The Design and Implementation of the Anykernel and Rump Kernels

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Pager window create+access time (s)
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mmap + munmap
alloc + copy + free
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Disjoint regions
Figure 3.9: Performance of page remapping vs. copying. Allocating a
pager window from anonymous memory and copying file pages to it for the purpose
of pageout by the vnode pager is faster than remapping memory backed by a file.
Additionally, the cost of copying is practically independent of the amount of noncontiguous
pages. With remapping, each disjoint region requires a separate call to
mmap().
The lowest level memory allocator in NetBSD is the UVM kernel memory allocator
(uvm_km). It is used to allocate memory on a pagelevel granularity. The standard implementation
in sys/uvm/uvm_km.c allocates a virtual memory address range and,
if requested, allocates physical memory for the range and maps it in. Since mapping
is incompatible with a rump kernel, we did a straightforward implementation which
allocates a page or contiguous page range with a hypercall.
The kmem, pool and pool cache allocators are general purpose allocators meant to
be used by drivers. Fundamentally, they operate by requesting pages from uvm_km
and handing memory out in requested size chunks. The flow of memory between
UVM and the allocators is dynamic, meaning if an allocator runs out of memory, it
will request more from UVM, and if there is a global memory shortage, the system
will attempt to reclaim cached memory from the allocators. We have extracted the