Re: [PATCH v2 0/2] kbuild: Cache exploratory calls to the compiler

From: Guenter Roeck
Date: Wed Oct 04 2017 - 21:46:15 EST

On Wed, Oct 4, 2017 at 3:37 PM, Douglas Anderson <dianders@xxxxxxxxxxxx> wrote:
> This two-patch series attempts to speed incremental builds of the
> kernel up by a bit. How much of a speedup you get depends a lot on
> your environment, specifically the speed of your workstation and how
> fast it takes to invoke the compiler.
> In the Chrome OS build environment you get a really big win. For an
> incremental build (via emerge) I measured a speedup from ~1 minute to
> ~35 seconds. ...but Chrome OS calls the compiler through a number of
> wrapper scripts and also calls the kernel make at least twice for an
> emerge (during compile stage and install stage), so it's a bit of a
> worst case.
> Perhaps a more realistic measure of the speedup others might see is
> running "time make help > /dev/null" outside of the Chrome OS build
> environment on my system. When I do this I see that it took more than
> 1.0 seconds before and less than 0.2 seconds after. So presumably
> this has the ability to shave ~0.8 seconds off an incremental build
> for most folks out there. While 0.8 seconds savings isn't huge, it
> does make incremental builds feel a lot snappier.
> Caveats from v1 still copied here, though with Masahiro Yamada's
> suggestions from v1 this is starting to feel a little more baked and
> I've even dropped the RFC from it (though extra testing still
> appreciated):
> Please note that I make no illusions of being a Makefile expert nor do
> I have any belief that I fully understand the Linux kernel build
> system. Please take this patch series as the start of a discussion
> about whether others feel like this type of speedup is worthwhile and
> how to best accomplish it. Specific things to note:
> - I'm happy to paint the bikeshed any color that maintainers want. If
> you'd like the cache named differently, in a slightly different
> format, or you want me to adjust the spacing / names of Makefile
> stuff then please just let me know.
> - If this is totally the wrong approach and you have a better idea
> then let me know. If you want something that's super complicated to
> explain then feel free to post a replacement patch and I'm happy to
> test.
> - This patch definitely needs extra testing. I've tested it on a very
> limited build environment and it seems to be working fine, but I
> could believe that with some weird compiler options or on certain
> architectures you might need some extra escaping here and there.

Not being a makefile expert, I can't say anything about the merits of
this series. However, it does pass a run on my test farm at So. I'll give it a

Tested-by: Guenter Roeck <linux@xxxxxxxxxxxx>


> Changes in v2:
> - Abstract at a different level (like shell-cached) per Masahiro Yamada
> - Include ld-version, which I missed the first time
> Douglas Anderson (2):
> kbuild: Add a cache for generated variables
> kbuild: Cache a few more calls to the compiler
> Documentation/kbuild/makefiles.txt | 21 +++++++++
> Makefile | 4 +-
> scripts/Kbuild.include | 90 ++++++++++++++++++++++++++++++++------
> 3 files changed, 99 insertions(+), 16 deletions(-)
> --