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slob: reduce external fragmentation by using three free lists

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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>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Matt Mackall 2008-02-04 22:29:37 -08:00 committed by Linus Torvalds
parent 679299b32d
commit 20cecbae44
1 changed files with 33 additions and 14 deletions
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47
mm/slob.c
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@ -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 @@ static inline void free_slob_page(struct slob_page *sp)
}
/*
* 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 @@ static inline int slob_page_free(struct slob_page *sp)
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 @@ static void *slob_alloc(size_t size, gfp_t gfp, int align, int node)
{
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 @@ static void *slob_alloc(size_t size, gfp_t gfp, int align, int node)
/* 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 @@ static void *slob_alloc(size_t size, gfp_t gfp, int align, int node)
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 @@ static void slob_free(void *block, int size)
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;
}