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    SubjectRe: [PATCHv2 11/11] writeback: prevent unnecessary bdi threads wakeups
    On Thu, Jul 22, 2010 at 09:48:24AM +0300, Artem Bityutskiy wrote:
    > 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.

    I can see what you mean, but I think the designs in core code should
    be made as efficient as possible _unless_ there is some complication
    in doing otherwise (not the other way around).

    This is producing 2 unrequired context switches, so I really would
    like to see it done properly. Setting up a timer is really pretty
    simple (or if you would care to implement a delayed process wakeup
    API, I think that would be useful -- I'm surprised there isn't one
    already).


    > > 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.


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