linux-kernel - Re: [PATCH 10/25] Unionfs: add un/likely conditionals on copyup ops

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Date:	Wed, 26 Sep 2007 11:43:21 -0400
From:	Erez Zadok <ezk@...sunysb.edu>
To:	Kyle Moffett <mrmacman_g4@....com>
Cc:	Erez Zadok <ezk@...sunysb.edu>,
	"Kok, Auke" <auke-jan.h.kok@...el.com>, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	viro@....linux.org.uk, hch@...radead.org
Subject: Re: [PATCH 10/25] Unionfs: add un/likely conditionals on copyup ops 

In message <2DDDD55D-4445-4565-9384-4E4BE7B41D7D@....com>, Kyle Moffett writes:
> On Sep 26, 2007, at 09:40:20, Erez Zadok wrote:
[...]
> > Recently we've done a full audit of the entire code, and added un/ 
> > likely where we felt that the chance of succeeding is 95% or better  
> > (e.g., error conditions that should rarely happen, and such).
> 
> Actually due to the performance penalty on some systems I think you  
> only want to use it if the chance of succeeding is 99% or better, as  
> the benefit if predicted is a cycle or two and the harm if  
> mispredicted can be more than 50 cycles, depending on the CPU.

*That's* the information I was looking for, Kyle: what's the estimated
probability I should be using as my guideline.  I used 95% (20/1 ratio), and
you're telling me I should use 99% (100/1 ratio).  The difference between
the number of cycles saved/added is very compelling.  Given that I certainly
agree with you that I'm using un/likely too much.  I'll re-evaluate and
update my patch series then.

Thanks,
Erez.
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