The Design and Implementation of the Anykernel and Rump Kernels

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rump kernel memory limit
relative performance
0.5MB 50%
1MB 90%
3MB 100%
unlimited (host container limit) 100%
Table 3.2: File system I/O performance vs. available memory. If memory
is extremely tight, the performance of the I/O system suffers. A few megabytes of
rump kernel memory was enough to allow file I/O processing at full media speed.
from secondary is not included. Our view is that paging anonymous memory
should be handled by the host. What is left is the vnode pager, i.e. moving
file contents between the memory cache and the file system.
• Wired memory is non-pageable, i.e. it is always present and mapped. Still,
it is critical to note that the host can page memory which is wired in the rump
kernel barring precautions such as a mlock() hypercall. Most wired memory
in NetBSD is in one form or another allocated from a pool as was described
in Section 3.4.2. During memory shortage, the pagedaemon requests the
allocators to return unused pages back to the system.
The pagedaemon is implemented in the uvm_pageout() routine in the source file
sys/rump/librump/rumpkern/vm.c. As mentioned earlier, it is invoked whenever
90% of the memory limit has been allocated from the host, or when a memory
allocation hypercall fails. The pagedaemon releases memory in stages, from the
ones most likely to bring benefit to the least likely. The use case the pagedaemon
was developed against was the ability to run file systems with a rump kernel with
limited memory. Measurements showing how memory capacity affects file system
performance are presented in Table 3.2.