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Modern Operating Systems by Herbert Bos and Andrew S. Tanenb...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf
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Modern Operating Systems by Herbert Bos and Andrew...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Modern Operating Systems by Herbert...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Page 1026
SEC. 12.3
IMPLEMENTATION
995
Exokernels
While layering has its supporters among system designers, another camp has
precisely the opposite view (Engler et al., 1995).
Their view is based on the
end-
to-end argument
(Saltzer et al., 1984).
This concept says that if something has to
be done by the user program itself, it is wasteful to do it in a lower layer as well.
Consider an application of that principle to remote file access.
If a system is
worried about data being corrupted in transit, it should arrange for each file to be
checksummed at the time it is written and the checksum stored along with the file.
When a file is transferred over a network from the source disk to the destination
process, the checksum is transferred, too, and also recomputed at the receiving end.
If the two disagree, the file is discarded and transferred again.
This check is more accurate than using a reliable network protocol since it also
catches disk errors, memory errors, software errors in the routers, and other errors
besides bit transmission errors. The end-to-end argument says that using a reliable
network protocol is then not necessary, since the endpoint (the receiving process)
has enough information to verify the correctness of the file. The only reason for
using a reliable network protocol in this view is for efficiency, that is, catching and
repairing transmission errors earlier.
The end-to-end argument can be extended to almost all of the operating sys-
tem. It argues for not having the operating system do anything that the user pro-
gram can do itself. For example, why have a file system?
Just let the user read and
write a portion of the raw disk in a protected way.
Of course, most users like hav-
ing files, but the end-to-end argument says that the file system should be a library
procedure linked with any program that needs to use files. This approach allows
different programs to have different file systems.
This line of reasoning says that
all the operating system should do is securely allocate resources (e.g., the CPU and
the disks) among the competing users. The Exokernel is an operating system built
according to the end-to-end argument (Engler et al., 1995).
Microkernel-Based Client-Server Systems
A compromise between having the operating system do everything and the op-
erating system do nothing is to have the operating system do a little bit. This de-
sign leads to a microkernel with much of the operating system running as user-
level server processes, as illustrated in Fig. 12-3. This is the most modular and
flexible of all the designs. The ultimate in flexibility is to have each device driver
also run as a user process, fully protected against the kernel and other drivers, but
even having the device drivers run in the kernel adds to the modularity.
When the device drivers are in the kernel, they can access the hardware device
registers directly. When they are not, some mechanism is needed to provide access
to them.
If the hardware permits, each driver process could be given access to only
those I/O devices it needs. For example, with memory-mapped I/O, each driver
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