Re: Help with the high res timers

[john stultz]

On Wed, 2005-05-04 at 10:46 -0700, George Anzinger wrote:
> > Well, as long as the HZ period is close to the timer-interval unit
> > length, this is true. However if the timer-interval unit is smaller,
> > multiple bucket entries would be expired. The performance considerations
> > here are being looked at and this may be an area where the concepts in
> > HRT might help (having a HRT specific sub-bucket).
> 
> This is where we get in trouble with HR timers.  For a HR timer, we need to know 
> how to get a timer to expire (i.e. appear in the call back) at a well defined 
> and presise time (leaving aside latency issues).  The above discription allows 
> timers to be put in buckets without (as near as I can tell) making transparent 
> exactly when the bucket will be emptied, only saying that it will be after the 
> latest timer in the bucket is due.

<snip>

> 
> Think of it this way.  Decompose a HR timer into corse and fine units (you 
> choose, but here let say jiffies and nanoseconds).  Now we want the normal timer 
> system to handle the jiffies part of the time and to turn the timer over to the 
> HR timer code to take care of the nanosecond remainder.  If the jiffie part is 
> late, depending on the nanosecond part, it could make the timer late (i.e for 
> low values of the nanosecond part).  For high values of the nanosecond part, we 
> can compenstate...
> 
> This decomposition makes a lot of sense, by the way, for, at least, the 
> following reasons:
> 1) it keeps the most of the HR issues out of the normal timer code,
> 2) it keeps high res and low res timer in the correct time order, i.e. a low res 
> timer for jiffie X will expire prior to a high res timer for jiffie X + Y 
> nanoseconds.
> 3) handling the high res timer list is made vastly easier as it will only need 
> to have a rather small number of timers in it at any given time (i.e. those that 
> are to expire prior to the next corse timer tick).


Hmmm. Ok I think I see what your getting at. Let me know where I go
wrong:

1. Since HR soft-timers are a special case, their absolute nanosecond
expire values (exp_ns) are decomposed into a low-res portion and a high-
res portion. In your case it is units of jiffies (exp_jf) and
arch_cycles (exp_ac) respectively.

2. Since jiffies units map directly to a periodic tick, one can set a
regular soft-timer to expire at exp_jf. Then when it is expired, it is
added to a separate HR-timers list to expire in exp_ac  arch_cycles
units.

3. With the new-timeofday rework and Nish's soft-timers code, the soft-
timers bucket entries map to actual nanosecond time values, rather then
ticks. This causes a problem with your two level (regular periodic and
hr-timer) timer-lists because since entries don't strictly map to
interrupts, you don't how to decompose the nanosecond expiration into
low-res and high-res portions.


Here is my possible solution: Still keeping Nish's soft-timer rework
where we use nanoseconds instead of ticks or jiffies, provide an
expected interrupt-period value, which gives you the maximum interval
length between ticks. Thus you schedule a regular-low-res timer for the
nanosecond value (exp_ns - expected_interrupt_period). When that timer
expires, you move the timer to the HR timer list and schedule an
interrupt for the remaining time.

Let me know how that sounds.

thanks
-john

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