Re: [PATCH 23/23] kbuild: use pigz for gzip compression if available

[Lorenzo Stoakes (ARM)]

On Wed, Sep 09, 2026 at 09:19:12PM -0700, Nathan Chancellor wrote:
> > The gzip step of a kernel build is very lengthily, especially for larger
> > builds such as allmodconfig.
> >
> > gzip itself cannot be run in parallel, however an alternative tool exists
> > that can, providing the same feature set as gzip itself - pigz - which
> > works as a drop-in replacement.
> >
> > On a 128-core Threadripper, gzip -9 of a 36 MiB x86-64 vmlinux.bin takes
> > 1.6s, and with pigz it takes 0.09s, so the performance increase is
> > significant.
> >
> > It is already possible to specify the KGZIP environmental variable to make
> > use of pigz, however it seems sensible to make use of pigz if it is
> > available.
> >
> > Therefore default to using pigz if it is available on the system upon which
> > the kernel is being built, otherwise fall back to gzip.
> >
> > The output is byte-for-byte identical between pigz/gzip invocations, but
> > gzip and pigz do not produce the same stream as one another.
> >
> > Therefore, for reproducible builds, the same set of tools should be used.
> >
> > This seems to already be an implicit requirement in any case, but update
> > the reproducible build documentation to make this clear.
> >
> > Also update the kbuild documentation to reflect the change.
> >
> > Every x86 build ends with the compression of vmlinux.bin, 36MB for
> > defconfig and over 200MB for allmodconfig, no-op builds are unchanged.
> >
> > Whole build, 128-thread Threadripper 9980X, best of N runs:
> >
> >                                          before   after     delta
> >                                          -------------------------------
> >   x86 defconfig, touch mm/vma.c, gcc        7.4s     5.4s     -2.0s (-27%)
> >   x86 defconfig, touch mm/vma.c, clang      6.6s     4.9s     -1.7s (-26%)
> >   x86 defconfig, clean, gcc                26.2s    24.4s     -1.8s (-7%)
> >   x86 defconfig, clean, clang              25.8s    24.3s     -1.5s (-6%)
> >   x86 allmodconfig, touch mm/vma.c, gcc    23.7s    15.3s     -8.4s (-35%)
> >   x86 allmodconfig, touch mm/vma.c, clang  22.1s    15.2s     -6.9s (-31%)
> >   x86 allmodconfig, clean, gcc            291.4s   275.2s     -16.2s (-6%)
> >   x86 allmodconfig, clean, clang          278.3s   265.7s     -12.6s (-5%)
> >
> > Link: https://zlib.net/pigz/
> > Assisted-by: LLM
> > Signed-off-by: Lorenzo Stoakes (ARM) <ljs@xxxxxxxxxx>
> ...
> > diff --git a/Makefile b/Makefile
> > index a0d29dee81d7..efa6a2d77e6b 100644
> > --- a/Makefile
> > +++ b/Makefile
> > @@ -561,7 +561,7 @@ PERL		= perl
> >  PYTHON3		= python3
> >  CHECK		= sparse
> >  BASH		= bash
> > -KGZIP		= gzip
> > +KGZIP		:= $(if $(shell command -v pigz 2>/dev/null),pigz,gzip)
>
> This should be '?=', right? Sashiko also notes a concern around parallel

Actually make KGZIP=foo will override this still. But it doesn't take an
environment variable for make -e.

However, that's consistent with other variables there like KBZIP2 etc.

Probably the wording in the docs should be tweaked!

> module installation with gzip module compression since there is no
> thread limitation, which could bring a little risk to doing this by
> default? Might not be that big of a deal though?

Yeah, will modify so it single threads for things that aren't vmlinux on
respin!

>
> --
> Cheers,
> Nathan
>

--
Cheers, Lorenzo