Design elements of the FreeBSD VM system
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# 3 SWAP Layers

Private data pages are initially either copy-on-write or zero-fill pages. When a change, and therefore a copy, is made, the original backing object (usually a file) can no longer be used to save a copy of the page when the VM system needs to reuse it for other purposes. This is where SWAP comes in. SWAP is allocated to create backing store for memory that does not otherwise have it. FreeBSD allocates the swap management structure for a VM Object only when it is actually needed. However, the swap management structure has had problems historically:

- Under FreeBSD 3.X the swap management structure preallocates an array that encompasses the entire object requiring swap backing store--even if only a few pages of that object are swap-backed. This creates a kernel memory fragmentation problem when large objects are mapped, or processes with large runsizes (RSS) fork.

- Also, in order to keep track of swap space, a list of holes is kept in kernel memory, and this tends to get severely fragmented as well. Since the list of holes is a linear list, the swap allocation and freeing performance is a non-optimal O(n)-per-page.

- It requires kernel memory allocations to take place during the swap freeing process, and that creates low memory deadlock problems.

- The problem is further exacerbated by holes created due to the interleaving algorithm.

- Also, the swap block map can become fragmented fairly easily resulting in non-contiguous allocations.

- Kernel memory must also be allocated on the fly for additional swap management structures when a swapout occurs.

It is evident from that list that there was plenty of room for improvement. For FreeBSD 4.X, I completely rewrote the swap subsystem:

- Swap management structures are allocated through a hash table rather than a linear array giving them a fixed allocation size and much finer granularity.

- Rather then using a linearly linked list to keep track of swap space reservations, it now uses a bitmap of swap blocks arranged in a radix tree structure with free-space hinting in the radix node structures. This effectively makes swap allocation and freeing an O(1) operation.

- The entire radix tree bitmap is also preallocated in order to avoid having to allocate kernel memory during critical low memory swapping operations. After all, the system tends to swap when it is low on memory so we should avoid allocating kernel memory at such times in order to avoid potential deadlocks.

- To reduce fragmentation the radix tree is capable of allocating large contiguous chunks at once, skipping over smaller fragmented chunks.

I did not take the final step of having an allocating hint pointer that would trundle through a portion of swap as allocations were made in order to further guarantee contiguous allocations or at least locality of reference, but I ensured that such an addition could be made.

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