linux-kernel - Re: [PATCH v7 04/17] tracing: add static function tracer support for MIPS

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Date:	Wed, 11 Nov 2009 13:15:02 +0000 (GMT)
From:	"Maciej W. Rozycki" <macro@...ux-mips.org>
To:	Wu Zhangjin <wuzhangjin@...il.com>
cc:	David Daney <ddaney@...iumnetworks.com>, linux-mips@...ux-mips.org,
	linux-kernel@...r.kernel.org, zhangfx@...ote.com, zhouqg@...il.com,
	Ralf Baechle <ralf@...ux-mips.org>, rostedt@...dmis.org,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...e.hu>,
	Nicholas Mc Guire <der.herr@...r.at>,
	Richard Sandiford <rdsandiford@...glemail.com>,
	Patrik Kluba <kpajko79@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Michal Simek <monstr@...str.eu>
Subject: Re: [PATCH v7 04/17] tracing: add static function tracer support
 for MIPS

On Wed, 11 Nov 2009, Wu Zhangjin wrote:

> > -mlong-calls really degrades performance.  I have seen things like 6% 
> > drop in network packet forwarding rates with -mlong-calls.
> > 
> 
> so much drop? seems only two instructions added for it: lui, addi. from
> this view point, I think the -fno-omit-frame-pointer(add, sd, move...)
> will also bring with much drop.

 No, register jumps cannot be predicted -- this is where the performance 
goes on any serious processor -- the two extra instructions are nothing 
compared to that.  OTOH frame pointer calculations are pure arithmetic, so 
you only lose time incurred by the instructions themselves.

  Maciej
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