On Wed, 2010-11-17 at 12:27 +0800, Wu Fengguang wrote:
> +void bdi_update_write_bandwidth(struct backing_dev_info *bdi,
> + unsigned long *bw_time,
> + s64 *bw_written)
> +{
> + unsigned long written;
> + unsigned long elapsed;
> + unsigned long bw;
> + unsigned long w;
> +
> + if (*bw_written == 0)
> + goto snapshot;
> +
> + elapsed = jiffies - *bw_time;
> + if (elapsed < HZ/100)
> + return;
> +
> + /*
> + * When there lots of tasks throttled in balance_dirty_pages(), they
> + * will each try to update the bandwidth for the same period, making
> + * the bandwidth drift much faster than the desired rate (as in the
> + * single dirtier case). So do some rate limiting.
> + */
> + if (jiffies - bdi->write_bandwidth_update_time < elapsed)
> + goto snapshot;
Why this goto snapshot and not simply return? This is the second call
(bdi_update_bandwidth equivalent).
If you were to leave the old bw_written/bw_time in place the next loop
around in wb_writeback() would see a larger delta..
I guess this funny loop in wb_writeback() is also the reason you've got
a single function and not the get/update like separation
> + written = percpu_counter_read(&bdi->bdi_stat[BDI_WRITTEN]) - *bw_written;
> + bw = (HZ * PAGE_CACHE_SIZE * written + elapsed/2) / elapsed;
> + w = min(elapsed / (HZ/100), 128UL);
> + bdi->write_bandwidth = (bdi->write_bandwidth * (1024-w) + bw * w) >> 10;
> + bdi->write_bandwidth_update_time = jiffies;
> +snapshot:
> + *bw_written = percpu_counter_read(&bdi->bdi_stat[BDI_WRITTEN]);
> + *bw_time = jiffies;
> +}
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