Messages in this thread | | | Date | Mon, 7 May 2007 13:30:46 +0200 (CEST) | From | Esben Nielsen <> | Subject | Re: [patch] CFS scheduler, -v8 |
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On Sun, 6 May 2007, Linus Torvalds wrote:
> > > On Sun, 6 May 2007, Ingo Molnar wrote: >> >> * Linus Torvalds <torvalds@linux-foundation.org> wrote: >> >>> So the _only_ valid way to handle timers is to >>> - either not allow wrapping at all (in which case "unsigned" is better, >>> since it is bigger) >>> - or use wrapping explicitly, and use unsigned arithmetic (which is >>> well-defined in C) and do something like "(long)(a-b) > 0". >> >> hm, there is a corner-case in CFS where a fix like this is necessary. >> >> CFS uses 64-bit values for almost everything, and the majority of values >> are of 'relative' nature with no danger of overflow. (They are signed >> because they are relative values that center around zero and can be >> negative or positive.) > > Well, I'd like to just worry about that for a while. > > You say there is "no danger of overflow", and I mostly agree that once > we're talking about 64-bit values, the overflow issue simply doesn't > exist, and furthermore the difference between 63 and 64 bits is not really > relevant, so there's no major reason to actively avoid signed entries. > > So in that sense, it all sounds perfectly sane. And I'm definitely not > sure your "292 years after bootup" worry is really worth even considering. >
I would hate to tell mission control for Mankind's first mission to another star to reboot every 200 years because "there is no need to worry about it."
As a matter of principle an OS should never need a reboot (with exception for upgrading). If you say you have to reboot every 200 years, why not every 100? Every 50? .... Every 45 days (you know what I am referring to :-) ?
> When we're really so well off that we expect the hardware and software > stack to be stable over a hundred years, I'd start to think about issues > like that, in the meantime, to me worrying about those kinds of issues > just means that you're worrying about the wrong things. > > BUT. > > There's a fundamental reason relative timestamps are difficult and almost > always have overflow issues: the "long long in the future" case as an > approximation of "infinite timeout" is almost always relevant. > > So rather than worry about the system staying up 292 years, I'd worry > about whether people pass in big numbers (like some MAX_S64 approximation) > as an approximation for "infinite", and once you have things like that, > the "64 bits never overflows" argument is totally bogus. > > There's a damn good reason for using only *absolute* time. The whole > "signed values of relative time" may _sound_ good, but it really sucks in > subtle and horrible ways! >
I think you are wrong here. The only place you need absolute time is a for the clock (CLOCK_REALTIME). You waste CPU using a 64 bit representation when you could have used a 32 bit. With a 32 bit implementation you are forced to handle the corner cases with wrap around and too big arguments up front. With a 64 bit you hide those problems.
I think CFS would be best off using a 32 bit timer counting in micro seconds. That would wrap around in 72 minuttes. But as the timers are relative you will never be able to specify a timer larger than 36 minuttes in the future. But 36 minuttes is redicolously long for a scheduler and a simple test limiting time values to that value would not break anything.
Esben
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