On Tue, Mar 08, 2011 at 11:31:13PM +0100, Jan Kara wrote:
> This patch changes balance_dirty_pages() throttling so that the function does
> not submit writes on its own but rather waits for flusher thread to do enough
> writes. This has an advantage that we have a single source of IO allowing for
> better writeback locality. Also we do not have to reenter filesystems from a
> non-trivial context.
>
> The waiting is implemented as follows: Whenever we decide to throttle a task in
> balance_dirty_pages(), task adds itself to a list of tasks that are throttled
> against that bdi and goes to sleep waiting to receive specified amount of page
> IO completions. Once in a while (currently HZ/10, later the interval should be
> autotuned based on observed IO completion rate), accumulated page IO
> completions are distributed equally among waiting tasks.
>
> This waiting scheme has been chosen so that waiting time in
> balance_dirty_pages() is proportional to
> number_waited_pages * number_of_waiters.
> In particular it does not depend on the total number of pages being waited for,
> thus providing possibly a fairer results. Note that the dependency on the
> number of waiters is inevitable, since all the waiters compete for a common
> resource so their number has to be somehow reflected in waiting time.
>
> CC: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> CC: Christoph Hellwig <hch@xxxxxxxxxxxxx>
> CC: Dave Chinner <david@xxxxxxxxxxxxx>
> CC: Wu Fengguang <fengguang.wu@xxxxxxxxx>
> CC: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
> Signed-off-by: Jan Kara <jack@xxxxxxx>
> ---
> include/linux/backing-dev.h | 7 +
> include/linux/writeback.h | 1 +
> include/trace/events/writeback.h | 65 +++++++-
> mm/backing-dev.c | 8 +
> mm/page-writeback.c | 345 +++++++++++++++++++++++++-------------
> 5 files changed, 310 insertions(+), 116 deletions(-)
>
[..]
> +/*
> + * 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'.
> + * If we're over `background_thresh' then the writeback threads are woken to
> + * perform some writeout.
> + */
> +static void balance_dirty_pages(struct address_space *mapping,
> + unsigned long write_chunk)
> +{
> + struct backing_dev_info *bdi = mapping->backing_dev_info;
> + struct balance_waiter bw;
> + struct dirty_limit_state st;
> + int dirty_exceeded = check_dirty_limits(bdi, &st);
> +
> + if (dirty_exceeded < DIRTY_MAY_EXCEED_LIMIT ||
> + (dirty_exceeded == DIRTY_MAY_EXCEED_LIMIT &&
> + !bdi_task_limit_exceeded(&st, current))) {
> + if (bdi->dirty_exceeded &&
> + dirty_exceeded < DIRTY_MAY_EXCEED_LIMIT)
> + bdi->dirty_exceeded = 0;
> /*
> - * Increase the delay for each loop, up to our previous
> - * default of taking a 100ms nap.
> + * In laptop mode, we wait until hitting the higher threshold
> + * before starting background writeout, and then write out all
> + * the way down to the lower threshold. So slow writers cause
> + * minimal disk activity.
> + *
> + * In normal mode, we start background writeout at the lower
> + * background_thresh, to keep the amount of dirty memory low.
> */
> - pause <<= 1;
> - if (pause > HZ / 10)
> - pause = HZ / 10;
> + if (!laptop_mode && dirty_exceeded == DIRTY_EXCEED_BACKGROUND)
> + bdi_start_background_writeback(bdi);
> + return;
> }
>
> - /* Clear dirty_exceeded flag only when no task can exceed the limit */
> - if (!min_dirty_exceeded && bdi->dirty_exceeded)
> - bdi->dirty_exceeded = 0;
> + if (!bdi->dirty_exceeded)
> + bdi->dirty_exceeded = 1;
Will it make sense to move out bdi_task_limit_exceeded() check in a
separate if condition statement as follows. May be this is little
easier to read.
if (dirty_exceeded < DIRTY_MAY_EXCEED_LIMIT) {
if (bdi->dirty_exceeded)
bdi->dirty_exceeded = 0;
if (!laptop_mode && dirty_exceeded == DIRTY_EXCEED_BACKGROUND)
bdi_start_background_writeback(bdi);
return;
}
if (dirty_exceeded == DIRTY_MAY_EXCEED_LIMIT &&
!bdi_task_limit_exceeded(&st, current))
return;
/* Either task is throttled or we crossed global dirty ratio */
if (!bdi->dirty_exceeded)
bdi->dirty_exceeded = 1;
Thanks
Vivek
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