[PATCH] mm: vmscan: fix IO/refault regression in cache workingset transition

[PATCH] mm: vmscan: fix IO/refault regression in cache workingset transition

[Johannes Weiner]

Since 59dc76b0d4df ("mm: vmscan: reduce size of inactive file list")
we noticed bigger IO spikes during changes in cache access patterns.

The patch in question shrunk the inactive list size to leave more room
for the current workingset in the presence of streaming IO. However,
workingset transitions that previously happened on the inactive list
are now pushed out of memory and incur more refaults to complete.

This patch disables active list protection when refaults are being
observed. This accelerates workingset transitions, and allows more of
the new set to establish itself from memory, without eating into the
ability to protect the established workingset during stable periods.

Fixes: 59dc76b0d4df ("mm: vmscan: reduce size of inactive file list")
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: <stable@vger.kernel.org> # 4.7+
---
 include/linux/memcontrol.h | 64 +++++++++++++++++++++++++++++--
 include/linux/mmzone.h     |  2 +
 mm/memcontrol.c            | 24 ++++--------
 mm/vmscan.c                | 94 ++++++++++++++++++++++++++++++++++++----------
 mm/workingset.c            |  7 +++-
 5 files changed, 150 insertions(+), 41 deletions(-)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index c5ebb32fef49..cfa91a3ca0ca 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -57,6 +57,9 @@ enum mem_cgroup_stat_index {
 	MEMCG_SLAB_RECLAIMABLE,
 	MEMCG_SLAB_UNRECLAIMABLE,
 	MEMCG_SOCK,
+	MEMCG_WORKINGSET_REFAULT,
+	MEMCG_WORKINGSET_ACTIVATE,
+	MEMCG_WORKINGSET_NODERECLAIM,
 	MEMCG_NR_STAT,
 };
 
@@ -495,6 +498,40 @@ extern int do_swap_account;
 void lock_page_memcg(struct page *page);
 void unlock_page_memcg(struct page *page);
 
+static inline unsigned long mem_cgroup_read_stat(struct mem_cgroup *memcg,
+						 enum mem_cgroup_stat_index idx)
+{
+	long val = 0;
+	int cpu;
+
+	for_each_possible_cpu(cpu)
+		val += per_cpu(memcg->stat->count[idx], cpu);
+
+	if (val < 0)
+		val = 0;
+
+	return val;
+}
+
+static inline void mem_cgroup_update_stat(struct mem_cgroup *memcg,
+				   enum mem_cgroup_stat_index idx, int val)
+{
+	if (!mem_cgroup_disabled())
+		this_cpu_add(memcg->stat->count[idx], val);
+}
+
+static inline void mem_cgroup_inc_stat(struct mem_cgroup *memcg,
+				   enum mem_cgroup_stat_index idx)
+{
+	mem_cgroup_update_stat(memcg, idx, 1);
+}
+
+static inline void mem_cgroup_dec_stat(struct mem_cgroup *memcg,
+				   enum mem_cgroup_stat_index idx)
+{
+	mem_cgroup_update_stat(memcg, idx, -1);
+}
+
 /**
  * mem_cgroup_update_page_stat - update page state statistics
  * @page: the page
@@ -509,14 +546,14 @@ void unlock_page_memcg(struct page *page);
  *   if (TestClearPageState(page))
  *     mem_cgroup_update_page_stat(page, state, -1);
  *   unlock_page(page) or unlock_page_memcg(page)
+ *
+ * Kernel pages are an exception to this, since they'll never move.
  */
 static inline void mem_cgroup_update_page_stat(struct page *page,
 				 enum mem_cgroup_stat_index idx, int val)
 {
-	VM_BUG_ON(!(rcu_read_lock_held() || PageLocked(page)));
-
 	if (page->mem_cgroup)
-		this_cpu_add(page->mem_cgroup->stat->count[idx], val);
+		mem_cgroup_update_stat(page->mem_cgroup, idx, val);
 }
 
 static inline void mem_cgroup_inc_page_stat(struct page *page,
@@ -741,6 +778,27 @@ static inline bool mem_cgroup_oom_synchronize(bool wait)
 	return false;
 }
 
+static inline unsigned long mem_cgroup_read_stat(struct mem_cgroup *memcg,
+						 enum mem_cgroup_stat_index idx)
+{
+	return 0;
+}
+
+static inline void mem_cgroup_update_stat(struct mem_cgroup *memcg,
+				   enum mem_cgroup_stat_index idx, int val)
+{
+}
+
+static inline void mem_cgroup_inc_stat(struct mem_cgroup *memcg,
+				   enum mem_cgroup_stat_index idx)
+{
+}
+
+static inline void mem_cgroup_dec_stat(struct mem_cgroup *memcg,
+				   enum mem_cgroup_stat_index idx)
+{
+}
+
 static inline void mem_cgroup_update_page_stat(struct page *page,
 					       enum mem_cgroup_stat_index idx,
 					       int nr)
diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 618499159a7c..ebaccd4e7d8c 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -230,6 +230,8 @@ struct lruvec {
 	struct zone_reclaim_stat	reclaim_stat;
 	/* Evictions & activations on the inactive file list */
 	atomic_long_t			inactive_age;
+	/* Refaults at the time of last reclaim cycle */
+	unsigned long			refaults;
 #ifdef CONFIG_MEMCG
 	struct pglist_data *pgdat;
 #endif
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 490d5b4676c1..108d5b097db1 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -569,23 +569,6 @@ mem_cgroup_largest_soft_limit_node(struct mem_cgroup_tree_per_node *mctz)
  * common workload, threshold and synchronization as vmstat[] should be
  * implemented.
  */
-static unsigned long
-mem_cgroup_read_stat(struct mem_cgroup *memcg, enum mem_cgroup_stat_index idx)
-{
-	long val = 0;
-	int cpu;
-
-	/* Per-cpu values can be negative, use a signed accumulator */
-	for_each_possible_cpu(cpu)
-		val += per_cpu(memcg->stat->count[idx], cpu);
-	/*
-	 * Summing races with updates, so val may be negative.  Avoid exposing
-	 * transient negative values.
-	 */
-	if (val < 0)
-		val = 0;
-	return val;
-}
 
 static unsigned long mem_cgroup_read_events(struct mem_cgroup *memcg,
 					    enum mem_cgroup_events_index idx)
@@ -5244,6 +5227,13 @@ static int memory_stat_show(struct seq_file *m, void *v)
 	seq_printf(m, "pgmajfault %lu\n",
 		   events[MEM_CGROUP_EVENTS_PGMAJFAULT]);
 
+	seq_printf(m, "workingset_refault %lu\n",
+		   stat[MEMCG_WORKINGSET_REFAULT]);
+	seq_printf(m, "workingset_activate %lu\n",
+		   stat[MEMCG_WORKINGSET_ACTIVATE]);
+	seq_printf(m, "workingset_nodereclaim %lu\n",
+		   stat[MEMCG_WORKINGSET_NODERECLAIM]);
+
 	return 0;
 }
 
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 58615bb27f2f..b3f62cf37097 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2006,6 +2006,8 @@ static void shrink_active_list(unsigned long nr_to_scan,
  * Both inactive lists should also be large enough that each inactive
  * page has a chance to be referenced again before it is reclaimed.
  *
+ * If that fails and refaulting is observed, the inactive list grows.
+ *
  * The inactive_ratio is the target ratio of ACTIVE to INACTIVE pages
  * on this LRU, maintained by the pageout code. A zone->inactive_ratio
  * of 3 means 3:1 or 25% of the pages are kept on the inactive list.
@@ -2022,12 +2024,15 @@ static void shrink_active_list(unsigned long nr_to_scan,
  *   10TB     320        32GB
  */
 static bool inactive_list_is_low(struct lruvec *lruvec, bool file,
-						struct scan_control *sc, bool trace)
+				 struct mem_cgroup *memcg,
+				 struct scan_control *sc, bool actual_reclaim)
 {
-	unsigned long inactive_ratio;
-	unsigned long inactive, active;
-	enum lru_list inactive_lru = file * LRU_FILE;
 	enum lru_list active_lru = file * LRU_FILE + LRU_ACTIVE;
+	struct pglist_data *pgdat = lruvec_pgdat(lruvec);
+	enum lru_list inactive_lru = file * LRU_FILE;
+	unsigned long inactive, active;
+	unsigned long inactive_ratio;
+	unsigned long refaults;
 	unsigned long gb;
 
 	/*
@@ -2040,27 +2045,43 @@ static bool inactive_list_is_low(struct lruvec *lruvec, bool file,
 	inactive = lruvec_lru_size(lruvec, inactive_lru, sc->reclaim_idx);
 	active = lruvec_lru_size(lruvec, active_lru, sc->reclaim_idx);
 
-	gb = (inactive + active) >> (30 - PAGE_SHIFT);
-	if (gb)
-		inactive_ratio = int_sqrt(10 * gb);
+	if (memcg)
+		refaults = mem_cgroup_read_stat(memcg,
+						MEMCG_WORKINGSET_ACTIVATE);
 	else
-		inactive_ratio = 1;
+		refaults = node_page_state(pgdat, WORKINGSET_ACTIVATE);
+
+	/*
+	 * When refaults are being observed, it means a new workingset
+	 * is being established. Disable active list protection to get
+	 * rid of the stale workingset quickly.
+	 */
+	if (file && actual_reclaim && lruvec->refaults != refaults) {
+		inactive_ratio = 0;
+	} else {
+		gb = (inactive + active) >> (30 - PAGE_SHIFT);
+		if (gb)
+			inactive_ratio = int_sqrt(10 * gb);
+		else
+			inactive_ratio = 1;
+	}
 
-	if (trace)
-		trace_mm_vmscan_inactive_list_is_low(lruvec_pgdat(lruvec)->node_id,
-				sc->reclaim_idx,
-				lruvec_lru_size(lruvec, inactive_lru, MAX_NR_ZONES), inactive,
-				lruvec_lru_size(lruvec, active_lru, MAX_NR_ZONES), active,
-				inactive_ratio, file);
+	if (actual_reclaim)
+		trace_mm_vmscan_inactive_list_is_low(pgdat->node_id, sc->reclaim_idx,
+			lruvec_lru_size(lruvec, inactive_lru, MAX_NR_ZONES), inactive,
+			lruvec_lru_size(lruvec, active_lru, MAX_NR_ZONES), active,
+			inactive_ratio, file);
 
 	return inactive * inactive_ratio < active;
 }
 
 static unsigned long shrink_list(enum lru_list lru, unsigned long nr_to_scan,
-				 struct lruvec *lruvec, struct scan_control *sc)
+				 struct lruvec *lruvec, struct mem_cgroup *memcg,
+				 struct scan_control *sc)
 {
 	if (is_active_lru(lru)) {
-		if (inactive_list_is_low(lruvec, is_file_lru(lru), sc, true))
+		if (inactive_list_is_low(lruvec, is_file_lru(lru),
+					 memcg, sc, true))
 			shrink_active_list(nr_to_scan, lruvec, sc, lru);
 		return 0;
 	}
@@ -2169,7 +2190,7 @@ static void get_scan_count(struct lruvec *lruvec, struct mem_cgroup *memcg,
 	 * lruvec even if it has plenty of old anonymous pages unless the
 	 * system is under heavy pressure.
 	 */
-	if (!inactive_list_is_low(lruvec, true, sc, false) &&
+	if (!inactive_list_is_low(lruvec, true, memcg, sc, false) &&
 	    lruvec_lru_size(lruvec, LRU_INACTIVE_FILE, sc->reclaim_idx) >> sc->priority) {
 		scan_balance = SCAN_FILE;
 		goto out;
@@ -2320,7 +2341,7 @@ static void shrink_node_memcg(struct pglist_data *pgdat, struct mem_cgroup *memc
 				nr[lru] -= nr_to_scan;
 
 				nr_reclaimed += shrink_list(lru, nr_to_scan,
-							    lruvec, sc);
+							    lruvec, memcg, sc);
 			}
 		}
 
@@ -2387,7 +2408,7 @@ static void shrink_node_memcg(struct pglist_data *pgdat, struct mem_cgroup *memc
 	 * Even if we did not try to evict anon pages at all, we want to
 	 * rebalance the anon lru active/inactive ratio.
 	 */
-	if (inactive_list_is_low(lruvec, false, sc, true))
+	if (inactive_list_is_low(lruvec, false, memcg, sc, true))
 		shrink_active_list(SWAP_CLUSTER_MAX, lruvec,
 				   sc, LRU_ACTIVE_ANON);
 }
@@ -2710,6 +2731,26 @@ static void shrink_zones(struct zonelist *zonelist, struct scan_control *sc)
 	sc->gfp_mask = orig_mask;
 }
 
+static void snapshot_refaults(struct mem_cgroup *root_memcg, pg_data_t *pgdat)
+{
+	struct mem_cgroup *memcg;
+
+	memcg = mem_cgroup_iter(root_memcg, NULL, NULL);
+	do {
+		unsigned long refaults;
+		struct lruvec *lruvec;
+
+		if (memcg)
+			refaults = mem_cgroup_read_stat(memcg,
+						MEMCG_WORKINGSET_ACTIVATE);
+		else
+			refaults = node_page_state(pgdat, WORKINGSET_ACTIVATE);
+
+		lruvec = mem_cgroup_lruvec(pgdat, memcg);
+		lruvec->refaults = refaults;
+	} while ((memcg = mem_cgroup_iter(root_memcg, memcg, NULL)));
+}
+
 /*
  * This is the main entry point to direct page reclaim.
  *
@@ -2730,6 +2771,9 @@ static unsigned long do_try_to_free_pages(struct zonelist *zonelist,
 					  struct scan_control *sc)
 {
 	int initial_priority = sc->priority;
+	pg_data_t *last_pgdat;
+	struct zoneref *z;
+	struct zone *zone;
 retry:
 	delayacct_freepages_start();
 
@@ -2756,6 +2800,15 @@ static unsigned long do_try_to_free_pages(struct zonelist *zonelist,
 			sc->may_writepage = 1;
 	} while (--sc->priority >= 0);
 
+	last_pgdat = NULL;
+	for_each_zone_zonelist_nodemask(zone, z, zonelist, sc->reclaim_idx,
+					sc->nodemask) {
+		if (zone->zone_pgdat == last_pgdat)
+			continue;
+		last_pgdat = zone->zone_pgdat;
+		snapshot_refaults(sc->target_mem_cgroup, zone->zone_pgdat);
+	}
+
 	delayacct_freepages_end();
 
 	if (sc->nr_reclaimed)
@@ -3040,7 +3093,7 @@ static void age_active_anon(struct pglist_data *pgdat,
 	do {
 		struct lruvec *lruvec = mem_cgroup_lruvec(pgdat, memcg);
 
-		if (inactive_list_is_low(lruvec, false, sc, true))
+		if (inactive_list_is_low(lruvec, false, memcg, sc, true))
 			shrink_active_list(SWAP_CLUSTER_MAX, lruvec,
 					   sc, LRU_ACTIVE_ANON);
 
@@ -3287,6 +3340,7 @@ static int balance_pgdat(pg_data_t *pgdat, int order, int classzone_idx)
 		pgdat->kswapd_failures++;
 
 out:
+	snapshot_refaults(NULL, pgdat);
 	/*
 	 * Return the order kswapd stopped reclaiming at as
 	 * prepare_kswapd_sleep() takes it into account. If another caller
diff --git a/mm/workingset.c b/mm/workingset.c
index eda05c71fa49..51c6f61d4cea 100644
--- a/mm/workingset.c
+++ b/mm/workingset.c
@@ -269,7 +269,6 @@ bool workingset_refault(void *shadow)
 	lruvec = mem_cgroup_lruvec(pgdat, memcg);
 	refault = atomic_long_read(&lruvec->inactive_age);
 	active_file = lruvec_lru_size(lruvec, LRU_ACTIVE_FILE, MAX_NR_ZONES);
-	rcu_read_unlock();
 
 	/*
 	 * The unsigned subtraction here gives an accurate distance
@@ -290,11 +289,15 @@ bool workingset_refault(void *shadow)
 	refault_distance = (refault - eviction) & EVICTION_MASK;
 
 	inc_node_state(pgdat, WORKINGSET_REFAULT);
+	mem_cgroup_inc_stat(memcg, MEMCG_WORKINGSET_REFAULT);
 
 	if (refault_distance <= active_file) {
 		inc_node_state(pgdat, WORKINGSET_ACTIVATE);
+		mem_cgroup_inc_stat(memcg, MEMCG_WORKINGSET_ACTIVATE);
+		rcu_read_unlock();
 		return true;
 	}
+	rcu_read_unlock();
 	return false;
 }
 
@@ -472,6 +475,8 @@ static enum lru_status shadow_lru_isolate(struct list_head *item,
 	if (WARN_ON_ONCE(node->exceptional))
 		goto out_invalid;
 	inc_node_state(page_pgdat(virt_to_page(node)), WORKINGSET_NODERECLAIM);
+	mem_cgroup_inc_page_stat(virt_to_page(node),
+				 MEMCG_WORKINGSET_NODERECLAIM);
 	__radix_tree_delete_node(&mapping->page_tree, node,
 				 workingset_update_node, mapping);
 
-- 
2.12.1

Re: [PATCH] mm: vmscan: fix IO/refault regression in cache workingset transition

[Andrew Morton]

On Tue,  4 Apr 2017 18:00:52 -0400 Johannes Weiner <hannes@cmpxchg.org> wrote:

> Since 59dc76b0d4df ("mm: vmscan: reduce size of inactive file list")
> we noticed bigger IO spikes during changes in cache access patterns.
> 
> The patch in question shrunk the inactive list size to leave more room
> for the current workingset in the presence of streaming IO. However,
> workingset transitions that previously happened on the inactive list
> are now pushed out of memory and incur more refaults to complete.
> 
> This patch disables active list protection when refaults are being
> observed. This accelerates workingset transitions, and allows more of
> the new set to establish itself from memory, without eating into the
> ability to protect the established workingset during stable periods.
> 
> Fixes: 59dc76b0d4df ("mm: vmscan: reduce size of inactive file list")
> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
> Cc: <stable@vger.kernel.org> # 4.7+

That's a pretty large patch and the problem has been there for a year. 
I'm not sure that it's 4.11 material, let alone -stable.  Care to
explain further?

Re: [PATCH] mm: vmscan: fix IO/refault regression in cache workingset transition

[Johannes Weiner]

On Tue, Apr 04, 2017 at 03:07:03PM -0700, Andrew Morton wrote:
> On Tue,  4 Apr 2017 18:00:52 -0400 Johannes Weiner <hannes@cmpxchg.org> wrote:
> 
> > Since 59dc76b0d4df ("mm: vmscan: reduce size of inactive file list")
> > we noticed bigger IO spikes during changes in cache access patterns.
> > 
> > The patch in question shrunk the inactive list size to leave more room
> > for the current workingset in the presence of streaming IO. However,
> > workingset transitions that previously happened on the inactive list
> > are now pushed out of memory and incur more refaults to complete.
> > 
> > This patch disables active list protection when refaults are being
> > observed. This accelerates workingset transitions, and allows more of
> > the new set to establish itself from memory, without eating into the
> > ability to protect the established workingset during stable periods.
> > 
> > Fixes: 59dc76b0d4df ("mm: vmscan: reduce size of inactive file list")
> > Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
> > Cc: <stable@vger.kernel.org> # 4.7+
> 
> That's a pretty large patch and the problem has been there for a year. 
> I'm not sure that it's 4.11 material, let alone -stable.  Care to
> explain further?

The problem statement is a little terse, my apologies.

The workloads that were measurably affected for us were hit pretty bad
by it, with refault/majfault rates doubling and tripling during cache
transitions, and the machines sustaining half-hour periods of 100% IO
utilization, where they'd previously have sub-minute peaks at 60-90%.

Stateful services that handle user data tend to be more conservative
with kernel upgrades. As a result we hit most page cache issues with
some delay, as was the case here.

The severity seemed to warrant a stable tag, but I agree that holding
out until 4.11.1 is probably better, given the invasiveness of this.

Re: [PATCH] mm: vmscan: fix IO/refault regression in cache workingset transition

[Rik van Riel]

[-- Attachment #1: Type: text/plain, Size: 934 bytes --]

On Tue, 2017-04-04 at 18:00 -0400, Johannes Weiner wrote:

> +
> +	/*
> +	 * When refaults are being observed, it means a new
> workingset
> +	 * is being established. Disable active list protection to
> get
> +	 * rid of the stale workingset quickly.
> +	 */

This looks a little aggressive. What is this
expected to do when you have multiple workloads
sharing the same LRU, and one of the workloads
is doing refaults, while the other workload is
continuing to use the same working set as before?

I have been trying to wrap my mind around that for
the past day or so, and figure I should just ask
the question :)

> +	if (file && actual_reclaim && lruvec->refaults != refaults)
> {
> +		inactive_ratio = 0;
> +	} else {
> +		gb = (inactive + active) >> (30 - PAGE_SHIFT);
> +		if (gb)
> +			inactive_ratio = int_sqrt(10 * gb);
> +		else
> +			inactive_ratio = 1;
> +	}

-- 
All rights reversed

[-- Attachment #2: This is a digitally signed message part --]
[-- Type: application/pgp-signature, Size: 473 bytes --]

Re: [PATCH] mm: vmscan: fix IO/refault regression in cache workingset transition

[Johannes Weiner]

On Wed, Apr 05, 2017 at 06:11:04PM -0400, Rik van Riel wrote:
> On Tue, 2017-04-04 at 18:00 -0400, Johannes Weiner wrote:
> 
> > +
> > +	/*
> > +	 * When refaults are being observed, it means a new
> > workingset
> > +	 * is being established. Disable active list protection to
> > get
> > +	 * rid of the stale workingset quickly.
> > +	 */
> 
> This looks a little aggressive. What is this
> expected to do when you have multiple workloads
> sharing the same LRU, and one of the workloads
> is doing refaults, while the other workload is
> continuing to use the same working set as before?

It is aggressive, but it seems to be a trade-off between three things:
maximizing workingset protection during stable periods; minimizing
repeat refaults during workingset transitions; both of those when the
LRU is shared.

The data point we would need to balance optimally between these cases
is whether the active list is hot or stale, but we only have that once
we disable active list protection and challenge those pages.

The more conservative we go about this, the more IO cost to establish
the incoming workingset pages.

I actually did experiment with this. Instead of disabling active list
protection entirely, I reverted to the more conservative 50/50 ratio
during refaults. The 50/50 split addressed the regression, but the
aggressive behavior fared measurably better across three different
services I tested this on (one of them *is* multi-workingset, but the
jobs are cgrouped so they don't *really* share LRUs).

That win was intriguing, but it would be bad if it came out of the
budget of truly shared LRUs (for which I have no quantification).

Since this is a regression fix, it would be fair to be conservative
and use the 50/50 split for transitions here; keep the more adaptive
behavior for a future optimization.

What do you think?

Re: [PATCH] mm: vmscan: fix IO/refault regression in cache workingset transition

[Rik van Riel]

On Thu, 2017-04-06 at 10:49 -0400, Johannes Weiner wrote:
> On Wed, Apr 05, 2017 at 06:11:04PM -0400, Rik van Riel wrote:
> > On Tue, 2017-04-04 at 18:00 -0400, Johannes Weiner wrote:
> > 
> > > +
> > > +	/*
> > > +	 * When refaults are being observed, it means a new
> > > workingset
> > > +	 * is being established. Disable active list protection
> > > to
> > > get
> > > +	 * rid of the stale workingset quickly.
> > > +	 */
> > 
> > This looks a little aggressive. What is this
> > expected to do when you have multiple workloads
> > sharing the same LRU, and one of the workloads
> > is doing refaults, while the other workload is
> > continuing to use the same working set as before?
> 
> That win was intriguing, but it would be bad if it came out of the
> budget of truly shared LRUs (for which I have no quantification).
> 
> Since this is a regression fix, it would be fair to be conservative
> and use the 50/50 split for transitions here; keep the more adaptive
> behavior for a future optimization.
> 
> What do you think?

Lets try your patch, and see what happens.
After all, it only affects the file cache,
and does not lead to anonymous pages being
swapped out and causing major pain.

A fast workload transition seems like it
could be in everybody's best interest.

If this approach leads to trouble, we can
always try to soften it later.

One potential way of softening would be to
look at the number of refaults, vs the
number of working set re-confirmations, and
determine a target based on that.