The Design and Implementation of the Anykernel and Rump Kernels

100
A rump kernel can be configured with either a specified amount of memory or an
unlimited amount of memory. In the former case the rump kernel will disallow
runtime memory allocations going over the limit. When 90% of the allocation limit
has been reached, the pagedaemon will be invoked to push out memory content
which has not been used recently. If no limit is specified, a rump kernel can grow
up to potentially consuming all memory and swap space on the host. The latter is
the default, since in our experience it suits all but a few use cases. For example,
in virtually all unit test use cases the memory limit has no consequence, since a
short-lived rump kernel will not have a chance to grow large enough for it to matter.
Furthermore, a POSIX per-process memory size limit (rlimit) will usually be hit
before one process can have a serious impact on the host. Hitting this limit causes
the memory allocation hypercall to fail. If the memory allocation hypercall fails,
the rump kernel treats the situation the same as reaching the soft-configured limit
and invokes the pagedaemon, which will hopefully flush out the cache and allow the
current allocation to succeed.
To free memory, the pagedaemon must locate memory resources which are not in
use and release them. There are fundamentally two types of memory: pageable and
wired.
• Pageable memory means that a memory page can be paged out. Paging
is done using the pager construct that the NetBSD VM (UVM) inherited
from the Mach VM [99] via the 4.4BSD VM. A pager has the capability to
move the contents of the page in and out of secondary storage. NetBSD
currently supports three classes of pagers: anonymous, vnode and device.
Device pagers map device memory, so they can be left out of a discussion
concerning RAM. We extract the standard UVM anonymous memory object
implementation (sys/uvm/uvm_aobj.c) mainly because the tmpfs file system
requires anonymous memory objects. However, we compile uvm_aobj.c
without defining VMSWAP, i.e. the code for support moving memory to and