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    SubjectRe: [PATCHv2 11/11] writeback: prevent unnecessary bdi threads wakeups
    Hi Nick,

    On Thu, 2010-07-22 at 13:19 +1000, Nick Piggin wrote:
    > > out:
    > > spin_unlock(&inode_lock);
    > > +
    > > + if (wakeup_bdi) {
    > > + spin_lock(&bdi->wb_lock);
    > > + if (!bdi->wb.task)
    > > + wake_up_process(default_backing_dev_info.wb.task);
    > > + else
    > > + wake_up_process(bdi->wb.task);
    > > + spin_unlock(&bdi->wb_lock);
    > > + }
    > > }
    > We really want to wake up the bdi right away when first dirtying
    > the inode? I haven't looked at where the state of the bdi code is
    > now, but isn't it better to have a a delay there?

    Yes, I guess we want to wake up the bdi thread after 5 secs (assuming
    default settings). I could implement a "wake_up_process_delayed"
    function which would use a timer, but I think it is not necessary to
    introduce these complications. We can just wake-up the bdi thread, it'll
    find out there is nothing to do, and will go sleep for 5 secs. I think
    this is good enough.

    IOW, delayed wake-up is not worth the trouble.

    > And rather than spreading details of how bdi tasks are managed
    > would you consider putting this into its own function?

    Sure, will do.

    > Other than that, I like your patches.

    Thanks :-)

    > Out of interest, is 5 seconds
    > very detremental to power usage? What is a reasonable goal for
    > wakeups? (eg. 95%+ of possible efficiency)

    I cannot tell for sure. In Nokia N900 phone we use OMAP3 and we have
    dynamic OFF-mode, so we switch off the CPU and peripherals completely
    when there is nothing to do, and SDRAM stays in low-power auto-refresh
    mode. Every useless wake-up makes us do a lot of job re-constructing the
    CPU state. I cannot tell the numbers, but I'm CCing Tero, who is working
    on OMAP3 PM and makes a lot of battery current measurements, he can
    provide some numbers.

    Best Regards,
    Artem Bityutskiy (Артём Битюцкий)

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