Lecture 4
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Code examples on csserver in directory
/home/hwang/cs470/lecture04
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Homework 1 posted, due next Wednesday
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Questions?
Wednesday, January 18
CS 470 Operating Systems - Lecture 4
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Outline
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Message passing
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Message queues
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Cross-network IPC
Wednesday, January 18
CS 470 Operating Systems - Lecture 4
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Shared Memory
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Recall that in shared memory, cooperating
processes map same physical shared memory
segment into their logical memory.
This is very efficient, since access is the same
as for non-shared memory. But using shared
memory requires synchronization (Chapter 6).
Like all software engineering problems, we can
solve this problem by adding a layer of
abstraction.
Wednesday, January 18
CS 470 Operating Systems - Lecture 4
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Message Passing
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The message passing technique for providing
IPC has the following characteristics:
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Messages are used to exchange information
Each send/receive is a system call
Synchronization is provided automatically, since
processes do not share space.
Since messages are copied, they are mostly
used for small amounts of data exchange.
They are especially good for distributed
communication. E.g., a chat program
Wednesday, January 18
CS 470 Operating Systems - Lecture 4
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Message Passing
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There are at least two operations: send and
receive. Lots of design issues in implementing
message passing.
Are the messages fixed size or variable sized?
Advantage/disadvantages of each?
Is communication direct or indirect? Direct is
like sending mail to a specific address. Indirect
is like sending mail to a post office box.
Wednesday, January 18
CS 470 Operating Systems - Lecture 4
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Message Passing
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Direct - "name" is given in calls: send (P, msg)
or receive (Q, msg). Why might this be useful?
Indirect - "mailbox" (or "port") is created that is
used in calls: send (M, msg) or receive (M,
msg). How does this behave differently than
using names?
Wednesday, January 18
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Message Passing
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If indirect, are mailboxes stored/owned in
process space or in OS space? What are the
differences?
Process A
Process B
OS Kernel
Wednesday, January 18
Process A
M
Process B
OS Kernel
CS 470 Operating Systems - Lecture 4
M
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Message Passing
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Is communication synchronous or
asynchronous? Either for send or receive. Can
be any of the four possible combinations. In
particular, if both send and receive block, have
a rendezvous (implemented directly by Ada
tasks).
How is message storage handled? Is there a
fixed size buffer? Is buffering automatic?
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Zero capacity - no waiting messages
Bounded - sender blocks when queue is full
Unbounded - sender never blocks
Wednesday, January 18
CS 470 Operating Systems - Lecture 4
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Message Passing
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Mach OS uses message passing for all IPC
including system calls to the OS. The main
issue is that local communication is slow
because data must be transferred from user
space to kernel space and back.
Solve this by using shared memory to
implement mailboxes. The sender's message
area is mapped into the receiver's message
area.
Wednesday, January 18
CS 470 Operating Systems - Lecture 4
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Producer/Consumer Processes
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Producer
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Consumer
Item nextP;
Item nextC;
while (true) {
nextP = MakeItem();
while(numItems ==
BUFSIZE)
; // do nothing
buffer[in] = nextP;
in = (in+1) % BUFSIZE;
numItems++;
}
while (true) {
while(numItems == 0)
; // do nothing
nextC = buffer[out];
out = (out+1) % BUFSIZE;
numItems­­;
UseItem(nextC);
}
Wednesday, January 18
CS 470 Operating Systems - Lecture 4
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System V Message Queues
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An example implementation using System V
(SysV) message queues is shown in file shmprod-cons.cpp
System V message queue routines are defined
in library <sys/msg.h>. They have syntax
similar to the shared memory routines.
Both the producer (parent process) and
consumer (child process) are in this file, and it
uses signal handlers to clean up before exit.
Message queue is used as the buffer.
Wednesday, January 18
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System V Message Queues
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System V message queues are owned by the
OS, and have automatic bounded buffering.
Maximum queue size is set by the system, but
can be modified by the superuser. Default is
synchronous, but can be made asynchronous.
Programmer defines the format of a message
as a struct. It must start with a long that stores
the type of the message followed by the
message data which must consist of plain
types. E.g., the Message struct.
Wednesday, January 18
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System V Message Queues
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The msgget( ) routine is used to create a
message queue and has prototype:
int msgget(key_t key, int flags);
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The routine returns a queue id. Examples:
// Create a named queue
id=msgget(19,IPC_CREAT|IPC_EXCL|0660);
// Access existing queue by name
id=msgget(19, 0);
// Create unnamed queue
id=msgget(IPC_PRIVATE, 0660);
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System V Message Queues
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msgsnd( ) adds a message to the queue; its
prototype is:
void msgsnd(int id, void *msg,
size_t len, int flg);
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id is the queue identifier. msg must be a
pointer to a message struct. len is the size of
the data portion of the msg struct in bytes.
Note that len does not include the size of the
type field. flg is usually 0 or IPC_NOWAIT. By
default msgsnd( ) will block if the queue is full
unless the IPC_NOWAIT flag is set.
Wednesday, January 18
CS 470 Operating Systems - Lecture 4
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System V Message Queues
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msgrcv( ) is used to get a message and has
prototype:
void msgrcv(int id, void *msg,
size_t len, size_t type, int flg);
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id is the queue identifier. msg must be a
pointer to a message struct just as for
msgsnd( ). len is the size of the data area in
the receiving struct. If the message is longer
than len, the message is removed from the
queue and the call fails. (This can be modified
via msgctl( )).
Wednesday, January 18
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System V Message Queues
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The type parameter allows the receiver to read
the next message of a particular type.
If type is 0, the first message is read.
If type is greater than 0, the first message in
the queue of that type is read. (Unless flg
contains MSG_EXCEPT, then the first
message NOT of that type is read.)
Wednesday, January 18
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System V Message Queues
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If type is less than 0, the first message in the
queue with the lowest type less than or equal
to the absolute value of type is read.
The flg parameter is usually either 0 or
IPC_NOWAIT (if you do not want msgrcv( ) to
block until a message becomes available).
Wednesday, January 18
CS 470 Operating Systems - Lecture 4
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System V Message Queues
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The msgctl( ) routine is used to change
permissions on a queue, get information about
the queue, and to delete the queue:
// Remove the queue
ret = msgctl(id, IPC_RMID, 0);
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Unlike shared memory, the queue is removed
immediately. Any processes that are waiting on
the queue are awakened and receive an error
return.
Wednesday, January 18
CS 470 Operating Systems - Lecture 4
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Cross-Network IPC
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Most system implement the POSIX socket
interface to provide the low-level mechanism for
connecting two processes and transferring
data.
Sockets are tricky to use. Again abstract up a
level and provide remote procedure calls
(RPCs) or remote method invocation (RMI).
Wednesday, January 18
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Remote Procedure Call (RPC)
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An RPC looks like a regular call, but is just a
stub that wrap system calls. Some issues:
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How to marshal/unmarshal (value) parameters
/return values, that is, convert data from local
representation to an external data representation
(XDR). In particular, linked structures need to be
"flattened" and reassembled.
How to find the port of the remote procedure. Fixed
names or a matchmaker process.
How to handle failure, both of procedure and
network. At most once vs. exactly once semantics.
Wednesday, January 18
CS 470 Operating Systems - Lecture 4
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Remote Procedure Call (RPC)
Client
Server
P(arg1, ...)
void P
(int arg1,...)
Arguments
Arguments
Client stub
Server stub
Marshalled
arguments
Marshalled
arguments
RPC
system call
RPC
system call
Data
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Remote Method Invocation (RMI)
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An RMI (used in Java) is similar to an RPC. In
addition, references to non-local objects can
be sent as parameters.
Wednesday, January 18
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