The Design and Implementation of the Anykernel and Rump Kernels

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• Copying the large file was measured to consist of 98.5% asynchronous data
writes. Memory mapped I/O is almost twice as slow as read/write, since as
explained in Section 3.9.2, the relevant parts of the image must be paged in
before they can be overwritten and thus I/O bandwidth requirement is double.
Unbuffered userspace read/write is 1.5% slower than the kernel mount.
• Copying a directory tree is a mix of directory metadata and file data operations
and one third of the I/O is done synchronously in this case. The
memory mapped case does not suffer as badly as the large copy, as locality
is better. The rump read/write case performs 10% better than the kernel
due to a buffered backend. The tradeoff is increased memory use. In the
unbuffered case the problem of not being able to issue synchronous write
operations while an asynchronous one is in progress shows.
Notably, we did not look into modifying the host kernel to provide more finegrained
interfaces for selective cache flushing and I/O to character devices. For now, we
maintain that performance for the typical workload is acceptable when compared to
a kernel mount. We still emphasize that due to the anykernel a kernel mount can
be used for better performance where it is safe to do so.
Journaled FFS
We measured the large file copy and copy of a directory structure tests similarly
as for traditional FFS. We did not measure read-only operation since journaling is
meant to speed up write operations. We did not measure the MMIO rump kernel
block device or the character device backend, since they were already shown to be
inferior. The tests were done as in the previous section, with the exception that the
file system mounted with -o log. The results are presented in Figure 4.18.