Re: Help with the high res timers

[George Anzinger <george@mvista.com>]

Chris Friesen wrote:
> Nishanth Aravamudan wrote:
> 
>> If I understand your point correctly, I think this is achieved by
>> TIMERINTERVAL_BITS in my patch (not to claim my patch is function, but
>> conceptually). No matter what you actually request, the best you can do
>> is 2^TIMERINTERVAL_BITS nanoseconds, and usually worse because the
>> tick-rate and timerinterval length do not necessarily line up.
> 
> 
> My point is simply that the timer for the next interval should start at 
> the time the timer expires, not the time that userspace picks up the 
> prior expiration.  Throttling the timer rate should be done at the time 
> of timer request rather than timer expiry.
> 
> If I have usec-accuracy in the timer subsystem, I should be able to set 
> a timer with an interval of 9.999ms and have it remain accurate over 
> time (subject to scheduler jitter, of course).  N timer intervals later 
> my timer should expire at (original_time + N*9.999ms + jitter).  In this 
> case the error is roughly constant with time.
> 
> If the timer doesn't start counting the next interval until the user 
> detects expiry, I'm going to get some non-zero addition to *each* 
> interval such that my timers will not remain accurate over long periods 
> of time.  In this case N timer intervals later my timer will expire at 
> (original_time + N*(9.999ms + jitter)) which is a very different thing. 
>  Since jitter will always be positive, the error increases with time.

You can, and we do in the main line kernel, keep this accuracy even with missed 
timer expiries.  When a timer is restarted, the actual requested expire time is 
recalculated.  It is adjusted ONLY by the requested repeat time until it results 
in a time that has yet to come and the number of repeat times it takes is logged 
in the overrun count for user examination.
> 
> Chris
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-- 
George Anzinger   george@mvista.com
High-res-timers:  http://sourceforge.net/projects/high-res-timers/