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

* 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

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