Slab allocation

Slab allocation is a memory management mechanism intended for the efficient memory allocation of kernel objects. It eliminates 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. It is analogous to an object pool, but only applies to memory, not other resources.

Slab allocation was first introduced in the Solaris 5.4 kernel by Jeff Bonwick. It is now widely used by many Unix and Unix-like operating systems including FreeBSD and Linux.

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, overhead costs of initialization can result in significant performance drops. 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 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:

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 :

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".

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