Re: No 100 HZ timer !

[george anzinger]

Rik van Riel wrote:
> 
> On Wed, 1 Aug 2001, george anzinger wrote:
> > Chris Friesen wrote:
> > > george anzinger wrote:
> > >
> > > > The testing I have done seems to indicate a lower overhead on a lightly
> > > > loaded system, about the same overhead with some load, and much more
> > > > overhead with a heavy load.  To me this seems like the wrong thing to
> > >
> > > What about something that tries to get the best of both worlds?  How about a
> > > tickless system that has a max frequency for how often it will schedule?  This
> >
> > How would you do this?  Larger time slices?  But _most_ context
> > switches are not related to end of slice.  Refuse to switch?
> > This just idles the cpu.
> 
> Never set the next hit of the timer to (now + MIN_INTERVAL).
> 
> This way we'll get to run the current task until the timer
> hits or until the current task voluntarily gives up the CPU.

The overhead under load is _not_ the timer interrupt, it is the context
switch that needs to set up a "slice" timer, most of which never
expire.  During a kernel compile on an 800MHZ PIII I am seeing ~300
context switches per second (i.e. about every 3 ms.)   Clearly the
switching is being caused by task blocking.  With the ticked system the
"slice" timer overhead is constant.
> 
> We can check for already-expired timers in schedule().

Delaying "alarm" timers is bad form.  
Especially for someone who is working on high-res-timers :)

George