Compiling kernels faster (was Re: v4.10: kernel stack frame pointer .. has bad value (null))

From: Pavel Machek <pavel@ucw.cz>
To: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
	Josh Poimboeuf <jpoimboe@redhat.com>,
	Andy Lutomirski <luto@amacapital.net>,
	kernel list <linux-kernel@vger.kernel.org>,
	Ingo Molnar <mingo@kernel.org>,
	Andrew Lutomirski <luto@kernel.org>,
	Borislav Petkov <bp@alien8.de>, Brian Gerst <brgerst@gmail.com>,
	Denys Vlasenko <dvlasenk@redhat.com>, Peter Anvin <hpa@zytor.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Thomas Gleixner <tglx@linutronix.de>
Subject: Compiling kernels faster (was Re: v4.10: kernel stack frame pointer .. has bad value (null))
Date: Fri, 10 Mar 2017 14:17:51 +0100	[thread overview]
Message-ID: <20170310131750.GB11875@amd> (raw)
In-Reply-To: <CAMuHMdUmHEjrLoB=S+wto6Z4mP7-DxkO83FWn37soQ6sXSSGDQ@mail.gmail.com>

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On Thu 2017-03-09 13:16:09, Geert Uytterhoeven wrote:
> Hi Pavel,
> 
> On Thu, Mar 9, 2017 at 11:56 AM, Pavel Machek <pavel@ucw.cz> wrote:
> > On Thu 2017-03-09 10:38:46, Geert Uytterhoeven wrote:
> >> On Wed, Mar 8, 2017 at 10:22 PM, Pavel Machek <pavel@ucw.cz> wrote:
> >> > Well, I have fast CPUs, but most of the time they just compile
> >> > stuff. Especially bisect is compile-heavy. I suspect going back to
> >> > gcc-3.2 would bring me bigger advantages than CPU upgrade...
> >>
> >> I hope you do use ccache or distcc?
> >
> > I tried to use distcc before, but it was rather hard to maintain. No
> > ccache here. Hmm. I guess ccache really makes sense for bisect.
> 
> Yes it does. So if you're not using it yet, do the below, today, not tomorrow.
> 
> If your distro supports it, prepend /usr/lib/ccache/ to your $PATH.
> Create symlinks from the names of your favorite cross-compilers
> to /usr/bin/ccache, and make sure they are early in your $PATH.
> 
> That's it! Enjoy!

Hmm. Installed, and seems to work. OTOH, compilation now seems to
produce 2-3MB/sec writing on spinning rust, and CPUs are no longer
fully loaded. (make -j 7 on 2 core HT machine). Any io load sends the
CPU utilization to cca 50% range... Compilation goes up from 9:13 to
11:40... to 23 minutes depending on situation. I guess it is still
worth it for the bisect, but it looks like ccache really needs an ssd.

On the other hand, switching to -O1 is really easy, and gets 15% or so
improvement.

Hmm. And killing chromium matters a lot for a compile time. I hate
modern web :-(.

Best regards,							Pavel

--- a/Makefile
+++ b/Makefile
@@ -639,9 +639,9 @@ ifdef CONFIG_CC_OPTIMIZE_FOR_SIZE
 KBUILD_CFLAGS  += -Os $(call cc-disable-warning,maybe-uninitialized,)
 else
 ifdef CONFIG_PROFILE_ALL_BRANCHES
-KBUILD_CFLAGS  += -O2 $(call cc-disable-warning,maybe-uninitialized,)
+KBUILD_CFLAGS  += -O1 $(call cc-disable-warning,maybe-uninitialized,)
 else
-KBUILD_CFLAGS   += -O2
+KBUILD_CFLAGS   += -O1
 endif
 endif

-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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