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    SubjectRe: [PATCH 3/6] writeback: avoid unnecessary calculation of bdi dirty thresholds
    On Tue, Aug 03, 2010 at 11:03:42PM +0800, Peter Zijlstra wrote:
    > On Sun, 2010-07-11 at 10:06 +0800, Wu Fengguang wrote:
    > > plain text document attachment (writeback-less-bdi-calc.patch)
    > > Split get_dirty_limits() into global_dirty_limits()+bdi_dirty_limit(),
    > > so that the latter can be avoided when under global dirty background
    > > threshold (which is the normal state for most systems).
    >
    > The patch looks OK, although esp with the proposed comments in the
    > follow up email, bdi_dirty_limit() gets a bit confusing wrt to how and
    > what the limit is.
    >
    > Maybe its clearer to not call task_dirty_limit() from bdi_dirty_limit(),
    > that way the comment can focus on the device write request completion
    > proportion thing.

    Done, thanks.

    > > +unsigned long bdi_dirty_limit(struct backing_dev_info *bdi,
    > > + unsigned long dirty)
    > > +{
    > > + u64 bdi_dirty;
    > > + long numerator, denominator;
    > >
    > > + /*
    > > + * Calculate this BDI's share of the dirty ratio.
    > > + */
    > > + bdi_writeout_fraction(bdi, &numerator, &denominator);
    > >
    > > + bdi_dirty = (dirty * (100 - bdi_min_ratio)) / 100;
    > > + bdi_dirty *= numerator;
    > > + do_div(bdi_dirty, denominator);
    > >
    > > + bdi_dirty += (dirty * bdi->min_ratio) / 100;
    > > + if (bdi_dirty > (dirty * bdi->max_ratio) / 100)
    > > + bdi_dirty = dirty * bdi->max_ratio / 100;
    > > +
    > + return bdi_dirty;
    > > }
    >
    > And then add the call to task_dirty_limit() here:

    Done. I omitted adding task_dirty_limit() to the bdi_dirty_limit()
    inside bdi_debug_stats_show() -- looks unnecessary there.

    > > +++ linux-next/mm/backing-dev.c 2010-07-11 08:53:44.000000000 +0800
    > > @@ -83,7 +83,8 @@ static int bdi_debug_stats_show(struct s
    > > nr_more_io++;
    > > spin_unlock(&inode_lock);
    > >
    > > - get_dirty_limits(&background_thresh, &dirty_thresh, &bdi_thresh, bdi);
    > > + global_dirty_limits(&background_thresh, &dirty_thresh);
    > > + bdi_thresh = bdi_dirty_limit(bdi, dirty_thresh);
    > + bdi_thresh = task_dirty_limit(current, bdi_thresh);
    >
    > And add a comment to task_dirty_limit() as well, explaining its reason
    > for existence (protecting light/slow dirtying tasks from heavier/fast
    > ones).

    Comments updated as below. Any suggestions/corrections?

    Thanks,
    Fengguang

    Subject: writeback: add comment to the dirty limits functions
    From: Wu Fengguang <fengguang.wu@intel.com>
    Date: Thu Jul 15 09:54:25 CST 2010

    Document global_dirty_limits(), bdi_dirty_limit() and task_dirty_limit().

    Cc: Christoph Hellwig <hch@infradead.org>
    Cc: Dave Chinner <david@fromorbit.com>
    Cc: Jens Axboe <axboe@kernel.dk>
    Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
    Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
    ---
    mm/page-writeback.c | 31 ++++++++++++++++++++++++++++---
    1 file changed, 28 insertions(+), 3 deletions(-)

    --- linux-next.orig/mm/page-writeback.c 2010-08-03 23:14:19.000000000 +0800
    +++ linux-next/mm/page-writeback.c 2010-08-05 00:37:17.000000000 +0800
    @@ -261,11 +261,18 @@ static inline void task_dirties_fraction
    }

    /*
    - * scale the dirty limit
    + * task_dirty_limit - scale down dirty throttling threshold for one task
    *
    * task specific dirty limit:
    *
    * dirty -= (dirty/8) * p_{t}
    + *
    + * To protect light/slow dirtying tasks from heavier/fast ones, we start
    + * throttling individual tasks before reaching the bdi dirty limit.
    + * Relatively low thresholds will be allocated to heavy dirtiers. So when
    + * dirty pages grow large, heavy dirtiers will be throttled first, which will
    + * effectively curb the growth of dirty pages. Light dirtiers with high enough
    + * dirty threshold may never get throttled.
    */
    static unsigned long task_dirty_limit(struct task_struct *tsk,
    unsigned long bdi_dirty)
    @@ -390,6 +397,15 @@ unsigned long determine_dirtyable_memory
    return x + 1; /* Ensure that we never return 0 */
    }

    +/**
    + * global_dirty_limits - background-writeback and dirty-throttling thresholds
    + *
    + * Calculate the dirty thresholds based on sysctl parameters
    + * - vm.dirty_background_ratio or vm.dirty_background_bytes
    + * - vm.dirty_ratio or vm.dirty_bytes
    + * The dirty limits will be lifted by 1/4 for PF_LESS_THROTTLE (ie. nfsd) and
    + * runtime tasks.
    + */
    void global_dirty_limits(unsigned long *pbackground, unsigned long *pdirty)
    {
    unsigned long background;
    @@ -424,8 +440,17 @@ void global_dirty_limits(unsigned long *
    *pdirty = dirty;
    }

    -unsigned long bdi_dirty_limit(struct backing_dev_info *bdi,
    - unsigned long dirty)
    +/**
    + * bdi_dirty_limit - @bdi's share of dirty throttling threshold
    + *
    + * Allocate high/low dirty limits to fast/slow devices, in order to prevent
    + * - starving fast devices
    + * - piling up dirty pages (that will take long time to sync) on slow devices
    + *
    + * The bdi's share of dirty limit will be adapting to its throughput and
    + * bounded by the bdi->min_ratio and/or bdi->max_ratio parameters, if set.
    + */
    +unsigned long bdi_dirty_limit(struct backing_dev_info *bdi, unsigned long dirty)
    {
    u64 bdi_dirty;
    long numerator, denominator;

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