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: <55F71408.4070702@gmail.com>
Date:	Mon, 14 Sep 2015 14:38:00 -0400
From:	Austin S Hemmelgarn <ahferroin7@...il.com>
To:	Christoph Lameter <cl@...ux.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 2015-09-14 14:27, Christoph Lameter wrote:
> 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.
>
Ah, yes, there is that too (like I tried to say, and messed up my 
grammar in doing so, I'm no expert), although on processors that 
actually have a reasonable amount of cache, this is not usually 
something most people would notice without a benchmark except on a very 
slow processor (HPC workloads and gamers notwithstanding of course).


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