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    SubjectRe: [PATCH 4/5] writeback: per task dirty rate limit
    On Sat, Aug 06, 2011 at 10:35:31PM +0800, Andrea Righi wrote:
    > On Sat, Aug 06, 2011 at 04:44:51PM +0800, Wu Fengguang wrote:
    > > Add two fields to task_struct.
    > >
    > > 1) account dirtied pages in the individual tasks, for accuracy
    > > 2) per-task balance_dirty_pages() call intervals, for flexibility
    > >
    > > The balance_dirty_pages() call interval (ie. nr_dirtied_pause) will
    > > scale near-sqrt to the safety gap between dirty pages and threshold.
    > >
    > > XXX: The main problem of per-task nr_dirtied is, if 10k tasks start
    > > dirtying pages at exactly the same time, each task will be assigned a
    > > large initial nr_dirtied_pause, so that the dirty threshold will be
    > > exceeded long before each task reached its nr_dirtied_pause and hence
    > > call balance_dirty_pages().
    > >
    > > Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
    >
    > A minor nitpick below.
    >
    > Reviewed-by: Andrea Righi <andrea@betterlinux.com>

    Thank you.

    > > +/*
    > > * balance_dirty_pages() must be called by processes which are generating dirty
    > > * data. It looks at the number of dirty pages in the machine and will force
    > > * the caller to perform writeback if the system is over `vm_dirty_ratio'.
    >
    > I think we should also fix the comment of balance_dirty_pages(), now
    > that it's IO-less for the caller. Maybe something like:
    >
    > /*
    > * balance_dirty_pages() must be called by processes which are generating dirty
    > * data. It looks at the number of dirty pages in the machine and will force
    > * the caller to wait once crossing the dirty threshold. If we're over
    > * `background_thresh' then the writeback threads are woken to perform some
    > * writeout.
    > */

    Good catch! I'll add this change to the next patch:

    /*
    * balance_dirty_pages() must be called by processes which are generating dirty
    * data. It looks at the number of dirty pages in the machine and will force
    - * the caller to perform writeback if the system is over `vm_dirty_ratio'.
    + * the caller to wait once crossing the (background_thresh + dirty_thresh) / 2.
    * If we're over `background_thresh' then the writeback threads are woken to
    * perform some writeout.
    */

    Thanks,
    Fengguang


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