Re: [PATCH 03/10] writeback: Do not congestion sleep if there are no congested BDIs or significant writeback

[Minchan Kim <minchan.kim@gmail.com>]

On Mon, Sep 13, 2010 at 5:55 PM, Mel Gorman <mel@csn.ul.ie> wrote:
> On Mon, Sep 13, 2010 at 12:37:44AM +0900, Minchan Kim wrote:
>> > > > > > <SNIP>
>> > > > > >
>> > > > > > + * in sleeping but cond_resched() is called in case the current process has
>> > > > > > + * consumed its CPU quota.
>> > > > > > + */
>> > > > > > +long wait_iff_congested(struct zone *zone, int sync, long timeout)
>> > > > > > +{
>> > > > > > +   long ret;
>> > > > > > +   unsigned long start = jiffies;
>> > > > > > +   DEFINE_WAIT(wait);
>> > > > > > +   wait_queue_head_t *wqh = &congestion_wqh[sync];
>> > > > > > +
>> > > > > > +   /*
>> > > > > > +    * If there is no congestion, check the amount of writeback. If there
>> > > > > > +    * is no significant writeback and no congestion, just cond_resched
>> > > > > > +    */
>> > > > > > +   if (atomic_read(&nr_bdi_congested[sync]) == 0) {
>> > > > > > +           unsigned long inactive, writeback;
>> > > > > > +
>> > > > > > +           inactive = zone_page_state(zone, NR_INACTIVE_FILE) +
>> > > > > > +                           zone_page_state(zone, NR_INACTIVE_ANON);
>> > > > > > +           writeback = zone_page_state(zone, NR_WRITEBACK);
>> > > > > > +
>> > > > > > +           /*
>> > > > > > +            * If less than half the inactive list is being written back,
>> > > > > > +            * reclaim might as well continue
>> > > > > > +            */
>> > > > > > +           if (writeback < inactive / 2) {
>> > > > >
>> > > > > I am not sure this is best.
>> > > > >
>> > > >
>> > > > I'm not saying it is. The objective is to identify a situation where
>> > > > sleeping until the next write or congestion clears is pointless. We have
>> > > > already identified that we are not congested so the question is "are we
>> > > > writing a lot at the moment?". The assumption is that if there is a lot
>> > > > of writing going on, we might as well sleep until one completes rather
>> > > > than reclaiming more.
>> > > >
>> > > > This is the first effort at identifying pointless sleeps. Better ones
>> > > > might be identified in the future but that shouldn't stop us making a
>> > > > semi-sensible decision now.
>> > >
>> > > nr_bdi_congested is no problem since we have used it for a long time