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    SubjectRe: [RFC PATCH 0/6] Convert all tasklets to workqueues

    > the context-switch argument i'll believe if i see numbers. You'll
    > probably need in excess of tens of thousands of irqs/sec to even be able
    > to measure its overhead. (workqueues are driven by nice kernel threads
    > so there's no TLB overhead, etc.)

    It was authors of the patch who were supposed to give some numbers,
    at least one or two, just to prove the concept. :-)

    According to my measurements (maybe, wrong) on 2.5GHz P4 tasklet
    schedule and execution eats ~300ns, workqueue eats ~4usec.
    On my 1.8GHz PM notebook (UP kernel), the numbers are 170ns and 1.2usec.

    Formally looking awful, this result is positive: tasklets are almost
    never used in hot paths. I am sure only about one such place: acenic
    driver uses tasklet to refill rx queue. This generates not more than
    3000 tasklet schedules per second. Even on P4 it pure workqueue schedule
    will eat ~1% of bare cpu ticks.

    Anyway, all the uses of tasklet should be verified:

    The most dubios place is popular Neterion 10Gbit driver, which uses
    tasklet like acenic. But at 10Gbit, multiply acenic numbers and panic. :-)

    Also, there exists some hardware which uses tasklets even harder,
    but I have no idea what real frequencies are: f.e. sundance.

    The case with acenic/s2io is quite special: normally network drivers
    refill queues in irq handlers. It was Jes Sorensen observation
    that offloading refilling from irq improves performance, I do not
    remember numbers. Probably, switching to workqueues will not affect
    performance at all, probably it will just collapse, no idea.

    > ... workqueues are also possibly much more scalable

    I cannot figure out - scale in what direction? :-)

    > (percpu workqueues
    > are easy without changing anything in your code but the call where you
    > create the workqueue).

    I do not see how it is related to scalability. And the statement
    does not even make sense. The patch already uses per-cpu workqueue
    for tasklets, otherwise it would be a disaster: guaranteed cpu non-locality.

    Tasklet is single thread by definition and purpose. Those a few places
    where people used tasklets to do per-cpu jobs (RCU f.e.) exist just because
    they had troubles with allocating new softirq. Workqueues do not make
    any difference: tasklet is not workqueue, it is work_struct, and you
    still will have to allocate array of per-cpu work structs, everything
    remains the same.

    > the only remaining argument is latency:

    You could set realtime prioriry by default, not a poor nice -5.
    If some network adapters were killed just because I run some task
    with nice --22, it would be just ridiculous.

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      Last update: 2007-06-28 16:43    [W:0.023 / U:0.284 seconds]
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