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    SubjectRe: [PATCH 11/32] nohz/cpuset: Don't turn off the tick if rcu needs it
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    On Tue, 2012-03-27 at 20:19 -0500, Christoph Lameter wrote:
    > On Tue, 27 Mar 2012, Steven Rostedt wrote:
    >
    > > > Obviously compiling a kernel with preemptiong introduces additional
    > > > overhead to guarantee more deterministic behavior. Additional overhead
    > > > increases latencies generated by the OS in general. Compile a kernel
    > > > without preemption and it will run faster and thus have lower latencies.
    > >
    > > I call that "lower overhead".
    >
    > Good marketing but it does not change the facts.

    I see we are mixing the paint for the bike shed.

    >
    > > "Latency is a measure of time delay experienced in a system, the precise
    > > definition of which depends on the system and the time being measured.
    > > Latencies may have different meaning in different contexts."
    > >
    > > That last sentence is key. So lets avoid the term "latency" as it
    > > obviously has a different meaning to the both of us.
    > >
    > > Instead, lets use "determinism" (what we call latency in the realtime
    > > world) and "overhead" (what you seem to see as latency caused by the
    > > kernel).
    >
    > I sure wish you would be using the term determinism instead of "latency".
    >
    > Overhead causes latency and the definition that you quoted is what I am
    > talking about. Latencies are the delays in processing experienced by the
    > application through the speed of system calls and by interruptions of
    > a user space process by the kernel for various reasons.

    I could also argue that a non-preempt kernel has a large latency as
    well. Although it may have good through put for one task, another task
    may suffer from a large latency waiting for a lower priority task to get
    out of a system call.

    You say tomAYto I say tomAHto.

    Read the article: http://en.wikipedia.org/wiki/Latency_%28engineering%29

    Especially the section about: Computer hardware and operating system latency

    You'll see that it describes latency much closer to my definition than
    yours.

    Heck, google "operating system latency" and you'll see a lot of talk
    about reaction times and how fast the hardware can do its job. I don't
    see anything about the time a system call takes.

    -- Steve





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