Re: [patch 8/8] Immediate Values - Documentation

[Randy Dunlap <randy.dunlap@oracle.com>]

Mathieu Desnoyers wrote:
> * Randy Dunlap (randy.dunlap@oracle.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:
> > > +1462.09±18.87 - 150.92±1.01 = 1311.17±18.90 cycles
> > > +
> > > +Getppid with memory pressure:
> > > +17113.33±1655.92 - 578.22±269.51 = 16535.11±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 * 555.93±12.74 = 1111.86±25.48 cycles
> > > +
> > > +Therefore,
> > > +
> > > +1111.86±25.48 / 16535.11±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: 0.0672±0.0070 * 100% = 6.72±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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