linux-kernel - Re: [PATCH 2/4, v2] x86: enlightenment for ticket spin locks

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Message-Id: <4C2B24B30200007800008C07@vpn.id2.novell.com>
Date:	Wed, 30 Jun 2010 10:04:19 +0100
From:	"Jan Beulich" <JBeulich@...ell.com>
To:	"Peter Zijlstra" <peterz@...radead.org>
Cc:	<jeremy.fitzhardinge@...rix.com>, <mingo@...e.hu>,
	<tglx@...utronix.de>, "Ky Srinivasan" <KSrinivasan@...ell.com>,
	<linux-kernel@...r.kernel.org>, <hpa@...or.com>
Subject: Re: [PATCH 2/4, v2] x86: enlightenment for ticket spin locks -
	 Xen implementation

>>> On 30.06.10 at 10:56, Peter Zijlstra <peterz@...radead.org> wrote:
> On Wed, 2010-06-30 at 09:52 +0100, Jan Beulich wrote:
>> > why is that a rwlock?, those things are useless.
>> 
>> Because potentially each CPU's lock gets acquired for reading during
>> unlock, while only the locking CPU's one needs to be acquired for
>> writing during lock. 
> 
> Can you say: scalability nightmare? but then its Xen code so who cares..

Yes, this is a scalability problem. And yes, I'm open to suggestions.
But no, I didn't have any better idea myself. And for the time being
I don't foresee this to be a problem, as VMs tend to have much fewer
CPUs than physical machines. And I think a performance win of 25%
justifies a sub-optimal implementation (with the perspective of people
smarter than me replacing it with a better one over time).

Jan

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