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

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Date:	Fri, 07 Sep 2007 07:24:43 -0700
From:	Randy Dunlap <randy.dunlap@...cle.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
CC:	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [patch 8/8] Immediate Values - Documentation

Mathieu Desnoyers wrote:
> * 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 ?

I think that most people on lkml say that UTF-8 is OK in text doc files.


>>> +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
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