linux-kernel - Re: [patch 8/8] Immediate Values

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Message-ID: <20070907122353.GB9735@Krystal>
Date:	Fri, 7 Sep 2007 08:23:55 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Randy Dunlap <randy.dunlap@...cle.com>
Cc:	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [patch 8/8] Immediate Values - Documentation

* Randy Dunlap (randy.dunlap@...cle.com) wrote:
[fixing the rest of the comments in the patch]
> What character is this that I cannot read (not displayed properly
> by my email client maybe)?  <something> after 644.09 and before the
> +- symbol, repeated just before all of the +- symbols.
> 

This file seems to be UTF-8 and the mail has been send by quilt. Is it
ok or should I change this to +/- just to make sure ?


> 
> > +Getppid without memory pressure:
> > +1462.09Â±18.87 - 150.92Â±1.01 = 1311.17Â±18.90 cycles
> > +
> > +Getppid with memory pressure:
> > +17113.33Â±1655.92 - 578.22Â±269.51 = 16535.11Â±1677.71 cycles
> > +
> > +Therefore, if we add 2 markers not based on immediate values to the getppid
> > +code, which would add 2 memory reads, we would add
> > +2 * 555.93Â±12.74 = 1111.86Â±25.48 cycles
> > +
> > +Therefore,
> > +
> > +1111.86Â±25.48 / 16535.11Â±1677.71 = 0.0672
> > + relative error: sqrt(((25.48/1111.86)^2)+((1677.71/16535.11)^2))
> > +                     = 0.1040
> > + absolute error: 0.1040 * 0.0672 = 0.0070
> > +
> > +Therefore: 0.0672Â±0.0070 * 100% = 6.72Â±0.70 %
> > +
> > +We can therefore affirm that adding 2 markers to getppid, on a system with high
> > +memory pressure, would have a performance hit of at least 6.0% on the system
> > +call time, all within the uncertainty limits of these tests. The same applies to
> > +other kernel code paths. The smaller those code paths are, the highest the
> > +impact ratio will be.
> > +
> > +Therefore, not only is it interesting to use the immediate values to dynamically
> > +activate dormant code such as the markers, but I think it should also be
> > +considered as a replacement for many of the "read mostly" static variables.
> 
> 
> ---
> ~Randy
> *** Remember to use Documentation/SubmitChecklist when testing your code ***
> 

-- 
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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