[PATCH] ZVC: Scale thresholds depending on the size of the system

The ZVC counter update threshold is currently set to a fixed value of 32.
This patch sets up the threshold depending on the number of processors and
the sizes of the zones in the system.

With the current threshold of 32, I was able to observe slight contention
when more than 130-140 processors concurrently updated the counters. The
contention vanished when I either increased the threshold to 64 or used
Andrew's idea of overstepping the interval (see ZVC overstep patch).

However, we saw contention again at 220-230 processors. So we need higher
values for larger systems.

But the current default is already a bit of an overkill for smaller
systems. Some systems have tiny zones where precision matters. For
example i386 and x86_64 have 16M DMA zones and either 900M ZONE_NORMAL or
ZONE_DMA32. These are even present on SMP and NUMA systems.

The patch here sets up a threshold based on the number of processors in the
system and the size of the zone that these counters are used for. The
threshold should grow logarithmically, so we use fls() as an easy
approximation.

Results of tests on a system with 1024 processors (4TB RAM)

The following output is from a test allocating 1GB of memory concurrently
on each processor (Forking the process. So contention on mmap_sem and the
pte locks is not a factor):

X MIN
TYPE: CPUS WALL WALL SYS USER TOTCPU
fork 1 0.552 0.552 0.540 0.012 0.552
fork 4 0.552 0.548 2.164 0.036 2.200
fork 16 0.564 0.548 8.812 0.164 8.976
fork 128 0.580 0.572 72.204 1.208 73.412
fork 256 1.300 0.660 310.400 2.160 312.560
fork 512 3.512 0.696 1526.836 4.816 1531.652
fork 1020 20.024 0.700 17243.176 6.688 17249.863

So a threshold of 32 is fine up to 128 processors. At 256 processors contention
becomes a factor.

Overstepping the counter (earlier patch) improves the numbers a bit:

fork 4 0.552 0.548 2.164 0.040 2.204
fork 16 0.552 0.548 8.640 0.148 8.788
fork 128 0.556 0.548 69.676 0.956 70.632
fork 256 0.876 0.636 212.468 2.108 214.576
fork 512 2.276 0.672 997.324 4.260 1001.584
fork 1020 13.564 0.680 11586.436 6.088 11592.523

Still contention at 512 and 1020. Contention at 1020 is down by a third.
256 still has a slight bit of contention.

After this patch the counter threshold will be set to 125 which reduces
contention significantly:

fork 128 0.560 0.548 69.776 0.932 70.708
fork 256 0.636 0.556 143.460 2.036 145.496
fork 512 0.640 0.548 284.244 4.236 288.480
fork 1020 1.500 0.588 1326.152 8.892 1335.044

[[e-mail]: !SMP build fix]
Signed-off-by: Christoph Lameter <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

include/linux/mmzone.h

···
 struct per_cpu_pageset {
 	struct per_cpu_pages pcp[2];	/* 0: hot.  1: cold */
 #ifdef CONFIG_SMP
+	s8 stat_threshold;
 	s8 vm_stat_diff[NR_VM_ZONE_STAT_ITEMS];
 #endif
 } ____cacheline_aligned_in_smp;

mm/vmstat.c

···
 #include <linux/config.h>
 #include <linux/mm.h>
 #include <linux/module.h>
+#include <linux/cpu.h>
 
 void __get_zone_counts(unsigned long *active, unsigned long *inactive,
 			unsigned long *free, struct pglist_data *pgdat)
···
 
 #ifdef CONFIG_SMP
 
-#define STAT_THRESHOLD 32
+static int calculate_threshold(struct zone *zone)
+{
+	int threshold;
+	int mem;	/* memory in 128 MB units */
+
+	/*
+	 * The threshold scales with the number of processors and the amount
+	 * of memory per zone. More memory means that we can defer updates for
+	 * longer, more processors could lead to more contention.
+ 	 * fls() is used to have a cheap way of logarithmic scaling.
+	 *
+	 * Some sample thresholds:
+	 *
+	 * Threshold	Processors	(fls)	Zonesize	fls(mem+1)
+	 * ------------------------------------------------------------------
+	 * 8		1		1	0.9-1 GB	4
+	 * 16		2		2	0.9-1 GB	4
+	 * 20 		2		2	1-2 GB		5
+	 * 24		2		2	2-4 GB		6
+	 * 28		2		2	4-8 GB		7
+	 * 32		2		2	8-16 GB		8
+	 * 4		2		2	<128M		1
+	 * 30		4		3	2-4 GB		5
+	 * 48		4		3	8-16 GB		8
+	 * 32		8		4	1-2 GB		4
+	 * 32		8		4	0.9-1GB		4
+	 * 10		16		5	<128M		1
+	 * 40		16		5	900M		4
+	 * 70		64		7	2-4 GB		5
+	 * 84		64		7	4-8 GB		6
+	 * 108		512		9	4-8 GB		6
+	 * 125		1024		10	8-16 GB		8
+	 * 125		1024		10	16-32 GB	9
+	 */
+
+	mem = zone->present_pages >> (27 - PAGE_SHIFT);
+
+	threshold = 2 * fls(num_online_cpus()) * (1 + fls(mem));
+
+	/*
+	 * Maximum threshold is 125
+	 */
+	threshold = min(125, threshold);
+
+	return threshold;
+}
 
 /*
- * Determine pointer to currently valid differential byte given a zone and
- * the item number.
- *
- * Preemption must be off
+ * Refresh the thresholds for each zone.
  */
-static inline s8 *diff_pointer(struct zone *zone, enum zone_stat_item item)
+static void refresh_zone_stat_thresholds(void)
 {
-	return &zone_pcp(zone, smp_processor_id())->vm_stat_diff[item];
+	struct zone *zone;
+	int cpu;
+	int threshold;
+
+	for_each_zone(zone) {
+
+		if (!zone->present_pages)
+			continue;
+
+		threshold = calculate_threshold(zone);
+
+		for_each_online_cpu(cpu)
+			zone_pcp(zone, cpu)->stat_threshold = threshold;
+	}
 }
 
 /*
···
 void __mod_zone_page_state(struct zone *zone, enum zone_stat_item item,
 				int delta)
 {
-	s8 *p;
+	struct per_cpu_pageset *pcp = zone_pcp(zone, smp_processor_id());
+	s8 *p = pcp->vm_stat_diff + item;
 	long x;
 
-	p = diff_pointer(zone, item);
 	x = delta + *p;
 
-	if (unlikely(x > STAT_THRESHOLD || x < -STAT_THRESHOLD)) {
+	if (unlikely(x > pcp->stat_threshold || x < -pcp->stat_threshold)) {
 		zone_page_state_add(x, zone, item);
 		x = 0;
 	}
-
 	*p = x;
 }
 EXPORT_SYMBOL(__mod_zone_page_state);
···
  * No overflow check is necessary and therefore the differential can be
  * incremented or decremented in place which may allow the compilers to
  * generate better code.
- *
  * The increment or decrement is known and therefore one boundary check can
  * be omitted.
+ *
+ * NOTE: These functions are very performance sensitive. Change only
+ * with care.
  *
  * Some processors have inc/dec instructions that are atomic vs an interrupt.
  * However, the code must first determine the differential location in a zone
···
  */
 static void __inc_zone_state(struct zone *zone, enum zone_stat_item item)
 {
-	s8 *p = diff_pointer(zone, item);
+	struct per_cpu_pageset *pcp = zone_pcp(zone, smp_processor_id());
+	s8 *p = pcp->vm_stat_diff + item;
 
 	(*p)++;
 
-	if (unlikely(*p > STAT_THRESHOLD)) {
-		zone_page_state_add(*p + STAT_THRESHOLD / 2, zone, item);
-		*p = -STAT_THRESHOLD / 2;
+	if (unlikely(*p > pcp->stat_threshold)) {
+		int overstep = pcp->stat_threshold / 2;
+
+		zone_page_state_add(*p + overstep, zone, item);
+		*p = -overstep;
 	}
 }
 
···
 void __dec_zone_page_state(struct page *page, enum zone_stat_item item)
 {
 	struct zone *zone = page_zone(page);
-	s8 *p = diff_pointer(zone, item);
+	struct per_cpu_pageset *pcp = zone_pcp(zone, smp_processor_id());
+	s8 *p = pcp->vm_stat_diff + item;
 
 	(*p)--;
 
-	if (unlikely(*p < -STAT_THRESHOLD)) {
-		zone_page_state_add(*p - STAT_THRESHOLD / 2, zone, item);
-		*p = STAT_THRESHOLD /2;
+	if (unlikely(*p < - pcp->stat_threshold)) {
+		int overstep = pcp->stat_threshold / 2;
+
+		zone_page_state_add(*p - overstep, zone, item);
+		*p = overstep;
 	}
 }
 EXPORT_SYMBOL(__dec_zone_page_state);
···
 					   pageset->pcp[j].high,
 					   pageset->pcp[j].batch);
 			}
+#ifdef CONFIG_SMP
+			seq_printf(m, "\n  vm stats threshold: %d",
+					pageset->stat_threshold);
+#endif
 		}
 		seq_printf(m,
 			   "\n  all_unreclaimable: %u"
···
 
 #endif /* CONFIG_PROC_FS */
 
+#ifdef CONFIG_SMP
+/*
+ * Use the cpu notifier to insure that the thresholds are recalculated
+ * when necessary.
+ */
+static int __cpuinit vmstat_cpuup_callback(struct notifier_block *nfb,
+		unsigned long action,
+		void *hcpu)
+{
+	switch (action) {
+		case CPU_UP_PREPARE:
+		case CPU_UP_CANCELED:
+		case CPU_DEAD:
+			refresh_zone_stat_thresholds();
+			break;
+		default:
+			break;
+	}
+	return NOTIFY_OK;
+}
+
+static struct notifier_block __cpuinitdata vmstat_notifier =
+	{ &vmstat_cpuup_callback, NULL, 0 };
+
+int __init setup_vmstat(void)
+{
+	refresh_zone_stat_thresholds();
+	register_cpu_notifier(&vmstat_notifier);
+	return 0;
+}
+module_init(setup_vmstat)
+#endif