很久很久以前：一個叫做Mark Hemment的哥兒們寫了Slab。在接下來的一些年里，其他人對Slab進行了完善。一年半以前，SLOB問世了。SLOB的目標是針對嵌入式系統的，主要是適用於那些內存非常有限的系統，比如32MB以下的內存，它不太注重large smp系統，雖然最近在這方面有一些小的改進。幾個月之前，SLUB閃亮登場。它基本上屬於對Slab的重設計(redesign)，但是代碼更少，並且能更好的適應large NUMA系統。SLUB被很認為是Slab和Slob的取代者，大概在2.6.24/2.6.25將會被同志們拋棄。而SLUB將是未來Linux Kernel中的首選。
Linux內核真是變化太快了，內存管理這塊就是一個好例子。
本來Linux內核只有Slab的，現在好了，Slab多了兩個兄弟：Slob和Slub。瞧！這就是內核的命名風格，讓你光看名字就糊塗了！這也是我這兩天讀內核源代碼的深刻體會，什么cache啊，cache_cache啊，free_area啊，繞不暈你才怪呢～！
以前搞不懂這三個到底什么關系，為什么要有這三個。今天搜了一下，明白了一些。簡單的說：Slab是基礎，是最早從Sun OS那引進的；Slub是在Slab上進行的改進，在大型機上表現出色（不知道在普通PC上如何），據說還被IA-64作為默認；而Slob是針對小型系統設計的，當然了，主要是嵌入式。相關文章如下：
Anatomy of the Linux slab allocator
The SLUB allocator
The SLOB allocator
這也正好體現了一個Linux內核開發一貫的思想：提供一種機制，而不是一種策略（Provide mechanism not policy）。其它軟件開發又何嘗不是如此呢？
----------------------------------------------------------華麗的分割線之The SLOB allocator----------------------------------------------------------
slob: introduce the SLOB allocator
configurable replacement for slab allocator
This adds a CONFIG_SLAB option under CONFIG_EMBEDDED. When CONFIG_SLAB
is disabled, the kernel falls back to using the 'SLOB' allocator.
SLOB is a traditional K&R/UNIX allocator with a SLAB emulation layer,
similar to the original Linux kmalloc allocator that SLAB replaced.
It's signicantly smaller code and is more memory efficient. But like
all similar allocators, it scales poorly and suffers from
fragmentation more than SLAB, so it's only appropriate for small
systems.
It's been tested extensively in the Linux-tiny tree. I've also
stress-tested it with make -j 8 compiles on a 3G SMP+PREEMPT box (not
recommended).
Here's a comparison for otherwise identical builds, showing SLOB
saving nearly half a megabyte of RAM:
$ size vmlinux*
text data bss dec hex filename
3336372 529360 190812 4056544 3de5e0 vmlinux-slab
3323208 527948 190684 4041840 3dac70 vmlinux-slob
$ size mm/{slab,slob}.o
text data bss dec hex filename
13221 752 48 14021 36c5 mm/slab.o
1896 52 8 1956 7a4 mm/slob.o
/proc/meminfo:
SLAB SLOB delta
MemTotal: 27964 kB 27980 kB +16 kB
MemFree: 24596 kB 25092 kB +496 kB
Buffers: 36 kB 36 kB 0 kB
Cached: 1188 kB 1188 kB 0 kB
SwapCached: 0 kB 0 kB 0 kB
Active: 608 kB 600 kB -8 kB
Inactive: 808 kB 812 kB +4 kB
HighTotal: 0 kB 0 kB 0 kB
HighFree: 0 kB 0 kB 0 kB
LowTotal: 27964 kB 27980 kB +16 kB
LowFree: 24596 kB 25092 kB +496 kB
SwapTotal: 0 kB 0 kB 0 kB
SwapFree: 0 kB 0 kB 0 kB
Dirty: 4 kB 12 kB +8 kB
Writeback: 0 kB 0 kB 0 kB
Mapped: 560 kB 556 kB -4 kB
Slab: 1756 kB 0 kB -1756 kB
CommitLimit: 13980 kB 13988 kB +8 kB
Committed_AS: 4208 kB 4208 kB 0 kB
PageTables: 28 kB 28 kB 0 kB
VmallocTotal: 1007312 kB 1007312 kB 0 kB
VmallocUsed: 48 kB 48 kB 0 kB
VmallocChunk: 1007264 kB 1007264 kB 0 kB
(this work has been sponsored in part by CELF)
Signed-off-by: Matt Mackall <mpm@selenic.com>
Index: 2.6.14-slob/mm/slob.c
===================================================================
--- /dev/null 1970-01-01 00:00:00.000000000 +0000
+++ 2.6.14-slob/mm/slob.c 2005-11-01 09:42:01.000000000 -0800
@@ -0,0 +1,395 @@
+/*
+ * SLOB Allocator: Simple List Of Blocks
+ *
+ * Matt Mackall <mpm@selenic.com> 12/30/03
+ *
+ * How SLOB works:
+ *
+ * The core of SLOB is a traditional K&R style heap allocator, with
+ * support for returning aligned objects. The granularity of this
+ * allocator is 8 bytes on x86, though it's perhaps possible to reduce
+ * this to 4 if it's deemed worth the effort. The slob heap is a
+ * singly-linked list of pages from __get_free_page, grown on demand
+ * and allocation from the heap is currently first-fit.
+ *
+ * Above this is an implementation of kmalloc/kfree. Blocks returned
+ * from kmalloc are 8-byte aligned and prepended with a 8-byte header.
+ * If kmalloc is asked for objects of PAGE_SIZE or larger, it calls
+ * __get_free_pages directly so that it can return page-aligned blocks
+ * and keeps a linked list of such pages and their orders. These
+ * objects are detected in kfree() by their page alignment.
+ *
+ * SLAB is emulated on top of SLOB by simply calling constructors and
+ * destructors for every SLAB allocation. Objects are returned with
+ * the 8-byte alignment unless the SLAB_MUST_HWCACHE_ALIGN flag is
+ * set, in which case the low-level allocator will fragment blocks to
+ * create the proper alignment. Again, objects of page-size or greater
+ * are allocated by calling __get_free_pages. As SLAB objects know
+ * their size, no separate size bookkeeping is necessary and there is
+ * essentially no allocation space overhead.
+ */
+
+#include <linux/config.h>
+#include <linux/slab.h>
+#include <linux/mm.h>
+#include <linux/cache.h>
+#include <linux/init.h>
+#include <linux/module.h>
+#include <linux/timer.h>
+
+struct slob_block {
+ int units;
+ struct slob_block *next;
+};
+typedef struct slob_block slob_t;
+
+#define SLOB_UNIT sizeof(slob_t)
+#define SLOB_UNITS(size) (((size) + SLOB_UNIT - 1)/SLOB_UNIT)
+#define SLOB_ALIGN L1_CACHE_BYTES
+
+struct bigblock {
+ int order;
+ void *pages;
+ struct bigblock *next;
+};
+typedef struct bigblock bigblock_t;
+
+static slob_t arena = { .next = &arena, .units = 1 };
+static slob_t *slobfree = &arena;
+static bigblock_t *bigblocks;
+static DEFINE_SPINLOCK(slob_lock);
+static DEFINE_SPINLOCK(block_lock);
+
+static void slob_free(void *b, int size);
+
+static void *slob_alloc(size_t size, gfp_t gfp, int align)
+{
+ slob_t *prev, *cur, *aligned = 0;
+ int delta = 0, units = SLOB_UNITS(size);
+ unsigned long flags;
+
+ spin_lock_irqsave(&slob_lock, flags);
+ prev = slobfree;
+ for (cur = prev->next; ; prev = cur, cur = cur->next) {
+ if (align) {
+ aligned = (slob_t *)ALIGN((unsigned long)cur, align);
+ delta = aligned - cur;
+ }
+ if (cur->units >= units + delta) { /* room enough? */
+ if (delta) { /* need to fragment head to align? */
+ aligned->units = cur->units - delta;
+ aligned->next = cur->next;
+ cur->next = aligned;
+ cur->units = delta;
+ prev = cur;
+ cur = aligned;
+ }
+
+ if (cur->units == units) /* exact fit? */
+ prev->next = cur->next; /* unlink */
+ else { /* fragment */
+ prev->next = cur + units;
+ prev->next->units = cur->units - units;
+ prev->next->next = cur->next;
+ cur->units = units;
+ }
+
+ slobfree = prev;
+ spin_unlock_irqrestore(&slob_lock, flags);
+ return cur;
+ }
+ if (cur == slobfree) {
+ spin_unlock_irqrestore(&slob_lock, flags);
+
+ if (size == PAGE_SIZE) /* trying to shrink arena? */
+ return 0;
+
+ cur = (slob_t *)__get_free_page(gfp);
+ if (!cur)
+ return 0;
+
+ slob_free(cur, PAGE_SIZE);
+ spin_lock_irqsave(&slob_lock, flags);
+ cur = slobfree;
+ }
+ }
+}
+
+static void slob_free(void *block, int size)
+{
+ slob_t *cur, *b = (slob_t *)block;
+ unsigned long flags;
+
+ if (!block)
+ return;
+
+ if (size)
+ b->units = SLOB_UNITS(size);
+
+ /* Find reinsertion point */
+ spin_lock_irqsave(&slob_lock, flags);
+ for (cur = slobfree; !(b > cur && b < cur->next); cur = cur->next)
+ if (cur >= cur->next && (b > cur || b < cur->next))
+ break;
+
+ if (b + b->units == cur->next) {
+ b->units += cur->next->units;
+ b->next = cur->next->next;
+ } else
+ b->next = cur->next;
+
+ if (cur + cur->units == b) {
+ cur->units += b->units;
+ cur->next = b->next;
+ } else
+ cur->next = b;
+
+ slobfree = cur;
+
+ spin_unlock_irqrestore(&slob_lock, flags);
+}
+
+static int FASTCALL(find_order(int size));
+static int fastcall find_order(int size)
+{
+ int order = 0;
+ for ( ; size > 4096 ; size >>=1)
+ order++;
+ return order;
+}
+
+void *kmalloc(size_t size, gfp_t gfp)
+{
+ slob_t *m;
+ bigblock_t *bb;
+ unsigned long flags;
+
+ if (size < PAGE_SIZE - SLOB_UNIT) {
+ m = slob_alloc(size + SLOB_UNIT, gfp, 0);
+ return m ? (void *)(m + 1) : 0;
+ }
+
+ bb = slob_alloc(sizeof(bigblock_t), gfp, 0);
+ if (!bb)
+ return 0;
+
+ bb->order = find_order(size);
+ bb->pages = (void *)__get_free_pages(gfp, bb->order);
+
+ if (bb->pages) {
+ spin_lock_irqsave(&block_lock, flags);
+ bb->next = bigblocks;
+ bigblocks = bb;
+ spin_unlock_irqrestore(&block_lock, flags);
+ return bb->pages;
+ }
+
+ slob_free(bb, sizeof(bigblock_t));
+ return 0;
+}
+
+EXPORT_SYMBOL(kmalloc);
+
+void *kzalloc(size_t size, gfp_t flags)
+{
+ void *ret = kmalloc(size, flags);
+ if (ret)
+ memset(ret, 0, size);
+ return ret;
+}
+
+EXPORT_SYMBOL(kzalloc);
+
+void kfree(const void *block)
+{
+ bigblock_t *bb, **last = &bigblocks;
+ unsigned long flags;
+
+ if (!block)
+ return;
+
+ if (!((unsigned int)block & (PAGE_SIZE-1))) {
+ /* might be on the big block list */
+ spin_lock_irqsave(&block_lock, flags);
+ for (bb = bigblocks; bb; last = &bb->next, bb = bb->next) {
+ if (bb->pages == block) {
+ *last = bb->next;
+ spin_unlock_irqrestore(&block_lock, flags);
+ free_pages((unsigned long)block, bb->order);
+ slob_free(bb, sizeof(bigblock_t));
+ return;
+ }
+ }
+ spin_unlock_irqrestore(&block_lock, flags);
+ }
+
+ slob_free((slob_t *)block - 1, 0);
+ return;
+}
+
+EXPORT_SYMBOL(kfree);
+
+unsigned int ksize(const void *block)
+{
+ bigblock_t *bb;
+ unsigned long flags;
+
+ if (!block)
+ return 0;
+
+ if (!((unsigned int)block & (PAGE_SIZE-1))) {
+ spin_lock_irqsave(&block_lock, flags);
+ for (bb = bigblocks; bb; bb = bb->next)
+ if (bb->pages == block) {
+ spin_unlock_irqrestore(&slob_lock, flags);
+ return PAGE_SIZE << bb->order;
+ }
+ spin_unlock_irqrestore(&block_lock, flags);
+ }
+
+ return ((slob_t *)block - 1)->units * SLOB_UNIT;
+}
+
+struct kmem_cache_s {
+ unsigned int size, align;
+ const char *name;
+ void (*ctor)(void *, kmem_cache_t *, unsigned long);
+ void (*dtor)(void *, kmem_cache_t *, unsigned long);
+};
+
+kmem_cache_t *kmem_cache_create(const char *name, size_t size, size_t align,
+ unsigned long flags,
+ void (*ctor)(void*, kmem_cache_t *, unsigned long),
+ void (*dtor)(void*, kmem_cache_t *, unsigned long))
+{
+ kmem_cache_t *c;
+
+ c = slob_alloc(sizeof(kmem_cache_t), flags, 0);
+
+ if (c) {
+ c->name = name;
+ c->size = size;
+ c->ctor = ctor;
+ c->dtor = dtor;
+ /* ignore alignment unless it's forced */
+ c->align = (flags & SLAB_MUST_HWCACHE_ALIGN) ? SLOB_ALIGN : 0;
+ if (c->align < align)
+ c->align = align;
+ }
+
+ return c;
+}
+EXPORT_SYMBOL(kmem_cache_create);
+
+int kmem_cache_destroy(kmem_cache_t *c)
+{
+ slob_free(c, sizeof(kmem_cache_t));
+ return 0;
+}
+EXPORT_SYMBOL(kmem_cache_destroy);
+
+void *kmem_cache_alloc(kmem_cache_t *c, gfp_t flags)
+{
+ void *b;
+
+ if (c->size < PAGE_SIZE)
+ b = slob_alloc(c->size, flags, c->align);
+ else
+ b = (void *)__get_free_pages(flags, find_order(c->size));
+
+ if (c->ctor)
+ c->ctor(b, c, SLAB_CTOR_CONSTRUCTOR);
+
+ return b;
+}
+EXPORT_SYMBOL(kmem_cache_alloc);
+
+void kmem_cache_free(kmem_cache_t *c, void *b)
+{
+ if (c->dtor)
+ c->dtor(b, c, 0);
+
+ if (c->size < PAGE_SIZE)
+ slob_free(b, c->size);
+ else
+ free_pages((unsigned long)b, find_order(c->size));
+}
+EXPORT_SYMBOL(kmem_cache_free);
+
+unsigned int kmem_cache_size(kmem_cache_t *c)
+{
+ return c->size;
+}
+EXPORT_SYMBOL(kmem_cache_size);
+
+const char *kmem_cache_name(kmem_cache_t *c)
+{
+ return c->name;
+}
+EXPORT_SYMBOL(kmem_cache_name);
+
+static struct timer_list slob_timer = TIMER_INITIALIZER(
+ (void (*)(unsigned long))kmem_cache_init, 0, 0);
+
+void kmem_cache_init(void)
+{
+ void *p = slob_alloc(PAGE_SIZE, 0, PAGE_SIZE-1);
+
+ if (p)
+ free_page((unsigned int)p);
+
+ mod_timer(&slob_timer, jiffies + HZ);
+}
+
+atomic_t slab_reclaim_pages = ATOMIC_INIT(0);
+EXPORT_SYMBOL(slab_reclaim_pages);
+
+#ifdef CONFIG_SMP
+
+void *__alloc_percpu(size_t size, size_t align)
+{
+ int i;
+ struct percpu_data *pdata = kmalloc(sizeof (*pdata), GFP_KERNEL);
+
+ if (!pdata)
+ return NULL;
+
+ for (i = 0; i < NR_CPUS; i++) {
+ if (!cpu_possible(i))
+ continue;
+ pdata->ptrs[i] = kmalloc(size, GFP_KERNEL);
+ if (!pdata->ptrs[i])
+ goto unwind_oom;
+ memset(pdata->ptrs[i], 0, size);
+ }
+
+ /* Catch derefs w/o wrappers */
+ return (void *) (~(unsigned long) pdata);
+
+unwind_oom:
+ while (--i >= 0) {
+ if (!cpu_possible(i))
+ continue;
+ kfree(pdata->ptrs[i]);
+ }
+ kfree(pdata);
+ return NULL;
+}
+EXPORT_SYMBOL(__alloc_percpu);
+
+void
+free_percpu(const void *objp)
+{
+ int i;
+ struct percpu_data *p = (struct percpu_data *) (~(unsigned long) objp);
+
+ for (i = 0; i < NR_CPUS; i++) {
+ if (!cpu_possible(i))
+ continue;
+ kfree(p->ptrs[i]);
+ }
+ kfree(p);
+}
+EXPORT_SYMBOL(free_percpu);
+
+#endif
Makefile補丁略。。。
----------------------------------------------------------華麗的分割線之The SLUB allocator----------------------------------------------------------
The SLUB allocator
The slab allocator has been at the core of the kernel's memory management for many years. This allocator (sitting on top of the low-level page allocator) manages caches of objects of a specific size, allowing for fast and space-efficient allocations. Kernel hackers tend not to wander into the slab code because it's complex and because, for the most part, it works quite well.
Christoph Lameter is one of those people for whom the slab allocator does not work quite so well. Over time, he has come up with a list of complaints that is getting impressively long. The slab allocator maintains a number of queues of objects; these queues can make allocation fast but they also add quite a bit of complexity. Beyond that, the storage overhead tends to grow with the size of the system:
Beyond that, each slab (a group of one or more continuous pages from which objects are allocated) contains a chunk of metadata at the beginning which makes alignment of objects harder. The code for cleaning up caches when memory gets tight adds another level of complexity. And so on.
Christoph's response is the SLUB allocator, a drop-in replacement for the slab code. SLUB promises better performance and scalability by dropping most of the queues and related overhead and simplifying the slab structure in general, while retaining the current slab allocator interface.
In the SLUB allocator, a slab is simply a group of one or more pages neatly packed with objects of a given size. There is no metadata within the slab itself, with the exception that free objects are formed into a simple linked list. When an allocation request is made, the first free object is located, removed from the list, and returned to the caller.
Given the lack of per-slab metadata, one might well wonder just how that first free object is found. The answer is that the SLUB allocator stuffs the relevant information into the system memory map - the page structures associated with the pages which make up the slab. Making struct page larger is frowned upon in a big way, so the SLUB allocator makes this complicated structure even more so with the addition of another union. The end result is that struct page gets three new fields which only have meaning when the associated page is part of a slab:
void *freelist; short unsigned int inuse; short unsigned int offset;
For slab use, freelist points to the first free object within a slab, inuse is the number of objects which have been allocated from the slab, and offset tells the allocator where to find the pointer to the next free object. The SLUB allocator can use RCU to free objects, but, to do so, it must be able to put the "next object" pointer outside of the object itself; the offset pointer is the allocator's way of tracking where that pointer was put.
When a slab is first created by the allocator, it has no objects allocated from it. Once an object has been allocated, it becomes a "partial" slab which is stored on a list in the kmem_cache structure. Since this is a patch aimed at scalability, there is, in fact, one "partial" list for each NUMA node on the system. The allocator tries to keep allocations node-local, but it will reach across nodes before filling the system with partial slabs.
There is also a per-CPU array of active slabs, intended to prevent cache line bouncing even within a NUMA node. There is a special thread which runs (via a workqueue) which monitors the usage of per-CPU slabs; if a per-CPU slab is not being used, it gets put back onto the partial list for use by other processors.
If all objects within a slab are allocated, the allocator simply forgets about the slab altogether. Once an object in a full slab is freed, the allocator can relocate the containing slab via the system memory map and put it back onto the appropriate partial list. If all of the objects within a given slab (as tracked by the inuse counter) are freed, the entire slab is given back to the page allocator for reuse.
One interesting feature of the SLUB allocator is that it can combine slabs with similar object sizes and parameters. The result is fewer slab caches in the system (a 50% reduction is claimed), better locality of slab allocations, and less fragmentation of slab memory. The patch does note:
Causing bugs to stand out is generally considered to be a good thing, but wider use of the SLUB allocator could lead to some quirky behavior until those new bugs are stamped out.
Wider use may be in the cards: the SLUB allocator is in the -mm tree now and could hit the mainline as soon as 2.6.22. The simplified code is attractive, as is the claimed 5-10% performance increase. If merged, SLUB is likely to coexist with the current slab allocator (and the SLOB allocator intended for small systems) for some time. In the longer term, the current slab code may be approaching the end of its life.