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    SubjectRe: [PATCH] nohz1: Documentation
    On Thu, Mar 21, 2013 at 08:45:07AM -0700, Arjan van de Ven wrote:
    > On 3/20/2013 5:27 PM, Steven Rostedt wrote:
    > >I'm not sure I would recommend idle=poll either. It would certainly
    > >work, but it goes to the other extreme. You think NO_HZ=n drains a
    > >battery? Try idle=poll.
    >
    >
    > do not ever use idle=poll on anything production.. really bad idea.
    >
    > if you temporary cannot cope with the latency, you can use the PMQOS system
    > to limit (including going all the way to idle=poll).
    > but using idle=poll completely is very nasty for the hardware.
    >
    > In addition we should document that idle=poll will cost you peak performance,
    > possibly quite a bit.
    >
    > the same is true for the kernel paramter to some extend; it's there to work around
    > broken bioses/hardware/etc; if you have a latency/runtime requirement, it's much better
    > to use PMQOS for this from userspace.

    Thank you for the info, Arjan! Does the following capture the tradeoffs?

    o Dyntick-idle slows transitions to and from idle slightly.
    In practice, this has not been a problem except for the most
    aggressive real-time workloads, which have the option of disabling
    dyntick-idle mode, an option that most of them take. However,
    some workloads will no doubt want to use adaptive ticks to
    eliminate scheduling-clock-tick latencies. Here are some
    options for these workloads:

    o Use PMQOS from userspace to inform the kernel of your
    latency requirements (preferred).

    o Use the "idle=mwait" boot parameter.

    o Use the "intel_idle.max_cstate=" to limit the maximum
    depth C-state depth.

    o Use the "idle=poll" boot parameter. However, please note
    that use of this parameter can cause your CPU to overheat,
    which may cause thermal throttling to degrade your
    latencies --and that this degradation can be even worse
    than that of dyntick-idle.

    Thanx, Paul



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