linux-kernel - Re: Performance overhead of paravirt

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Date:	Fri, 22 May 2009 15:47:05 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
CC:	"Xin, Xiaohui" <xiaohui.xin@...el.com>,
	Chuck Ebbert <cebbert@...hat.com>, Ingo Molnar <mingo@...e.hu>,
	"Li, Xin" <xin.li@...el.com>,
	"Nakajima, Jun" <jun.nakajima@...el.com>,
	Nick Piggin <npiggin@...e.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Xen-devel <xen-devel@...ts.xensource.com>
Subject: Re: Performance overhead of paravirt_ops on native identified

Jeremy Fitzhardinge wrote:
> 
> I did a quick experiment to see how many sites this optimisation could 
> actually affect.  Firstly, it does absolutely nothing with frame 
> pointers enabled.  Arranging for no frame pointers is quite tricky, 
> since it means disabling all debugging, tracing and other things.
> 
> With no frame pointers, its about 26 of 5400 indirect calls are 
> immediately followed by ret (not all of those sites are pvops calls).  
> With preempt disabled, this goes up to 45 sites.
> 
> I haven't done any actual runtime tests, but a quick survey of the 
> affected sites shows that only a couple are performance-sensitive; 
> _spin_lock and _spin_lock_irq and _spin_lock_irqsave are the most obvious.
> 

OK, that doesn't seem like a very productive avenue.  Problem still
remains, obviously.

	-hpa

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