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    SubjectRe: [PATCH] bdi: use deferable timer for sync_supers task
    On Fri, Oct 08, 2010 at 09:57:04PM +0800, Wu Fengguang wrote:
    > On Fri, Oct 08, 2010 at 06:27:09PM +0800, Yong Wang wrote:
    > > On Fri, Oct 08, 2010 at 01:28:07PM +0300, Artem Bityutskiy wrote:
    > > > On Fri, 2010-10-08 at 18:27 +0800, Wu, Xia wrote:
    > > > > > However, when the next wake-up interrupt happens is not defined. It can
    > > > > > happen 1ms after, or 1 minute after, or 1 hour after. What Christoph
    > > > > > says is that there should be some guarantee that sb writeout starts,
    > > > > > say, within 5 to 10 seconds interval. Deferrable timers do not guarantee
    > > > > > this. But take a look at the range hrtimers - they do exactly this.
    > > > >
    > > > > If the system is in sleep state, is there any data which should be written?
    > > >
    > > > May be yes, may be no.
    > > >
    > >
    > > Thanks for the quick response, Artem. May I know what might need to be
    > > written out when system is really idle?
    >
    > system idle != no dirty inodes

    Ah sorry -- I missed the context. Please ignore the following
    paragraphs for sync_supers..

    Thanks,
    Fengguang

    > Imagine an application dirties 100MB data and quits. The system then
    > goes quiet for very long time. In this case we still want the flusher
    > thread to wake up within 30 seconds to flush the 100MB dirty data.
    > It's a contract that dirty data will be synced to disk after 30s
    > (which is the default value of /proc/sys/vm/dirty_expire_centisecs).
    >
    > Note that 30s is not an exact value. A dirty page may be synced to
    > disk when it's been dirtied for 35s. The 5s error comes from the
    > flusher wakeup interval (/proc/sys/vm/dirty_writeback_centisecs).
    >
    > Thanks,
    > Fengguang


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