The Advantages of using a Slab Allocator

When people take their first C programming courses, they are taught about the standard allocator named ‘malloc()‘, while when learning C++, we were first taught about its standard allocator, named ‘new‘.

These allocators work on the assumption that a program is running in user space, and may not always be efficient at allocating smaller chunks of memory. They assume that a standard method of managing the heap is in-place, where the heap of any one process is a part of that process’s memory-image, and partially managed by the kernel.

Not only that, but when we tell either of these standard operators to allocate a chunk of memory, the allocator recognizes the size of that chunk, prepends to the chunk of memory a binary representation of its size, and before returning a pointer to the allocated memory, subtracts the size of the binary representation, of the size originally requested by the programmer. Thus, the pointer returned by either of these allocators points directly to the memory which the programmer can use, even though the allocated chunk is larger, and preceded by a binary representation of its own size. That way, when the command is given to deallocate, all the deallocation-function needs to receive in principle, is a pointer to the allocated chunk, and the deallocation-function can then find the header that was inserted from there, to derive how much memory to delete.

I suppose that one conclusion to draw from this is, that even though it looks like a good exercise to teach programming students, the exercise of always allocating a 32-bit or a 64-bit object – i.e., a 4-byte or an 8-byte object – such as an integer, to obtain an 8-byte pointer to that integer, is actually not a good one, because in addition to the requested 8 bytes, an additional header is always being allocated, which may add 4 bytes if the maximum allocated size is a 32-bit number, or add 8 bytes if the maximum allocated size (of one chunk) is a 64-bit number.

Additionally, these allocators assume the support of the kernel, to a user-space process, the latter of which has a heap. On 64-bit systems that are ‘vmalloc‘-based, this requires the user-space application try to access virtual address ‘0x0000 0000 0000 0000‘, which intentionally results in a page-fault, and stops the process. The kernel then needs to examine why the page-fault occurred, and since this was a legitimate reason, needs to set up the virtual page-frame, of an address returned to the (restarted) user-space process, via the usual methods for returning values.

And so means also needed to exist, by which a kernel can manage memory more-efficiently, even under the assumption that the kernel does not have the sort of heap, that a user-space process does. And one main mechanism for doing so, is to use a slab allocator. It will allocate large numbers of small chunks, without requiring as much overhead to do so, as the standard user-space allocators did. In kernel-space, these slabs are the main replacement for a heap.

(Updated 06/20/2017 … )

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