> The attached (untested) patch reverts to the old algorithm > specifically for the case of rearming a periodic timer, leaving the > new algorithm with +1 in place for all other uses.
> Now, it's unclear to me whether this is correct, because it can have > the following effect. Suppose ticks happen on intervals of time T. > Then, owing to unpredictable and uncontrollable scheduling delays, > the following sequence of events may happen: > Time 0*T: timer_settime(.it_value =3D T, .it_interval =3D T), arms timer at > 1*T > Time 1*T + 0.9*T: timer expires, rearms timer at 2*T > Time 2*T + 0.1*T: timer expires, rearms timer at 3*T > The duration between these consecutive expirations of the timer is > 0.2*T, even though we asked for an interval of T. True. In my opinion that is the correct behaviour; userland requested timer ticks at multiples of T, so there is a conceptually infinite stream of (conceptual) ticks generated at those times. Those then get turned into real events when the kernel can manage it. But a delay for one of them should not affect any other, except for the case where one is delayed long enough to occur after another's ideal time, in which case I would consider it acceptable (though not required) to drop one of the two. > [...POSIX...] IMO if POSIX forbids the above, POSIX is broken and should, in this respect, be ignored. One reason for using facilities taking structs itimerval is for ticks to _not_ be delayed by delay of previous ticks. If POSIX cannot be ignored for whatever reason, I would argue that a new facility that _does_ provide undelayed ticks should be provided. (I'm partial to timer sockets, but I am hardly unbiased. :-) > On the other hand, if it's not correct to do that, I'm not sure > correct POSIX periodic timers can attain a target _average_ interval > between expirations [...] I would argue that it's misleading, to the point I would call it incorrect, to call such a thing "periodic". /~\ The ASCII Mouse \ / Ribbon Campaign X Against HTML mo...@rodents-montreal.org / \ Email! 7D C8 61 52 5D E7 2D 39 4E F1 31 3E E8 B3 27 4B
