Re: 4G SGI quad Xeon - memory-related slowdowns

[Mark Hemment <markhe@veritas.com>]

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Hi Paul,
 
> 2) Other block I/O output (eg dd if=/dev/zero of=/dev/sdi bs=4M) also
> run very slowly

What do you notice when running "top" and doing the above?
Does the "buff" value grow high (+700MB), with high CPU usage?

  If so, I think this might be down to nr_free_buffer_pages().

  This function includes the pages in all zones (including the HIGHMEM
zone) in its calculations, while only DMA and NORMAL zone pages are used
for buffers.  This upsets the result from 
balance_dirty_state() (fs/buffer.c), and as a result the required flushing
of buffers is only done as a result of running v low of pages in the DMA
and NORMAL zones.

  I've attached a "quick hack" I did for 2.4.0.  It doesn't completely
solve the problem, but moves it in the right direction.

  Please let me know if this helps.

Mark

[-- Attachment #2: buffer.patch --]
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diff -urN -X dontdiff linux-2.4.0/mm/page_alloc.c markhe-2.4.0/mm/page_alloc.c
--- linux-2.4.0/mm/page_alloc.c	Wed Jan  3 17:59:06 2001
+++ markhe-2.4.0/mm/page_alloc.c	Mon Jan 15 15:35:14 2001
@@ -583,6 +583,27 @@
 }
 
 /*
+ *	Free pages in zone "type", and the zones below it.
+ */
+unsigned int nr_free_pages_zone (int type)
+{
+	unsigned int sum;
+	zone_t *zone;
+	pg_data_t *pgdat = pgdat_list;
+
+	if (type >= MAX_NR_ZONES)
+		BUG();
+
+	sum = 0;
+	while (pgdat) {
+		for (zone = pgdat->node_zones; zone < pgdat->node_zones + type; zone++)
+			sum += zone->free_pages;
+		pgdat = pgdat->node_next;
+	}
+	return sum;
+}
+
+/*
  * Total amount of inactive_clean (allocatable) RAM:
  */
 unsigned int nr_inactive_clean_pages (void)
@@ -600,6 +621,25 @@
 	return sum;
 }
 
+unsigned int nr_inactive_clean_pages_zone(int type)
+{
+	unsigned int sum;
+	zone_t *zone;
+	pg_data_t *pgdat = pgdat_list;
+
+	if (type >= MAX_NR_ZONES)
+		BUG();
+	type++;
+
+	sum = 0;
+	while (pgdat) {
+		for (zone = pgdat->node_zones; zone < pgdat->node_zones + type; zone++)
+			sum += zone->inactive_clean_pages;
+		pgdat = pgdat->node_next;
+	}
+	return sum;
+}
+
 /*
  * Amount of free RAM allocatable as buffer memory:
  */
@@ -607,9 +647,9 @@
 {
 	unsigned int sum;
 
-	sum = nr_free_pages();
-	sum += nr_inactive_clean_pages();
-	sum += nr_inactive_dirty_pages;
+	sum = nr_free_pages_zone(ZONE_NORMAL);
+	sum += nr_inactive_clean_pages_zone(ZONE_NORMAL);
+	sum += nr_inactive_dirty_pages;	/* XXX */
 
 	/*
 	 * Keep our write behind queue filled, even if