* [PATCH 2 of 3] slob: reduce external fragmentation by using three free lists
2008-01-17 17:57 [PATCH 0 of 3] SLOB patches for 2.6.25 Matt Mackall
2008-01-17 17:57 ` [PATCH 1 of 3] slob: fix free block merging at head of subpage Matt Mackall
@ 2008-01-17 17:57 ` Matt Mackall
2008-01-17 17:57 ` [PATCH 3 of 3] slob: correct Kconfig description Matt Mackall
2 siblings, 0 replies; 4+ messages in thread
From: Matt Mackall @ 2008-01-17 17:57 UTC (permalink / raw)
To: Andrew Morton; +Cc: linux-kernel
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@selenic.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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