On Mon, Sep 13, 2010 at 5:55 PM, Mel Gorman <mel@xxxxxxxxx> 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.
>> > > But you added new rule about writeback.
>> > >
>> >
>> > Yes, I'm trying to add a new rule about throttling in the page allocator
>> > and from vmscan. As you can see from the results in the leader, we are
>> > currently sleeping more than we need to.
>>
>> I can see the about avoiding congestion_wait but can't find about
>> (writeback < incative / 2) hueristic result.
>>
>
> See the leader and each of the report sections entitled
> "FTrace Reclaim Statistics: congestion_wait". It provides a measure of
> how sleep times are affected.
>
> "congest waited" are waits due to calling congestion_wait. "conditional waited"
> are those related to wait_iff_congested(). As you will see from the reports,
> sleep times are reduced overall while callers of wait_iff_congested() still
> go to sleep. The reports entitled "FTrace Reclaim Statistics: vmscan" show
> how reclaim is behaving and indicators so far are that reclaim is not hurt
> by introducing wait_iff_congested().
I saw the result.
It was a result about effectiveness _both_ nr_bdi_congested and
(writeback < inactive/2).
What I mean is just effectiveness (writeback < inactive/2) _alone_.
If we remove (writeback < inactive / 2) check and unconditionally
return, how does the behavior changed?
Am I misunderstanding your report in leader?
--
Kind regards,
Minchan Kim
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