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Slab allocation is a memory management mechanism intended for the efficient memory allocation of kernel objects which displays the desirable property of eliminating fragmentation caused by allocations and deallocations. The technique is used to retain allocated memory that contains a data object of a certain type for reuse upon subsequent allocations of objects of the same type. Slab allocation was first introduced in the Solaris 2.4 kernel by Jeff Bonwick and now is widely used by many Unix and Unix-like operating systems including FreeBSD  and Linux  (where it was the default allocator until 2.6.23 when SLUB allocation became the default).
The primary motivation for slab allocation is that the initialization and destruction of kernel data objects can actually outweigh the cost of allocating memory for them.  As object creation and deletion are widely employed by the kernel, mitigating overhead costs of initialization can result in significant performance gains. The notion of object caching was therefore introduced in order to avoid the invocation of functions used to initialize object state.
With slab allocation, memory chunks suitable to fit data objects of certain type or size are preallocated.  The slab allocator keeps track of these chunks, known as caches, so that when a request to allocate memory for a data object of a certain type is received it can instantly satisfy the request with an already allocated slot. Destruction of the object, however, does not free up the memory, but only opens a slot which is put in the list of free slots by the slab allocator. The next call to allocate memory of the same size will return the now unused memory slot. This process eliminates the need to search for suitable memory space and greatly alleviates memory fragmentation. In this context a slab is one or more contiguous pages in the memory containing pre-allocated memory chunks.
Understanding the slab allocation algorithm requires defining and explaining some terms:
- Cache: cache represents a small amount of very fast memory. A cache is a storage for a specific type of object such as semaphores, process descriptors, file objects etc.
- Slab: slab represents a contiguous piece of memory, usually made of several physically contiguous pages. A cache is stored in one or more slabs. The slab is the actual container of data associated to objects of the specific kind of the containing cache.
When a program sets up a cache, it allocates a number of objects to the slabs associated with that cache. This number depends on the size of the associated slabs.
Slabs may exist in one of the following states :
- empty - all objects on a slab marked as free
- partial - slab consists of both used and free objects
- full - all objects on a slab marked as used
Initially, the system marks each slab as "empty". When the process calls for a new kernel object, the system tries to find a free location for that object on a partial slab in a cache for that type of object. If no such location exists, the system allocates a new slab from contiguous physical pages and assigns it to a cache. The new object gets allocated from this slab, and its location becomes marked as "partial".
The allocation takes place quickly, because the system builds the objects in advance and readily allocates them from a slab.
A slab is the amount that a cache can grow or shrink by. It represents one memory allocation to the cache from the machine, and whose size is customarily a multiple of the page size. A slab must contain a list of free buffers (or bufctls), as well as a list of the bufctls that have been allocated (in the case of a large slab size).
These are for caches that store objects that are not less than 1/8 of the page size for a given machine. The reason for the large slabs having a different layout from the small slabs is that it allows large slabs to pack better into page-size units, which helps with fragmentation. The slab contains a list of bufctls, which are simply controllers for each buffer that can be allocated (a buffer is the memory that the user of a slab allocator would use).
The small slabs contain objects that are less than 1/8 of the page size for a given machine. These small slabs need to be optimized further from the logical layout, by avoiding using bufctls (which would be just as large as the data itself and cause memory usage to be much greater). A small slab is exactly one page, and has a defined structure that allows bufctls to be avoided. The last part of the page contains the 'slab header', which is the information needed to retain the slab. Starting at the first address of that page, there are as many buffers as can be allocated without running into the slab header at the end of the page.
Instead of using bufctls, we use the buffers themselves to retain the free list links. This allows the small slab's bufctl to be bypassed. 
Systems using slab allocation
- AmigaOS (introduced in AmigaOS 4)
- DragonFly BSD (introduced in release 1.0)
- FreeBSD (introduced in 5.0)
- Haiku (introduced in alpha 2)
- HP-UX (introduced in 11i)
- Linux (introduced in kernel 2.2, many popular distributions now choose the SLUB allocation method over SLAB, but it is still available as an option) -- In Linux, slab allocation provides a kind of front-end to the zoned buddy allocator for those sections of the kernel that require more flexible memory allocation than the standard 4KB page size
- NetBSD (introduced in 4.0)
- Solaris (introduced in 2.4)
- The Perl 5 compiler uses a slab allocator for internal memory management
- Memcached uses slab allocation for memory management
- FreeBSD Kernel Developer's Manual
- M. Tim Jones, Anatomy of the Linux slab allocator
- |Kernel commit that made SLUB the default allocator in 2.6.23
- Jeff Bonwick,The Slab Allocator: An Object-Caching Kernel Memory Allocator (1994)
- Abraham Silberschatz et al.: Operating system concepts. Wiley: 2004. ISBN 0-471-69466-5
- Chris Cooper and Chris Moore, HP-UX 11i Internals, Upper Saddle River, New Jersey: Prentice Hall PTR, 2004, ISBN 0-13-032861-8, p. 334.
- "Perl5-Porters Weekly: 2012 June 17". Retrieved 18 November 2012.
- Bonwick, Jeff. "Magazines and Vmem: Extending the Slab Allocator to Many CPUs and Arbitrary Resources". USENIX 2001. Retrieved 18 November 2012.
- FreeBSD uma(9) manual page
- Anatomy of the Linux slab allocator a developerWorks article by M. Tim Jones
- The SLUB allocator comment about management of slabs in Linux by two different allocators: SLUB allocator and SLAB allocator
- Memory Compaction v7 (a Linux patch set from Mel Gorman dealing with SLAB fragmentation and compaction issues, 2 April 2010)
- Detecting kernel memory leaks Jonathan Corbet, Linux Weekly News, 2006; includes user comments on garbage collection
- Linux performance: is Linux becoming just too slow and bloated? On SLAB and SLUB. Free software magazine 2010.