6. Page Table Optimizations
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## 6. Page Table Optimizations

The page table optimizations make up the most contentious part of the FreeBSD VM design and they have shown some strain with the advent of serious use of `mmap()`. I think this is actually a feature of most BSDs though I am not sure when it was first introduced. There are two major optimizations. The first is that hardware page tables do not contain persistent state but instead can be thrown away at any time with only a minor amount of management overhead. The second is that every active page table entry in the system has a governing `pv_entry` structure which is tied into the `vm_page` structure. FreeBSD can simply iterate through those mappings that are known to exist while Linux must check all page tables that might contain a specific mapping to see if it does, which can achieve O(n^2) overhead in certain situations. It is because of this that FreeBSD tends to make better choices on which pages to reuse or swap when memory is stressed, giving it better performance under load. However, FreeBSD requires kernel tuning to accommodate large-shared-address-space situations such as those that can occur in a news system because it may run out of `pv_entry` structures.

Both Linux and FreeBSD need work in this area. FreeBSD is trying to maximize the advantage of a potentially sparse active-mapping model (not all processes need to map all pages of a shared library, for example), whereas Linux is trying to simplify its algorithms. FreeBSD generally has the performance advantage here at the cost of wasting a little extra memory, but FreeBSD breaks down in the case where a large file is massively shared across hundreds of processes. Linux, on the other hand, breaks down in the case where many processes are sparsely-mapping the same shared library and also runs non-optimally when trying to determine whether a page can be reused or not.

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