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From: David Laight <David.Laight@ACULAB.COM>
To: 'Joe Perches' <joe@perches.com>, Arnd Bergmann <arnd@arndb.de>,
	"Oleksandr Natalenko" <oleksandr@redhat.com>
Cc: "Jason A. Donenfeld" <Jason@zx2c4.com>,
	LKML <linux-kernel@vger.kernel.org>, X86 ML <x86@kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>
Subject: RE: [PATCH] Kconfig: default to CC_OPTIMIZE_FOR_PERFORMANCE_O3 for gcc >= 10
Date: Sun, 10 May 2020 18:58:24 +0000	[thread overview]
Message-ID: <eeb52e23496f4730bb115d1e8c00743e@AcuMS.aculab.com> (raw)
In-Reply-To: <c78e00a05d19a6823e48c7bd50144f0563908a1a.camel@perches.com>

From: Joe Perches
> Sent: 10 May 2020 18:45
> 
> On Sun, 2020-05-10 at 12:47 +0000, David Laight wrote:
> > From: Joe Perches
> > > Sent: 08 May 2020 16:06
> > > On Fri, 2020-05-08 at 13:49 +0200, Arnd Bergmann wrote:
> > > > Personally, I'm more interested in improving compile speed of the kernel
> > >
> > > Any opinion on precompiled header support?
> >
> > When ever I've been anywhere near it it is always a disaster.
> 
> A disaster? Why?

The only time I've had systems that used them they always got
out of step with the headers - probable due to #define changes.
If auto-generated by the compiler then parallel makes also
give problems.

> For a large commercial c only project, it worked well
> by reducing a combined multi-include file, similar to
> kernel.h here, to a single file.

Certainly reducing the number of directories searched
can make a big difference.

I've also compiled .so by merging all the sources into a
single file.

> That was before SSDs though and the file open times
> might have been rather larger then.

The real killer is lots of directory names in the -I <paths>
especially over NFS.

I've also looked at system call stats during a kernel compile.
open() dominated and my 'gut feeling' was that most were
failing opens.

I also suspect that modern compilers remember that an include
file contained an include guard - and don't even both looking
for it a second time.

	David

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)


  reply	other threads:[~2020-05-10 18:58 UTC|newest]

Thread overview: 20+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-05-07 22:45 Jason A. Donenfeld
2020-05-08  8:35 ` Peter Zijlstra
2020-05-08  9:02 ` Oleksandr Natalenko
2020-05-08 11:21   ` Jason A. Donenfeld
2020-05-08 11:33     ` Oleksandr Natalenko
2020-05-08 11:49       ` Arnd Bergmann
2020-05-08 12:07         ` Jason A. Donenfeld
2020-05-08 13:04           ` Arnd Bergmann
2020-05-08 15:06         ` Joe Perches
2020-05-08 15:09           ` Arnd Bergmann
2020-05-10 12:47           ` David Laight
2020-05-10 17:45             ` Joe Perches
2020-05-10 18:58               ` David Laight [this message]
2020-05-12  1:10             ` Masahiro Yamada
2020-05-11 21:57   ` [PATCH v2] " Jason A. Donenfeld
2020-05-12  0:04     ` Linus Torvalds
2020-05-12  0:09       ` Linus Torvalds
2020-05-12  0:43       ` Jason A. Donenfeld
2020-05-12  8:44       ` Richard Biener
2020-05-13 11:27 ` [PATCH] " Artem S. Tashkinov

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