On Fri, 8 Feb 2013 16:53:17 +0100 Frederic Weisbecker wrote: > 2013/2/7 Christoph Lameter : > > On Thu, 7 Feb 2013, Frederic Weisbecker wrote: > > > >> Not with hrtick. > > > > hrtick? Did we not already try that a couple of years back and it turned > > out that the overhead of constantly reprogramming a timer via the PCI bus > > was causing too much of a performance regression? > > Yeah Peter said that especially reprogramming the clock everytime we > call schedule() was killing the performances. Now may be on some > workloads, with the tick stopped, we can find some new results. > -- I was a little apprehensive when you started talking about multiple tasks in Adaptive NOHZ mode on a core but the more I started thinking about it, I realized that we might end up in a cooperative multitasking mode with no tick at all going. Multiple SCHED_FIFO threads could run until blocking and another would be picked. Depends on well behaved threads of course, so probably many cases of users shooting off some toes with this... Of course if you mix scheduling policies or have RT throttling turned on we'll need some sort of tick for preemption. But if we can keep the timer reprogramming down we may see some big wins for RT and HPC loads.