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SubjectRe: [PATCH 5/5] writeback: IO-less balance_dirty_pages()
On Tue, Aug 16, 2011 at 10:20:11AM +0800, Wu Fengguang wrote:

[..]
> + if (dirty_exceeded && !bdi->dirty_exceeded)
> bdi->dirty_exceeded = 1;
>
> bdi_update_bandwidth(bdi, dirty_thresh, background_thresh,
> nr_dirty, bdi_thresh, bdi_dirty,
> start_time);
>
> - /* Note: nr_reclaimable denotes nr_dirty + nr_unstable.
> - * Unstable writes are a feature of certain networked
> - * filesystems (i.e. NFS) in which data may have been
> - * written to the server's write cache, but has not yet
> - * been flushed to permanent storage.
> - * Only move pages to writeback if this bdi is over its
> - * threshold otherwise wait until the disk writes catch
> - * up.
> - */
> - trace_balance_dirty_start(bdi);
> - if (bdi_nr_reclaimable > task_bdi_thresh) {
> - pages_written += writeback_inodes_wb(&bdi->wb,
> - write_chunk);
> - trace_balance_dirty_written(bdi, pages_written);
> - if (pages_written >= write_chunk)
> - break; /* We've done our duty */
> + if (unlikely(!writeback_in_progress(bdi)))
> + bdi_start_background_writeback(bdi);
> +
> + base_rate = bdi->dirty_ratelimit;
> + pos_ratio = bdi_position_ratio(bdi, dirty_thresh,
> + background_thresh, nr_dirty,
> + bdi_thresh, bdi_dirty);
> + if (unlikely(pos_ratio == 0)) {
> + pause = MAX_PAUSE;
> + goto pause;
> }
> + task_ratelimit = (u64)base_rate *
> + pos_ratio >> RATELIMIT_CALC_SHIFT;

Hi Fenguaang,

I am little confused here. I see that you have already taken pos_ratio
into account in bdi_update_dirty_ratelimit() and wondering why to take
that into account again in balance_diry_pages().

We calculated the pos_rate and balanced_rate and adjusted the
bdi->dirty_ratelimit accordingly in bdi_update_dirty_ratelimit().

So why are we adjusting this pos_ratio() adjusted limit again with
pos_ratio(). Doesn't it become effectively following (assuming
one is decreasing the dirty rate limit).

base_rate = bdi->dirty_ratelimit
pos_rate = base_rate * pos_ratio();

write_bw
balance_rate = pos_rate * --------
dirty_bw
delta = max(pos_rate, balance_rate)
bdi->dirty_ratelimit = bdi->dirty_ratelimit - delta;

task_ratelimit = bdi->dirty_ratelimit * pos_ratio().

So we have already taken into account pos_ratio() while calculating new
bdi->dirty_ratelimit. Do we need to take that into account again.

Thanks
Vivek


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