[PATCH] swapin readahead

[Rik van Riel <H.H.vanRiel@phys.uu.nl>]

Hi,

here is a very first primitive version of as swapin
readahead patch. It seems to give much increased
throughput to swap and the desktop switch time has
decreased noticably.

The checks are all needed. The first two checks are there
to avoid annoying messages from swap_state.c :)) The third
check is to make sure we always keep at least as much
swapout bandwidth as swapin bandwidth. We need that to keep
the system alive under heavy circumstances.

I am now testing the patch quite heavily (200+ swap IOs/second)
without any errors showing up in my xconsole, so I guess that
means you can have fun too :)

cheers,

Rik -- now completely used to dvorak kbd layout...
+-------------------------------------------------------------------+
| Linux memory management tour guide.        H.H.vanRiel@phys.uu.nl |
| Scouting Vries cubscout leader.      http://www.phys.uu.nl/~riel/ |
+-------------------------------------------------------------------+

--- mm/page_alloc.c.orig	Thu Nov 26 11:26:49 1998
+++ mm/page_alloc.c	Thu Nov 26 23:48:42 1998
@@ -370,9 +370,28 @@
 	pte_t * page_table, unsigned long entry, int write_access)
 {
 	unsigned long page;
+	int i;
 	struct page *page_map;
+	unsigned long offset = SWP_OFFSET(entry);
+	struct swap_info_struct *swapdev = SWP_TYPE(entry) + swap_info;
 	
 	page_map = read_swap_cache(entry);
+
+	/*
+	 * Primitive swap readahead code. We simply read the
+	 * next 16 entries in the swap area. The break below
+	 * is needed or else the request queue will explode :)
+	 */
+	for (i = 1; i++ < 16;) {
+		offset++;
+		if (!swapdev->swap_map[offset] || offset >= swapdev->max
+				|| atomic_read(&nr_async_pages) >
+				pager_daemon.swap_cluster / 2)
+			break;
+		read_swap_cache_async(SWP_ENTRY(SWP_TYPE(entry), offset),
+0);
+			break;
+	}
 
 	if (pte_val(*page_table) != entry) {
 		if (page_map)
--- mm/page_io.c.orig	Thu Nov 26 11:26:49 1998
+++ mm/page_io.c	Thu Nov 26 11:30:43 1998
@@ -60,7 +60,7 @@
 	}
 
 	/* Don't allow too many pending pages in flight.. */
-	if (atomic_read(&nr_async_pages) > SWAP_CLUSTER_MAX)
+	if (atomic_read(&nr_async_pages) > pager_daemon.swap_cluster)
 		wait = 1;
 
 	p = &swap_info[type];

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