linux-kernel - Re: [llvmlinux] percpu | bitmap issue? (Cannot boot on bare metal due to a kernel NULL pointer dereference)

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Message-ID: <alpine.DEB.2.11.1509141325460.4192@east.gentwo.org>
Date:	Mon, 14 Sep 2015 13:27:01 -0500 (CDT)
From:	Christoph Lameter <cl@...ux.com>
To:	Austin S Hemmelgarn <ahferroin7@...il.com>
cc:	sedat.dilek@...il.com, Peter Zijlstra <peterz@...radead.org>,
	Baoquan He <bhe@...hat.com>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Tejun Heo <tj@...nel.org>, LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	David Rientjes <rientjes@...gle.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Thomas Graf <tgraf@...g.ch>, Ingo Molnar <mingo@...nel.org>,
	the arch/x86 maintainers <x86@...nel.org>
Subject: Re: [llvmlinux] percpu | bitmap issue? (Cannot boot on bare metal
 due to a kernel NULL pointer dereference)

On Mon, 14 Sep 2015, Austin S Hemmelgarn wrote:

> I can comment at least a little about the -Os aspect (although not I'm no
> expert on this in particular).  In general, for _most_ use cases, a kernel
> compiled with CONFIG_CC_OPTIMIZE_FOR_SIZE will run slower than one compiled
> without it.  On rare occasion though, it may actually run faster, the only
> cases I've seen where this happens are specialized uses that are very memory
> pressure dependent and run almost entirely in userspace with almost no
> syscalls (for example math related stuff operating on _very, very big_ (as in,
> >1 trillion elements) multidimensional matrices, with complex memory
> constraints), and even then it's usually a miniscule improvement in
> performance (generally less than 1%, which can of course be significant
> depending on how long it takes before the improvement).

Cache footprint depends on size which has a significant impact on
performance. In our experience the kernel (and any other code) is
generally faster if optimized for size.

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