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Date: Fri, 22 May 2009 15:47:05 -0700
From: "H. Peter Anvin" <hpa@zytor.com>
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To: Jeremy Fitzhardinge <jeremy@goop.org>
CC: "Xin, Xiaohui" <xiaohui.xin@intel.com>, Chuck Ebbert <cebbert@redhat.com>,
       Ingo Molnar <mingo@elte.hu>, "Li, Xin" <xin.li@intel.com>,
       "Nakajima, Jun" <jun.nakajima@intel.com>, Nick Piggin <npiggin@suse.de>,
       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
       Xen-devel <xen-devel@lists.xensource.com>
Subject: Re: Performance overhead of paravirt_ops on native identified
References: <4A0B62F7.5030802@goop.org> <20090521184233.3c3e97ad@dhcp-100-2-144.bos.redhat.com> <4A15DA4E.2090505@goop.org> <C85CEDA13AB1CF4D9D597824A86D2B9005760A96DA@PDSMSX501.ccr.corp.intel.com> <4A1629BA.9070309@goop.org> <C85CEDA13AB1CF4D9D597824A86D2B9005760A9940@PDSMSX501.ccr.corp.intel.com> <4A16D3DF.5000600@zytor.com> <4A172ADE.3010702@goop.org>
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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