linux-kernel - [PATCH 2 of 3] slob: reduce external fragmentation by using three free lists

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Date:	Thu, 17 Jan 2008 11:57:20 -0600
From:	Matt Mackall <mpm@...enic.com>
To:	Andrew Morton <apkm@...ux-foundation.com>
Cc:	linux-kernel@...r.kernel.org
Subject: [PATCH 2 of 3] slob: reduce external fragmentation by using three
	free lists

By putting smaller objects on their own list, we greatly reduce
overall external fragmentation and increase repeatability. This
reduces total SLOB overhead from > 50% to ~6% on a simple boot test.

Signed-off-by: Matt Mackall <mpm@...enic.com>

diff -r 771a5ab2c6b7 -r eff994add207 mm/slob.c
--- a/mm/slob.c	Wed Jan 16 18:14:29 2008 -0600
+++ b/mm/slob.c	Wed Jan 16 18:14:29 2008 -0600
@@ -12,10 +12,17 @@
  * allocator is as little as 2 bytes, however typically most architectures
  * will require 4 bytes on 32-bit and 8 bytes on 64-bit.
  *
- * The slob heap is a linked list of pages from alloc_pages(), and
- * within each page, there is a singly-linked list of free blocks (slob_t).
- * The heap is grown on demand and allocation from the heap is currently
- * first-fit.
+ * The slob heap is a set of linked list of pages from alloc_pages(),
+ * and within each page, there is a singly-linked list of free blocks
+ * (slob_t). The heap is grown on demand. To reduce fragmentation,
+ * heap pages are segregated into three lists, with objects less than
+ * 256 bytes, objects less than 1024 bytes, and all other objects.
+ *
+ * Allocation from heap involves first searching for a page with
+ * sufficient free blocks (using a next-fit-like approach) followed by
+ * a first-fit scan of the page. Deallocation inserts objects back
+ * into the free list in address order, so this is effectively an
+ * address-ordered first fit.
  *
  * Above this is an implementation of kmalloc/kfree. Blocks returned
  * from kmalloc are prepended with a 4-byte header with the kmalloc size.
@@ -110,9 +117,13 @@
 }
 
 /*
- * All (partially) free slob pages go on this list.
+ * All partially free slob pages go on these lists.
  */
-static LIST_HEAD(free_slob_pages);
+#define SLOB_BREAK1 256
+#define SLOB_BREAK2 1024
+static LIST_HEAD(free_slob_small);
+static LIST_HEAD(free_slob_medium);
+static LIST_HEAD(free_slob_large);
 
 /*
  * slob_page: True for all slob pages (false for bigblock pages)
@@ -140,9 +151,9 @@
 	return test_bit(PG_private, &sp->flags);
 }
 
-static inline void set_slob_page_free(struct slob_page *sp)
+static void set_slob_page_free(struct slob_page *sp, struct list_head *list)
 {
-	list_add(&sp->list, &free_slob_pages);
+	list_add(&sp->list, list);
 	__set_bit(PG_private, &sp->flags);
 }
 
@@ -294,12 +305,20 @@
 {
 	struct slob_page *sp;
 	struct list_head *prev;
+	struct list_head *slob_list;
 	slob_t *b = NULL;
 	unsigned long flags;
 
+	if (size < SLOB_BREAK1)
+		slob_list = &free_slob_small;
+	else if (size < SLOB_BREAK2)
+		slob_list = &free_slob_medium;
+	else
+		slob_list = &free_slob_large;
+
 	spin_lock_irqsave(&slob_lock, flags);
 	/* Iterate through each partially free page, try to find room */
-	list_for_each_entry(sp, &free_slob_pages, list) {
+	list_for_each_entry(sp, slob_list, list) {
 #ifdef CONFIG_NUMA
 		/*
 		 * If there's a node specification, search for a partial
@@ -321,9 +340,9 @@
 		/* Improve fragment distribution and reduce our average
 		 * search time by starting our next search here. (see
 		 * Knuth vol 1, sec 2.5, pg 449) */
-		if (prev != free_slob_pages.prev &&
-				free_slob_pages.next != prev->next)
-			list_move_tail(&free_slob_pages, prev->next);
+		if (prev != slob_list->prev &&
+				slob_list->next != prev->next)
+			list_move_tail(slob_list, prev->next);
 		break;
 	}
 	spin_unlock_irqrestore(&slob_lock, flags);
@@ -341,7 +360,7 @@
 		sp->free = b;
 		INIT_LIST_HEAD(&sp->list);
 		set_slob(b, SLOB_UNITS(PAGE_SIZE), b + SLOB_UNITS(PAGE_SIZE));
-		set_slob_page_free(sp);
+		set_slob_page_free(sp, slob_list);
 		b = slob_page_alloc(sp, size, align);
 		BUG_ON(!b);
 		spin_unlock_irqrestore(&slob_lock, flags);
@@ -387,7 +406,7 @@
 		set_slob(b, units,
 			(void *)((unsigned long)(b +
 					SLOB_UNITS(PAGE_SIZE)) & PAGE_MASK));
-		set_slob_page_free(sp);
+		set_slob_page_free(sp, &free_slob_small);
 		goto out;
 	}
 
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