Messages in this thread | | | Date | Tue, 08 May 2007 10:35:36 +1000 | From | Peter Williams <> | Subject | Re: [patch] CFS scheduler, -v8 |
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Esben Nielsen wrote: > > > 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 :-) ?
There's always going to be an upper limit on the representation of time. At least until we figure out how to implement infinity properly.
> >> 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.
As does the other method. A 32 bit signed offset with a 32 bit base is just a crude version of 64 bit absolute time.
> > 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.
Except if you're measuring sleep times. I think that you'll find lots of tasks sleep for more than 72 minutes.
Peter -- Peter Williams pwil3058@bigpond.net.au
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