Re: O(1) scheduler ver H6 - more straightforward timeslice macros

[Matti Aarnio <matti.aarnio@zmailer.org>]

On Sat, Jan 12, 2002 at 02:16:08PM -0500, James C. Owens wrote:
> Ingo,
> 
> I like the new scheduler. It seems like the timeslice macros in sched.h 
> could be more straighforward - i.e. instead of

   (I quote too much, but to illustriate the point...)

> #define PRIO_TO_TIMESLICE(p) \
>   ((( (MAX_USER_PRIO-1-USER_PRIO(p))*(MAX_TIMESLICE-MIN_TIMESLICE) + \
>      MAX_USER_PRIO-1) / MAX_USER_PRIO) + MIN_TIMESLICE)
> 
> #define RT_PRIO_TO_TIMESLICE(p) \
>   ((( (MAX_RT_PRIO-(p)-1)*(MAX_TIMESLICE-MIN_TIMESLICE) + \
>      MAX_RT_PRIO-1) / MAX_RT_PRIO) + MIN_TIMESLICE)
> 
> why not
> 
> #define PRIO_TO_TIMESLICE(p) \
>   (MAX_TIMESLICE - 
>    (USER_PRIO(p)/(MAX_USER_PRIO-1))*(MAX_TIMESLICE-MIN_TIMESLICE))
> 
> #define RT_PRIO_TO_TIMESLICE(p) \
>   (MAX_TIMESLICE - (p/(MAX_RT_PRIO-1))*(MAX_TIMESLICE-MIN_TIMESLICE))
> 
> 
> The second way seems simpler to me, and really illustrates what you are 
> doing in a more straightforward manner.

   Except that the math is INTEGER, not floating-point,
   which means that this way you loose precission.

   You HAVE TO do multiplications first, only then (finally) the division.

   Depending the value-spaces, small-enough value-spaces might be
   turnable into table mappings.  However that has lots of dependencies
   in hardware architecture, e.g. memory access speeds, cache pollution,
   speed of multiply/divide operations, etc.

   If dividers/multipliers are constants, and powers of two, the math
   can happen with constant shifts, which are fast at all systems.
   If not, things get rather complicated.   (And thus a careless -1,
   or lack of one, may be costly.)

> I also cleaned up some of the comments. The sched.h diff between the H6 
> version of the scheduler applied to 2.4.18-pre3 and vanilla 2.4.18-pre3 
> follows: (Note that I changed the min and max timeslices to 20 and 100 
> for my own use.)
...

  "uni-diff please, use '-u' option for the diff"

> To lkml - please cc me on any response, as I do not subscribe to the 
> lkml - I read it via a news gateway.
> 
> 
> Jim Owens

/Matti Aarnio