[U-Boot] [PATCH] buildman: allow more incremental building

[Stephen Warren]

On 04/11/2016 06:04 AM, Simon Glass wrote:
> Hi,
>
> On 1 April 2016 at 07:35, Tom Rini <trini at konsulko.com> wrote:
>> On Mon, Mar 28, 2016 at 04:39:35PM -0600, Stephen Warren wrote:
>>
>>> From: Stephen Warren <swarren at nvidia.com>
>>>
>>> One use-case for buildman is to continually run it interactively after
>>> each small step in a large refactoring operation. This gives more
>>> immediate feedback than making a number of commits and then going back and
>>> testing them. For this to work well, buildman needs to be extremely fast.
>>> At present, a couple issues prevent it being as fast as it could be:
>>>
>>> 1) Each time buildman runs "make %_defconfig", it runs "make mrproper"
>>> first. This throws away all previous build results, requiring a
>>> from-scratch build. Optionally avoiding this would speed up the build, at
>>> the cost of potentially causing or missing some build issues.
>
> Does it actually cause problems? I think I have seen things go wrong
> before, which is why I added it, but I can't remember the detail.

I have not noticed any issues using -I and hence skipping the mrproper. 
It is possible you added the mrproper before the conversion to Kbuild; 
Kbuild seems to handle rebuilding after code changes much better than 
the old system, which definitely had some flaws that prevented 
incremental builds in many cases.

> Threads work by configuring and building the first commit for a board,
> then incrementally building each commit for that same board. So it
> seems like change should only be useful when all the boards you are
> building have a similar config.

No, because of point 2 below.

>>> 2) A build tree is created per thread rather than per board. When a thread
>>> switches between building different boards, this often causes many files
>>> to be rebuilt due to changing config options. Using a separate build tree
>>> for each board would avoid this. This does put more strain on the system's
>>> disk cache, but it is worth it on my system at least.
>
> I'm not sure why this is a win, given what I said above. I'm starting
> to feel that I don't understand what is going on.

Since each board gets built in a separate directory, there's no build 
churn due to the configuration being changed when threads switch between 
building different boards. Right now what happens on a thread (even 
without mrproper) is:

Thread 1 builds board X
- Some stuff gets rebuilt due to source changes
Thread 1 builds board Y
- Some stuff gets rebuilt due to source changes
- Lots more gets rebuilt because the config changed between the two 
boards, and bother were built in the same directory.

After this patch, the following happens:

Thread 1 builds board X
- Some stuff gets rebuilt due to source changes
Thread 1 builds board Y
- Some stuff gets rebuilt due to source changes
- Nothing else gets rebuilt

If the source changes happen not to affect some particular board, then 
this new scheme means that nothing gets rebuilt for it, which 
significantly speeds up the build.

>>> This commit adds two command-line options to implement the changes
>>> described above; -I ("--incremental") turns of "make mrproper" and -P
>>> ("--per-board-out-dir") creats a build directory per board rather than per
>>> thread.
>>>
>>> Tested:
>>>
>>>      ./tools/buildman/buildman.py tegra
>>>      ./tools/buildman/buildman.py -I -P tegra
>>>      ./tools/buildman/buildman.py -b tegra_dev tegra
>>>      ./tools/buildman/buildman.py -b tegra_dev -I -P tegra
>>>
>>> ... each once after deleting the buildman result/work directory, and once
>>> "incrementally" after a previous identical invocation.
>>>
>>> Signed-off-by: Stephen Warren <swarren at nvidia.com>
>>
>> Reviewed-by: Tom Rini <trini at konsulko.com>
>
> Oddly I don't seem to have received the original email, but I see this
> in patchwork.
>
> For me this cut build time from 6.25 minutes to 4.75 minutes for 30
> commits built for two similar boards on a 8-thread machine. It's a big
> win. I haven't yet tried it out on a a big build.
>
> Stephen can you please update the README with some of your commit
> notes? Other than that:
>
> Acked-by: Simon Glass <sjg at chromium.org>
> Tested-by: Simon Glass <sjg at chromium.org>