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    SubjectRe: [PATCH 05/18] writeback: per task dirty rate limit
    On Wed 07-09-11 01:27:38, Jan Kara wrote:
    > On Tue 06-09-11 17:47:10, Peter Zijlstra wrote:
    > > On Sun, 2011-09-04 at 09:53 +0800, Wu Fengguang wrote:
    > > > /*
    > > > + * After a task dirtied this many pages, balance_dirty_pages_ratelimited_nr()
    > > > + * will look to see if it needs to start dirty throttling.
    > > > + *
    > > > + * If dirty_poll_interval is too low, big NUMA machines will call the expensive
    > > > + * global_page_state() too often. So scale it near-sqrt to the safety margin
    > > > + * (the number of pages we may dirty without exceeding the dirty limits).
    > > > + */
    > > > +static unsigned long dirty_poll_interval(unsigned long dirty,
    > > > + unsigned long thresh)
    > > > +{
    > > > + if (thresh > dirty)
    > > > + return 1UL << (ilog2(thresh - dirty) >> 1);
    > > > +
    > > > + return 1;
    > > > +}
    > >
    > > Where does that sqrt come from?
    > He does 2^{log_2(x)/2} which, if done in real numbers arithmetics, would
    > result in x^{1/2}. Given the integer arithmetics, it might be twice as
    > small but still it's some approximation...
    Ah, now I realized that you probably meant to ask why does he use sqrt
    and not some other function... Sorry for the noise.

    Honza
    --
    Jan Kara <jack@suse.cz>
    SUSE Labs, CR


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