[PATCH] scale the initial value of min_free_kbytes

This tunable refers to the amount of free memory which the VM will attempt to
sustain.  It is mainly needed for atomic allocations (eg, networking
receive).

It is currently hardwired to 1024k, which is far too large for small machines
and too small for large machines.

Rework it to be 128k on tiny machines and 16M on huge machines.

include/linux/mmzone.h

@@ -284,8 +284,6 @@ struct ctl_table;
 struct file;
 int min_free_kbytes_sysctl_handler(struct ctl_table *, int, struct file *, 
 					  void *, size_t *);
-extern void setup_per_zone_pages_min(void);
-
 
 #ifdef CONFIG_NUMA
 #define MAX_NR_MEMBLKS	BITS_PER_LONG /* Max number of Memory Blocks */

init/main.c

@@ -396,7 +396,6 @@ asmlinkage void __init start_kernel(void)
 	lock_kernel();
 	printk(linux_banner);
 	setup_arch(&command_line);
-	setup_per_zone_pages_min();
 	setup_per_cpu_areas();
 
 	/*

mm/page_alloc.c

@@ -1589,7 +1589,7 @@ void __init page_alloc_init(void)
  *	that the pages_{min,low,high} values for each zone are set correctly 
  *	with respect to min_free_kbytes.
  */
-void setup_per_zone_pages_min(void)
+static void setup_per_zone_pages_min(void)
 {
 	unsigned long pages_min = min_free_kbytes >> (PAGE_SHIFT - 10);
 	unsigned long lowmem_pages = 0;
@@ -1632,6 +1632,45 @@ void setup_per_zone_pages_min(void)
 	}
 }
 
+/*
+ * Initialise min_free_kbytes.
+ *
+ * For small machines we want it small (128k min).  For large machines
+ * we want it large (16MB max).  But it is not linear, because network
+ * bandwidth does not increase linearly with machine size.  We use
+ *
+ *	min_free_kbytes = sqrt(lowmem_kbytes)
+ *
+ * which yields
+ *
+ * 16MB:	128k
+ * 32MB:	181k
+ * 64MB:	256k
+ * 128MB:	362k
+ * 256MB:	512k
+ * 512MB:	724k
+ * 1024MB:	1024k
+ * 2048MB:	1448k
+ * 4096MB:	2048k
+ * 8192MB:	2896k
+ * 16384MB:	4096k
+ */
+static int __init init_per_zone_pages_min(void)
+{
+	unsigned long lowmem_kbytes;
+
+	lowmem_kbytes = nr_free_buffer_pages() * (PAGE_SIZE >> 10);
+
+	min_free_kbytes = int_sqrt(lowmem_kbytes);
+	if (min_free_kbytes < 128)
+		min_free_kbytes = 128;
+	if (min_free_kbytes > 16384)
+		min_free_kbytes = 16384;
+	setup_per_zone_pages_min();
+	return 0;
+}
+module_init(init_per_zone_pages_min)
+
 /*
  * min_free_kbytes_sysctl_handler - just a wrapper around proc_dointvec() so 
  *	that we can call setup_per_zone_pages_min() whenever min_free_kbytes