On Wed, Sep 08, 2010 at 11:52:45PM +0900, Minchan Kim wrote:
> On Wed, Sep 08, 2010 at 12:04:03PM +0100, Mel Gorman wrote:
> > On Wed, Sep 08, 2010 at 12:25:33AM +0900, Minchan Kim wrote:
> > > > + * @zone: A zone to consider the number of being being written back from
> > > > + * @sync: SYNC or ASYNC IO
> > > > + * @timeout: timeout in jiffies
> > > > + *
> > > > + * Waits for up to @timeout jiffies for a backing_dev (any backing_dev) to exit
> > > > + * write congestion. If no backing_devs are congested then the number of
> > > > + * writeback pages in the zone are checked and compared to the inactive
> > > > + * list. If there is no sigificant writeback or congestion, there is no point
> > > and
> > >
> >
> > Why and? "or" makes sense because we avoid sleeping on either condition.
>
> if (nr_bdi_congested[sync]) == 0) {
> if (writeback < inactive / 2) {
> cond_resched();
> ..
> goto out
> }
> }
>
> for avoiding sleeping, above two condition should meet.
This is a terrible comment that is badly written. Is this any clearer?
/**
* wait_iff_congested - Conditionally wait for a backing_dev to become uncongested or a zone to complete writes
* @zone: A zone to consider the number of being being written back from
* @sync: SYNC or ASYNC IO
* @timeout: timeout in jiffies
*
* In the event of a congested backing_dev (any backing_dev) or a given @zone
* having a large number of pages in writeback, this waits for up to @timeout
* jiffies for either a BDI to exit congestion or a write to complete.
*
* If there is no congestion and few pending writes, then cond_resched()
* is called to yield the processor if necessary but otherwise does not
* sleep.
*/
> >
> > > > + * 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.
> Why I pointed out is that you added new rule and I hope let others know
> this change since they have a good idea or any opinions.
> I think it's a one of roles as reviewer.
>
Of course.
> >
> > > 1. Without considering various speed class storage, could we fix it as half of inactive?
> >
> > We don't really have a good means of identifying speed classes of
> > storage. Worse, we are considering on a zone-basis here, not a BDI
> > basis. The pages being written back in the zone could be backed by
> > anything so we cannot make decisions based on BDI speed.
>
> True. So it's why I have below question.
> As you said, we don't have enough information in vmscan.
> So I am not sure how effective such semi-sensible decision is.
>
What additional metrics would you apply than the ones I used in the
leader mail?
> I think best is to throttle in page-writeback well.
I do not think there is a problem as such in page writeback throttling.
The problem is that we are going to sleep without any congestion or without
writes in progress. We sleep for a full timeout in this case for no reason
and this is what I'm trying to avoid.
> But I am not a expert about that and don't have any idea. Sorry.
Don't be, this is something that needs thinking about!
> So I can't insist on my nitpick. If others don't have any objection,
> I don't mind this, either.
>
> Wu, Do you have any opinion?
>
> >
> > > 2. Isn't there any writeback throttling on above layer? Do we care of it in here?
> > >
> >
> > There are but congestion_wait() and now wait_iff_congested() are part of
> > that. We can see from the figures in the leader that congestion_wait()
> > is sleeping more than is necessary or smart.
> >
> > > Just out of curiosity.
> > >
> >
> > --
> > Mel Gorman
> > Part-time Phd Student Linux Technology Center
> > University of Limerick IBM Dublin Software Lab
>
> --
> Kind regards,
> Minchan Kim
>
--
Mel Gorman
Part-time Phd Student Linux Technology Center
University of Limerick IBM Dublin Software Lab
--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@xxxxxxxxxx For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>