* zstd has userspace tools in Debian stable but is not merged into the
  kernel yet
(other dists are probably similar)

Thus, it might be a good idea to keep lzo for a while longer.

Bare lzma can probably go -- xz filters are nice for binaries of archs it
knows (disabled otherwise), and lack of header requires hacks to find out
the payload's size.

So it's up to you guys: do you want me to drop lzo and/or lzma?
We can also drop them just for vmlinuz but not initrd.


Meow!
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