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

[Ingo Molnar <mingo@kernel.org>]

* Douglas Anderson <dianders@chromium.org> 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.

Very impressive!

> [...]  ...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.

I don't think that's a worst case: incremental builds are very commonly used 
during kernel development and kernel testing. (I'd even argue that the performnace 
of incremental builds is one of the most important features of a build system.)

That it's called twice in the Chrome OS build system does not change the 
proportion of the speedup.

> 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.

This is a huge deal!

FWIIW I have tested your patches and they work fine here. Here's the before/after 
performance testing of various styles of build times of the scheduler.

First the true worst case is a full rebuild:

[ before ]

  triton:~/tip> perf stat --null --repeat 3 --pre "make clean 2>/dev/null 2>&1" make kernel/sched/ >/dev/null

 Performance counter stats for 'make kernel/sched/' (3 runs):

       4.693974827 seconds time elapsed                                          ( +-  0.05% )

[ after ]

  triton:~/tip> perf stat --null --repeat 3 --pre "make clean 2>/dev/null 2>&1" make kernel/sched/ >/dev/null

 Performance counter stats for 'make kernel/sched/' (3 runs):

       4.391769610 seconds time elapsed                                          ( +-  0.21% )

Still a ~6% speedup which is nice to have.

Then the best case, a fully cached rebuild of a specific subsystem - which I 
personally do all the time when I don't remember whether I already built the 
kernel or not:

[ before ]

triton:~/tip> taskset 1 perf stat --null --pre "sync" --repeat 10 make kernel/sched/ >/dev/null

 Performance counter stats for 'make kernel/sched/' (10 runs):

       0.439517157 seconds time elapsed                                          ( +-  0.14% )

[ after ]

  triton:~/tip> taskset 1 perf stat --null --pre "sync" --repeat 10 make kernel/sched/ >/dev/null

 Performance counter stats for 'make kernel/sched/' (10 runs):

       0.148483807 seconds time elapsed                                          ( +-  0.57% )

A 300% speedup on my system!

So I wholeheartedly endorse the whole concept of caching build environment 
invariants:

  Tested-by: Ingo Molnar <mingo@kernel.org>

Thanks,

	Ingo