linux-kernel - Re: [Bug #11342] Linux 2.6.27-rc3: kernel BUG at mm/vmalloc.c

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Date:	Tue, 26 Aug 2008 13:01:02 -0700
From:	Mike Travis <travis@....com>
To:	Dave Jones <davej@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Mike Travis <travis@....com>,
	"Alan D. Brunelle" <Alan.Brunelle@...com>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Kernel Testers List <kernel-testers@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	"Siddha, Suresh B" <suresh.b.siddha@...el.com>,
	"Luck, Tony" <tony.luck@...el.com>, Jack Steiner <steiner@....com>,
	Christoph Lameter <cl@...ux-foundation.org>
Subject: Re: [Bug #11342] Linux 2.6.27-rc3: kernel BUG at mm/vmalloc.c -	bisected

Dave Jones wrote:
...
> 
> But yes, for this to be even remotely feasible, there has to be a negligable
> performance cost associated with it, which right now, we clearly don't have.
> Given that the number of people running 4096 CPU boxes even in a few years time
> will still be tiny, punishing the common case is obviously absurd.
> 
> 	Dave
> 

I did do some fairly extensive benchmarking between configs of NR_CPUS = 128 and
4096 and most performance hits were in the neighborhood of < 5% on systems with
8 cpus and 4GB of memory (our most common test system).  [But changing cpumask_t's
to be pointers instead of values will likely increase this.]  I've tried to be
very sensitive to this issue with all my previous changes, so convincing the distros
to set NR_CPUS=4096 would be as painless for them as possible. ;-)

Btw, huge count cpu systems I don't think are that far away.  I believe the nextgen
Larabbee chips will be geared towards HPC applications [instead of just GFX apps],
and putting 4 of these chips on a motherboard would add up to 512 cpu threads (1024
if they support hyperthreading.)

Thanks,
Mike
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