Message-ID: <20090609150619.GT18380@csn.ul.ie>
Date:	Tue, 9 Jun 2009 16:06:19 +0100
From:	Mel Gorman <mel@....ul.ie>
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Rik van Riel <riel@...hat.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	"Zhang, Yanmin" <yanmin.zhang@...el.com>,
	"linuxram@...ibm.com" <linuxram@...ibm.com>,
	linux-mm <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/3] Reintroduce zone_reclaim_interval for when
	zone_reclaim() scans and fails to avoid CPU spinning at 100% on NUMA

On Tue, Jun 09, 2009 at 09:38:04PM +0800, Wu Fengguang wrote:
> On Tue, Jun 09, 2009 at 05:40:50PM +0800, Mel Gorman wrote:
> > On Tue, Jun 09, 2009 at 05:07:35PM +0800, Wu Fengguang wrote:
> > > On Tue, Jun 09, 2009 at 04:31:54PM +0800, Mel Gorman wrote:
> > > > On Tue, Jun 09, 2009 at 04:25:39PM +0800, Wu Fengguang wrote:
> > > > > On Tue, Jun 09, 2009 at 04:14:25PM +0800, Mel Gorman wrote:
> > > > > > On Tue, Jun 09, 2009 at 09:58:22AM +0800, Wu Fengguang wrote:
> > > > > > > On Mon, Jun 08, 2009 at 09:01:28PM +0800, Mel Gorman wrote:
> > > > > > > > On NUMA machines, the administrator can configure zone_reclaim_mode that is a
> > > > > > > > more targetted form of direct reclaim. On machines with large NUMA distances,
> > > > > > > > zone_reclaim_mode defaults to 1 meaning that clean unmapped pages will be
> > > > > > > > reclaimed if the zone watermarks are not being met. The problem is that
> > > > > > > > zone_reclaim() can be in a situation where it scans excessively without
> > > > > > > > making progress.
> > > > > > > > 
> > > > > > > > One such situation is where a large tmpfs mount is occupying a large
> > > > > > > > percentage of memory overall. The pages do not get cleaned or reclaimed by
> > > > > > > > zone_reclaim(), but the lists are uselessly scanned frequencly making the
> > > > > > > > CPU spin at 100%. The scanning occurs because zone_reclaim() cannot tell
> > > > > > > > in advance the scan is pointless because the counters do not distinguish
> > > > > > > > between pagecache pages backed by disk and by RAM.  The observation in
> > > > > > > > the field is that malloc() stalls for a long time (minutes in some cases)
> > > > > > > > when this situation occurs.
> > > > > > > > 
> > > > > > > > Accounting for ram-backed file pages was considered but not implemented on
> > > > > > > > the grounds it would be introducing new branches and expensive checks into
> > > > > > > > the page cache add/remove patches and increase the number of statistics
> > > > > > > > needed in the zone. As zone_reclaim() failing is currently considered a
> > > > > > > > corner case, this seemed like overkill. Note, if there are a large number
> > > > > > > > of reports about CPU spinning at 100% on NUMA that is fixed by disabling
> > > > > > > > zone_reclaim, then this assumption is false and zone_reclaim() scanning
> > > > > > > > and failing is not a corner case but a common occurance
> > > > > > > > 
> > > > > > > > This patch reintroduces zone_reclaim_interval which was removed by commit
> > > > > > > > 34aa1330f9b3c5783d269851d467326525207422 [zoned vm counters: zone_reclaim:
> > > > > > > > remove /proc/sys/vm/zone_reclaim_interval] because the zone counters were
> > > > > > > > considered sufficient to determine in advance if the scan would succeed.
> > > > > > > > As unsuccessful scans can still occur, zone_reclaim_interval is still
> > > > > > > > required.
> > > > > > > 
> > > > > > > Can we avoid the user visible parameter zone_reclaim_interval?
> > > > > > > 
> > > > > > 
> > > > > > You could, but then there is no way of disabling it by setting it to 0
> > > > > > either. I can't imagine why but the desired behaviour might really be to
> > > > > > spin and never go off-node unless there is no other option. They might
> > > > > > want to set it to 0 for example when determining what the right value for
> > > > > > zone_reclaim_mode is for their workloads.
> > > > > > 
> > > > > > > That means to introduce some heuristics for it.
> > > > > > 
> > > > > > I suspect the vast majority of users will ignore it unless they are runing
> > > > > > zone_reclaim_mode at the same time and even then will probably just leave
> > > > > > it as 30 as a LRU scan every 30 seconds worst case is not going to show up
> > > > > > on many profiles.
> > > > > > 
> > > > > > > Since the whole point
> > > > > > > is to avoid 100% CPU usage, we can take down the time used for this
> > > > > > > failed zone reclaim (T) and forbid zone reclaim until (NOW + 100*T).
> > > > > > > 
> > > > > > 
> > > > > > i.e. just fix it internally at 100 seconds? How is that better than
> > > > > > having an obscure tunable? I think if this heuristic exists at all, it's
> > > > > > important that an administrator be able to turn it off if absolutly
> > > > > > necessary and so something must be user-visible.
> > > > > 
> > > > > That 100*T don't mean 100 seconds. It means to keep CPU usage under 1%:
> > > > > after busy scanning for time T, let's go relax for 100*T.
> > > > > 
> > > > 
> > > > Do I have a means of calculating what my CPU usage is as a result of
> > > > scanning the LRU list?
> > > > 
> > > > If I don't and the machine is busy, would I not avoid scanning even in
> > > > situations where it should have been scanned?
> > > 
> > > I guess we don't really care about the exact number for the ratio 100.
> > > If the box is busy, it automatically scales the effective ratio to 200
> > > or more, which I think is reasonable behavior.
> > > 
> > > Something like this.
> > > 
> > > Thanks,
> > > Fengguang
> > > 
> > > ---
> > >  include/linux/mmzone.h |    2 ++
> > >  mm/vmscan.c            |   11 +++++++++++
> > >  2 files changed, 13 insertions(+)
> > > 
> > > --- linux.orig/include/linux/mmzone.h
> > > +++ linux/include/linux/mmzone.h
> > > @@ -334,6 +334,8 @@ struct zone {
> > >  	/* Zone statistics */
> > >  	atomic_long_t		vm_stat[NR_VM_ZONE_STAT_ITEMS];
> > >  
> > > +	unsigned long		zone_reclaim_relax;
> > > +
> > >  	/*
> > >  	 * prev_priority holds the scanning priority for this zone.  It is
> > >  	 * defined as the scanning priority at which we achieved our reclaim
> > > --- linux.orig/mm/vmscan.c
> > > +++ linux/mm/vmscan.c
> > > @@ -2453,6 +2453,7 @@ int zone_reclaim(struct zone *zone, gfp_
> > >  	int ret;
> > >  	long nr_unmapped_file_pages;
> > >  	long nr_slab_reclaimable;
> > > +	unsigned long t;
> > >  
> > >  	/*
> > >  	 * Zone reclaim reclaims unmapped file backed pages and
> > > @@ -2475,6 +2476,11 @@ int zone_reclaim(struct zone *zone, gfp_
> > >  	if (zone_is_all_unreclaimable(zone))
> > >  		return 0;
> > >  
> > > +	if (time_in_range(zone->zone_reclaim_relax - 10000 * HZ,
> > > +			  jiffies,
> > > +			  zone->zone_reclaim_relax))
> > > +		return 0;
> > > +
> > 
> > So. zone_reclaim_relax is some value between now and 100 times the approximate
> > time it takes to scan the LRU list. This check ensures that we do not scan
> > multiple times within the same interval. Is that right?
> 
> Yes and no: zone_reclaim_relax is the *absolute* time for that.
> This check ensures that if we wasted T seconds doing a fruitless
> zone reclaim, zone reclaim won't be repeated in the following 100*T
> seconds - which is a coarse relax period.
> 
> Its simpler form is: time_before(jiffies, zone_reclaim_relax),
> if not considering wraparound issues.
> 

Ok

> > >  	/*
> > >  	 * Do not scan if the allocation should not be delayed.
> > >  	 */
> > > @@ -2493,7 +2499,12 @@ int zone_reclaim(struct zone *zone, gfp_
> > >  
> > >  	if (zone_test_and_set_flag(zone, ZONE_RECLAIM_LOCKED))
> > >  		return 0;
> > > +	t = jiffies;
> > >  	ret = __zone_reclaim(zone, gfp_mask, order);
> > > +	if (sc.nr_reclaimed == 0) {
> > > +		t = min_t(unsigned long, 10000 * HZ, 100 * (jiffies - t));
> > > +		zone->zone_reclaim_relax = jiffies + t;
> > > +	}
> > 
> > This appears to be a way of automatically selecting a value for
> > zone_reclaim_interval but is 100 times the length of time it takes to scan the
> > LRU list enough to avoid excessive scanning of the LRU lists by zone_reclaim?
> 
> Exactly.
> 
> > I don't know and unlike zone_reclaim_interval, we have no way for the
> > administrator to intervene in the event we get the calculation wrong.
> > 
> > Conceivably though, zone_reclaim_interval could automatically tune
> > itself based on a heuristic like this if the administrator does not give
> > a specific value. I think that would be an interesting follow on once
> > we've brought back zone_reclaim_interval and get a feeling for how often
> > it is actually used.
> 
> Well I don't think that's good practice. There are heuristic
> calculations all over the kernel. Shall we exporting parameters to
> user space just because we are not absolutely sure? Or shall we ship
> the heuristics and do adjustments based on feedbacks and only export
> parameters when we find _known cases_ that cannot be covered by pure
> heuristics?
> 

Good question - I don't have a satisfactory answer but I intuitively find
the zone_reclaim_interval easier to deal with than the heuristic.  That said,
I would prefer if neither was required.

In the patchset, I've added a counter for the number of times that the
scan-avoidance heuristic fails. If the tmpfs problem has been resolved
(patch with bug reporter, am awaiting test), I'll drop zone_reclaim_interval
altogether and we'll use the counter to detect if/when this situation
occurs again.

> > >  	zone_clear_flag(zone, ZONE_RECLAIM_LOCKED);
> > >  
> > >  	return ret;
> > > 
> > 
> > -- 
> > Mel Gorman
> > Part-time Phd Student                          Linux Technology Center
> > University of Limerick                         IBM Dublin Software Lab
> 

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab
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