Ben Cantrick (mackys) wrote,
Ben Cantrick

The Linux slab and SLUB memory allocation sub-systems.

Memory management is ultimately a zero-sum game of tradeoffs. You can develop an algorithm that uses little memory for management but takes more time to manage the available memory. You can also develop an algorithm that efficiently manages memory but uses a bit more memory. In the end, the requirements for the particular application drive the balance of the tradeoffs.

Early memory managers used a heap-based allocation strategy. In this method, a large block of memory (called the heap) is used to provide memory for user-defined purposes. When users need a block of memory, they make a request for a given size. The heap manager looks at the available memory (using a particular algorithm) and returns the block. Some of the algorithms used in this search are the first-fit (the first block encountered in the heap that satisfies the request), and the best-fit (the best block in the heap that satisfies the request). When the users are finished with the memory, they return it to the heap.

The fundamental problem with this heap-based allocation strategy is fragmentation. As blocks of memory are allocated, they are returned in different orders and at different times. This tends to leave holes in the heap requiring more time to efficiently manage the free memory. This algorithm tends to be memory efficient (allocating what's necessary) but requires more time to manage the heap.

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:


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.
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