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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
Showing 131-132 out of 1137
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 131
100
PROCESSES AND THREADS
CHAP. 2
An analogous situation exists with many other interactive programs. For exam-
ple, an electronic spreadsheet is a program that allows a user to maintain a matrix,
some of whose elements are data provided by the user. Other elements are com-
puted based on the input data using potentially complex formulas. When a user
changes one element, many other elements may have to be recomputed. By having
a background thread do the recomputation, the interactive thread can allow the user
to make additional changes while the computation is going on. Similarly, a third
thread can handle periodic backups to disk on its own.
Now consider yet another example of where threads are useful: a server for a
Website. Requests for pages come in and the requested page is sent back to the cli-
ent. At most Websites, some pages are more commonly accessed than other pages.
For example, Sony’s home page is accessed far more than a page deep in the tree
containing the technical specifications of any particular camera. Web servers use
this fact to improve performance by maintaining a collection of heavily used pages
in main memory to eliminate the need to go to disk to get them. Such a collection
is called a
cache
and is used in many other contexts as well.
We saw CPU caches
in Chap. 1, for example.
One way to organize the Web server is shown in Fig. 2-8(a). Here one thread,
the
dispatcher
, reads incoming requests for work from the network. After examin-
ing the request, it chooses an idle (i.e., blocked)
worker thread
and hands it the
request, possibly by writing a pointer to the message into a special word associated
with each thread. The dispatcher then wakes up the sleeping worker, moving it
from blocked state to ready state.
Dispatcher thread
Worker thread
Web page cache
Kernel
Network
connection
Web server process
User
space
Kernel
space
Figure 2-8.
A multithreaded Web server.
When the worker wakes up, it checks to see if the request can be satisfied from
the Web page cache, to which all threads have access. If not, it starts a
read
opera-
tion to get the page from the disk and blocks until the disk operation completes.
Page 132
SEC. 2.2
THREADS
101
When the thread blocks on the disk operation, another thread is chosen to run, pos-
sibly the dispatcher, in order to acquire more work, or possibly another worker that
is now ready to run.
This model allows the server to be written as a collection of sequential threads.
The dispatcher’s program consists of an infinite loop for getting a work request and
handing it off to a worker. Each worker’s code consists of an infinite loop consist-
ing of accepting a request from the dispatcher and checking the Web cache to see if
the page is present.
If so, it is returned to the client, and the worker blocks waiting
for a new request. If not, it gets the page from the disk, returns it to the client, and
blocks waiting for a new request.
A rough outline of the code is given in Fig. 2-9.
Here, as in the rest of this
book,
TRUE
is assumed to be the constant 1.
Also,
buf
and
page
are structures ap-
propriate for holding a work request and a Web page, respectively.
while (TRUE) {
while (TRUE) {
get
next
request(&buf);
wait
for
work(&buf)
handoff
work(&buf);
look
for
page
in
cache(&buf, &page);
}
if (page
not
in
cache(&page))
read
page
from
disk(&buf, &page);
return
page(&page);
}
(a)
(b)
Figure 2-9.
A rough outline of the code for Fig. 2-8. (a) Dispatcher thread.
(b) Worker thread.
Consider how the Web server could be written in the absence of threads. One
possibility is to have it operate as a single thread.
The main loop of the Web server
gets a request, examines it, and carries it out to completion before getting the next
one. While waiting for the disk, the server is idle and does not process any other
incoming requests.
If the Web server is running on a dedicated machine, as is
commonly the case, the CPU is simply idle while the Web server is waiting for the
disk. The net result is that many fewer requests/sec can be processed.
Thus,
threads gain considerable performance, but each thread is programmed sequential-
ly, in the usual way.
So far we have seen two possible designs: a multithreaded Web server and a
single-threaded Web server. Suppose that threads are not available but the system
designers find the performance loss due to single threading unacceptable.
If a
nonblocking version of the
read
system call is available, a third approach is pos-
sible. When a request comes in, the one and only thread examines it.
If it can be
satisfied from the cache, fine, but if not, a nonblocking disk operation is started.
The server records the state of the current request in a table and then goes and
gets the next event. The next event may either be a request for new work or a reply
from the disk about a previous operation.
If it is new work, that work is started.
If
it is a reply from the disk, the relevant information is fetched from the table and the
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