Messages in this thread | | | Date | Thu, 7 May 2009 11:01:12 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH] x86: Reduce the default HZ value |
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On Thu, May 07, 2009 at 07:38:24PM +0200, Peter Zijlstra wrote: > On Thu, 2009-05-07 at 10:36 -0700, Paul E. McKenney wrote: > > On Thu, May 07, 2009 at 07:18:38PM +0200, Peter Zijlstra wrote: > > > On Thu, 2009-05-07 at 19:13 +0200, Peter Zijlstra wrote: > > > > On Thu, 2009-05-07 at 19:09 +0200, Peter Zijlstra wrote: > > > > > On Thu, 2009-05-07 at 10:13 -0400, Christoph Lameter wrote: > > > > > > I think we need to reduce the general tick frequency to be as low as > > > > > > possible. With high resolution timers the tick frequency is just the > > > > > > frequency with which the timer interrupt disturbs a running application. > > > > > > > > > > > > Are there any benefits remaining from frequent timer interrupts? I would > > > > > > think that 60 HZ would be sufficient. > > > > > > > > > > > > It would be good if the kernel would be truly tickless. Scheduler events > > > > > > would be driven by the scheduling intervals and not the invokations of the > > > > > > scheduler softirq. > > > > > > > > > > The only thing that's driven by the softirq is load-balancing, there's > > > > > way more to the scheduler-tick than kicking that thing awake every so > > > > > often. > > > > > > > > > > The problem is that running the scheduler of off hrtimers is too > > > > > expensive. We have the code, we tried it, people complained. > > > > > > > > Therefore, decreasing the HZ value to say 50, we'd get a minimum > > > > involuntary preemption granularity of 20ms, something on the high end of > > > > barely usable. > > > > > > Another user is RCU, the grace period is tick driven, growing these > > > ticks by a factor 50 or so might require some tinkering with forced > > > grace periods when we notice our batch queues getting too long. > > > > One approach would be to enter nohz mode when running a CPU-bound > > application on a CPU that had nothing else (other than the idle task) > > on its runqueue and for which rcu_needs_cpu() returns zero. In this > > mode, RCU would need to be informed on each system call, perhaps with an > > rcu_kernel_enter() and rcu_kernel_exit() that work like rcu_irq_enter() > > and rcu_irq_exit() -- and that perhaps replace rcu_irq_enter() and > > rcu_irq_exit(). > > > > Then RCU would ignore any CPU that was executing a CPU-bound application, > > allowing the HZ to be dialed down as low as you like, or perhaps really > > entering something like nohz mode. > > Which would make syscall more expensive, not something you'd want to > do :-)
In general, I agree. However, in the case where you have a single CPU-bound task running in user mode, you don't care that much about syscall performance. So, yes, this would mean having yet another config variable that users running big CPU-bound scientific applications would need to worry about, which is not perfect either.
For whatever it is worth, the added overhead on entry would be something like the following:
void rcu_irq_enter(void) { struct rcu_dynticks *rdtp = &__get_cpu_var(rcu_dynticks);
if (rdtp->dynticks_nesting++) return; rdtp->dynticks++; WARN_ON_RATELIMIT(!(rdtp->dynticks & 0x1), &rcu_rs); smp_mb(); /* CPUs seeing ++ must see later RCU read-side crit sects */ }
On exit, a bit more:
void rcu_irq_exit(void) { struct rcu_dynticks *rdtp = &__get_cpu_var(rcu_dynticks);
if (--rdtp->dynticks_nesting) return; smp_mb(); /* CPUs seeing ++ must see prior RCU read-side crit sects */ rdtp->dynticks++; WARN_ON_RATELIMIT(rdtp->dynticks & 0x1, &rcu_rs);
/* If the interrupt queued a callback, get out of dyntick mode. */ if (__get_cpu_var(rcu_data).nxtlist || __get_cpu_var(rcu_bh_data).nxtlist) set_need_resched(); }
But I could move the callback check into call_rcu(), which would get the overhead of rcu_irq_exit() down to about that of rcu_irq_enter().
Thanx, Paul
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