The Design and Implementation of the Anykernel and Rump Kernels

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3. Accept that memory remapping support is necessary in a rump kernel and
handle the vnode pager using mmap() and munmap().
When comparing the first and the second option, the principle used is that memory
I/O is several orders of magnitude faster than device I/O. Therefore, anything which
affects device I/O should be avoided, especially if it might cause extra I/O operations
and thus option 1 is preferable over option 2.
To evaluate the first option against third option, we simulated pager conditions
and measured the amount of time it takes to construct a contiguous 64kB memory
window out of non-contiguous 4kB pages and to write the window out to a file
backed by a memory file system. The result for 1000 loops as a function of noncontiguous
pages is presented in Figure 3.9. We conclude that not only is copying
the technologically preferred option since it avoids the need for memory mapping
support on the host, it is also several times faster than mmap()/munmap() pairs.
It should be noted that a fourth option is to implement a separate vnode pager which
does not rely on mapping pages. This option was our initial approach. While the effort
produced a superficially working result, we could not get all corner cases to function
exactly the same as with the regular kernel — for example, the VOP_GETPAGES()
interface implemented by the vnode pager takes 8 different parameters and 14 different
flags. The lesson learnt from this attempt with the vnode pager reflects our
premise for the entire work: it is easy to write superficially working code, but getting
all corner cases right for complicated drivers is extremely difficult.
3.4.2 Memory Allocators
Although memory allocators are not strictly speaking part of the virtual memory
subsystem, they are related to memory so we describe them here.