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    SubjectRe: [PATCH] x86: Reduce the default HZ value
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    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 :-)



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