Does anyone here understand Linus Torvald's recent comments on FreeBSD?
Sure. There are different ways of moving data between the kernel and userland;
the classic mechanism involves copying data from a wired-down page in kernel
space allocated to network memory buffers to a userland page via copyin() and
copyout() (or equivalents).
Mach (and apparently the ZERO_COPY_SOCKETS option to FreeBSD) manipulate the VM
page table mappings to make that page visible in the process address space
rather than copying the sequence of bytes manually via a message-passing paradigm.
The former approach tends to be more efficient for small amounts of data,
especially for things smaller than one page of memory (ie, all non-jumbo network
traffic); the latter approach tends to better for things which are bigger in size.
The Mach VM has more overhead to its operations because the VMOs are more
complicated to work and a given workload will result in comparatively larger VMO
datastructures than the less-complicated approaches to doing VM. On the other
hand, Mach was the first or among the earliest platforms to support shared
libraries, dynamic loading of objects into user processes (dlopen vs. dso) and
into the kernel, and has somewhat better scaling in the face of gigabytes of RAM
and VM usage than most Unix flavors do (outside of Solaris, although FreeBSD is
pretty decent nowadays too).
Mach handles mapping shared libraries into VM via a technique called prebinding
that can minimize the work and memory overhead required for runtime symbol
relocation, which tends to big win if you are running a lot of, say, Java or
Perl processes that make extensive use of runtime class-loading, yet is flexible
enough to deal with collisions if needed (whereas the older fixed-VM shared
libraries were subject to evil nasty conflicts if your data segment grew too big
and overlapped a library's chosen address space, or if two libraries wanted to
be mapped into the same spot).
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