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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 176
SEC. 2.3
INTERPROCESS COMMUNICATION
145
At the other end of the spectrum, there are also design issues that are important
when the sender and receiver are on the same machine. One of these is perfor-
mance. Copying messages from one process to another is always slower than
doing a semaphore operation or entering a monitor. Much work has gone into mak-
ing message passing efficient.
The Producer-Consumer Problem with Message Passing
Now let us see how the producer-consumer problem can be solved with mes-
sage passing and no shared memory.
A solution is given in Fig. 2-36.
We assume
that all messages are the same size and that messages sent but not yet received are
buffered automatically by the operating system.
In this solution, a total of
N
mes-
sages is used, analogous to the
N
slots in a shared-memory buffer. The consumer
starts out by sending
N
empty messages to the producer. Whenever the producer
has an item to give to the consumer, it takes an empty message and sends back a
full one.
In this way, the total number of messages in the system remains constant
in time, so they can be stored in a given amount of memory known in advance.
If the producer works faster than the consumer, all the messages will end up
full, waiting for the consumer; the producer will be blocked, waiting for an empty
to come back.
If the consumer works faster, then the reverse happens: all the mes-
sages will be empties waiting for the producer to fill them up; the consumer will be
blocked, waiting for a full message.
Many variants are possible with message passing. For starters, let us look at
how messages are addressed.
One way is to assign each process a unique address
and have messages be addressed to processes.
A different way is to invent a new
data structure, called a
mailbox
.
A mailbox is a place to buffer a certain number
of messages, typically specified when the mailbox is created. When mailboxes are
used, the address parameters in the
send
and
receive
calls are mailboxes, not proc-
esses. When a process tries to send to a mailbox that is full, it is suspended until a
message is removed from that mailbox, making room for a new one.
For the producer-consumer problem, both the producer and consumer would
create mailboxes large enough to hold
N
messages. The producer would send mes-
sages containing actual data to the consumer’s mailbox, and the consumer would
send empty messages to the producer’s mailbox. When mailboxes are used, the
buffering mechanism is clear: the destination mailbox holds messages that have
been sent to the destination process but have not yet been accepted.
The other extreme from having mailboxes is to eliminate all buffering. When
this approach is taken, if the
send
is done before the
receive
, the sending process is
blocked until the
receive
happens, at which time the message can be copied direct-
ly from the sender to the receiver, with no buffering. Similarly, if the
receive
is
done first, the receiver is blocked until a
send
happens. This strategy is often
known as a
rendezvous
.
It is easier to implement than a buffered message scheme
but is less flexible since the sender and receiver are forced to run in lockstep.
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