linux-kernel - Re: [Xen-devel] Performance overhead of paravirt

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Date:	Thu, 14 May 2009 10:33:59 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Jan Beulich <JBeulich@...ell.com>
Cc:	Ingo Molnar <mingo@...e.hu>, Jeremy Fitzhardinge <jeremy@...p.org>,
	Jun Nakajima <jun.nakajima@...el.com>,
	Xiaohui Xin <xiaohui.xin@...el.com>, Xin Li <xin.li@...el.com>,
	Xen-devel <xen-devel@...ts.xensource.com>,
	Nick Piggin <npiggin@...e.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [Xen-devel] Performance overhead of paravirt_ops on
 nativeidentified

On Thu, 2009-05-14 at 09:05 +0100, Jan Beulich wrote:
> Wouldn't a third solution be to use ticket spinlocks everywhere, i.e. eliminate
> the current indirection, and replace it by an indirection for just the contention
> case? As I view it, the problem for Xen aren't really the ticket locks by
> themselves, but rather the extra spinning involved, which is of concern only
> if a lock is contended. We're using ticket locks quite happily in our kernels,
> with directed instead of global wakeup from the unlock path. The only open
> issue we currently have is that while for native keeping interrupts disabled
> while spinning may be acceptable (though I'm not sure how -rt folks are
> viewing this), in a pv environment one should really re-enable interrupts
> here due to the potentially much higher latency.

the -rt folks don't nearly have as many spinlocks, and for those we do
like ticket locks, because they are much fairer and give better worst
case contention behaviour.

Also, for the -rt folks, preempt disable is about as bad as irq disable.

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