Messages in this thread | | | Date | Wed, 15 Jun 2016 13:26:59 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [patch 00/20] timer: Refactor the timer wheel |
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On Wed, Jun 15, 2016 at 07:02:38PM +0200, Thomas Gleixner wrote: > On Wed, 15 Jun 2016, Paul E. McKenney wrote: > > On Mon, Jun 13, 2016 at 09:15:14AM -0700, Paul E. McKenney wrote: > > > On Mon, Jun 13, 2016 at 08:40:50AM -0000, Thomas Gleixner wrote: > > > > The current timer wheel has some drawbacks: > > > > > > > > 1) Cascading > > > > > > > > Cascading can be an unbound operation and is completely pointless in most > > > > cases because the vast majority of the timer wheel timers are canceled or > > > > rearmed before expiration. > > > > > > > > 2) No fast lookup of the next expiring timer > > > > > > > > In NOHZ scenarios the first timer soft interrupt after a long NOHZ period > > > > must fast forward the base time to current jiffies. As we have no way to > > > > find the next expiring timer fast, the code loops and increments the base > > > > time by one and checks for expired timers in each step. I've observed loops > > > > lasting 1 ms! > > > > > > > > There are some other issues caused by the above, but they are minor compare to > > > > those. > > > > > > For SMP configurations, this passes light rcutorture testing. For UP > > > builds, it complains about undefined symbols. Builds succeed with > > > the following kneejerk patch. Am retesting rcutorture. > > > > And with the patch below, testing goes as well with your patch stack as > > it does without it. So, with that patch (or equivalent): > > > > Tested-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> > > > > There were some complaints about increasing the size of the tiny > > configuration, FYI. > > I know. The extra storage space for the deferrable stuff makes it larger along > with the extra code for avoiding all the crap which the current wheel suffers > from :) Do the tiny people need NOHZ?
If it makes the code bigger, I would hope that they don't need it.
> > So, just out of curiosity, does anyone still run -rt on single-CPU systems? > > Of course :)
Hey, had to ask! ;-)
Thanx, Paul
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