add a per cpu minimum high watermark an tune batch size

make sure there's at least 1024 per cpu pages... a reasonably small
amount for todays system

Signed-off-by: Simão Gomes Viana <[email protected]>

Author: fenrus75

mm/page_alloc.c

@@ -5632,11 +5632,11 @@ static int zone_batchsize(struct zone *zone)
 
 	/*
 	 * The number of pages to batch allocate is either ~0.1%
-	 * of the zone or 1MB, whichever is smaller. The batch
+	 * of the zone or 4MB, whichever is smaller. The batch
 	 * size is striking a balance between allocation latency
 	 * and zone lock contention.
 	 */
-	batch = min(zone_managed_pages(zone) >> 10, SZ_1M / PAGE_SIZE);
+	batch = min(zone_managed_pages(zone) >> 10, 4 * SZ_1M / PAGE_SIZE);
 	batch /= 4;		/* We effectively *= 4 below */
 	if (batch < 1)
 		batch = 1;