On Mon 28-11-11 21:53:45, Wu Fengguang wrote: > We do "floating proportions" to let active devices to grow its target > share of dirty pages and stalled/inactive devices to decrease its target > share over time. > > It works well except in the case of "an inactive disk suddenly goes > busy", where the initial target share may be too small. To mitigate > this, bdi_position_ratio() has the below line to raise a small > bdi_thresh when it's safe to do so, so that the disk be feed with enough > dirty pages for efficient IO and in turn fast rampup of bdi_thresh: > > bdi_thresh = max(bdi_thresh, (limit - dirty) / 8); > > balance_dirty_pages() normally does negative feedback control which > adjusts ratelimit to balance the bdi dirty pages around the target. > In some extreme cases when that is not enough, it will have to block > the tasks completely until the bdi dirty pages drop below bdi_thresh. > > Acked-by: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx> > Signed-off-by: Wu Fengguang <fengguang.wu@xxxxxxxxx> Looks good. Acked-by: Jan Kara <jack@xxxxxxx> Honza > --- > mm/page-writeback.c | 16 ++++++++++++++-- > 1 file changed, 14 insertions(+), 2 deletions(-) > > --- linux-next.orig/mm/page-writeback.c 2011-11-23 10:57:41.000000000 +0800 > +++ linux-next/mm/page-writeback.c 2011-11-23 11:44:39.000000000 +0800 > @@ -411,8 +411,13 @@ void global_dirty_limits(unsigned long * > * > * Returns @bdi's dirty limit in pages. The term "dirty" in the context of > * dirty balancing includes all PG_dirty, PG_writeback and NFS unstable pages. > - * And the "limit" in the name is not seriously taken as hard limit in > - * balance_dirty_pages(). > + * > + * Note that balance_dirty_pages() will only seriously take it as a hard limit > + * when sleeping max_pause per page is not enough to keep the dirty pages under > + * control. For example, when the device is completely stalled due to some error > + * conditions, or when there are 1000 dd tasks writing to a slow 10MB/s USB key. > + * In the other normal situations, it acts more gently by throttling the tasks > + * more (rather than completely block them) when the bdi dirty pages go high. > * > * It allocates high/low dirty limits to fast/slow devices, in order to prevent > * - starving fast devices > @@ -594,6 +599,13 @@ static unsigned long bdi_position_ratio( > */ > if (unlikely(bdi_thresh > thresh)) > bdi_thresh = thresh; > + /* > + * It's very possible that bdi_thresh is close to 0 not because the > + * device is slow, but that it has remained inactive for long time. > + * Honour such devices a reasonable good (hopefully IO efficient) > + * threshold, so that the occasional writes won't be blocked and active > + * writes can rampup the threshold quickly. > + */ > bdi_thresh = max(bdi_thresh, (limit - dirty) / 8); > /* > * scale global setpoint to bdi's: > > -- Jan Kara <jack@xxxxxxx> SUSE Labs, CR -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
