Messages in this thread | | | Date | Wed, 04 May 2005 15:48:05 -0700 | From | George Anzinger <> | Subject | Re: Help with the high res timers |
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john stultz wrote: > 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 precise time (leaving aside latency issues). The above description 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 coarse 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 coarse 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.
Oh, I have been thinking along these same lines. I don't recall at the moment, but I depend on the timer retaining the absolute expire time as I set it. This is used both by the secondary timer, and by the rearm code. I need to look more closely at that. But this is picking things apart at a very low level and does not, I think, address timer ordering to the point where I feel completely safe. I.e. will such a scheme allow a LR time at time X to expire after a HR timer for time X+y. > > Let me know how that sounds. > > thanks > -john > > - > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/ >
-- George Anzinger george@mvista.com High-res-timers: http://sourceforge.net/projects/high-res-timers/ - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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