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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 1018
SEC. 12.2
INTERFACE DESIGN
987
to be sent and the reply to be requested with only one kernel trap. Everything else
is done by requesting some other process (e.g., the file-system process or the disk
driver) to do the work. The most recent version of MINIX added two additional
calls, both for asynchronous communication.
The
senda
call sends an asynchro-
nous message. The kernel will attempt to deliver the message, but the application
does not wait for this; it just keeps running. Similarly, the system uses the
notify
call to deliver short notifications. For instance, the kernel can notify a device driver
in user space that something happened—much like an interrupt. There is no mes-
sage associated with a notification. When the kernel delivers a notification to proc-
ess, all it does is flip a bit in a per-process bitmap indicating that something hap-
pened. Because it is so simple, it can be fast and the kernel does not need to worry
about what message to deliver if the process receives the same notification twice. It
is worth observing that while the number of calls is still very small, it is growing.
Bloat is inevitable. Resistance is futile.
Of course, these are just the kernel calls. Running a POSIX compliant system
on top of it, requires implementing a lot of POSIX system calls. But the beauty of
it is that they all map on just a tiny set of kernel calls. With a system that is (still)
so simple, there is a chance we may even get it right.
Amoeba is even simpler.
It has only one system call: perform remote proce-
dure call.
This call sends a message and waits for a reply.
It is essentially the
same as MINIX’
sendrec
.
Everything else is built on this one call. Whether or not
synchronous communication is the way to go is another matter, one that we will re-
turn to in Sec. 12.3.
Principle 3: Efficiency
The third guideline is efficiency of implementation. If a feature or system call
cannot be implemented efficiently, it is probably not worth having. It should also
be intuitively obvious to the programmer about how much a system call costs. For
example, UNIX programmers expect the
lseek
system call to be cheaper than the
read
system call because the former just changes a pointer in memory while the
latter performs disk I/O.
If the intuitive costs are wrong, programmers will write
inefficient programs.
12.2.2 Paradigms
Once the goals have been established, the design can begin. A good starting
place is thinking about how the customers will view the system.
One of the most
important issues is how to make all the features of the system hang together well
and present what is often called
architectural coherence
.
In this regard, it is im-
portant to distinguish two kinds of operating system ‘‘customers.’’ On the one
hand, there are the
users
, who interact with application programs; on the other are
the
programmers
, who write them. The former mostly deal with the GUI; the latter
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