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Re: [PATCH v7 04/17] tracing: add static function tracer support for MIP

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