SGI: Need Something like 1 Mutual Exclusion Semaphore for Multible Programs

[clusardi2k]

Hello,

I have a assignment just thrown onto my desk.

What is the easiest way to solve it? Below is a brief
description of the task.

There are multible programs which use the same library routine
which is an interface to what I'll call a service program.

The problem is that only 1 request at a time can be serviced by
the service program.

My boss told me to just use somehting such as using shared
memory (via memmap) and set a flag in there to 1 when a program
wants service. The various programs are to wait in a loop until
that flag becomes 0. Once the flag becomes 0 then set it to 1 along
with setting other parameters to send to the service program.

But, woun't this lead to programs setting that flag to 1 at the
same time causing loss of service etc.

The way I see it is the flag described above is a critical
section and must be protected by a mutual exclusion semaphore.

I don't see anything wrong with using shared memory for sending
other parameters to the service routine.

O'kay what should I do?

Thank you,
Christopher Lusardi

P.S.: I'm sorry if I'm posting this to the wrong group. :-[

 

[clusardi2k]

I can't use the following sequence because the programs that use the
library routine
reside on different computers.

sem_open
sem_init
sem_wait
sem_post
sem_post


The only thing I have to find out is how how do I declare the semaphore

to be in
shared memory.


Thanks,
Chris Lusardi

 

[CBFalconer]

I have a assignment just thrown onto my desk.
.... snip ...

P.S.: I'm sorry if I'm posting this to the wrong group. :-[

You are, in at least two of the groups you used. You also
neglected to set follow-ups to whatever home group you are using.
F'ups set.

 

[Tony 'Nicoya' Mantler]

: I have a assignment just thrown onto my desk.
:
: What is the easiest way to solve it? Below is a brief
: description of the task.
:
: There are multible programs which use the same library routine
: which is an interface to what I'll call a service program.
:
: The problem is that only 1 request at a time can be serviced by
: the service program.

There's a few options. You can, in order of decreasing preference:

* Fix your service program to queue requests
* Add an intermediary service program that queues requests to the service program
* Use some distributed API for locking (since you mention they're on different
machines), like maybe MPI, PVM, or something like RPC
* Use a reject-and-retry collision system

You could also possibly parallelize your service program so that it can serve
more than one request at a time, or make it transactional if it's not quite
parallelizable. It all depends on what your service program actually does.


Cheers - Tony 'Nicoya' Mantler

 

[CBFalconer]

I can't use the following sequence because the programs that use
the library routine reside on different computers.

sem_open
sem_init
sem_wait
sem_post
sem_post

The only thing I have to find out is how how do I declare the
semaphore to be in shared memory.

This has nothing to do with at least three of the groups to which
you cross-posted without troubling to find out what they were
about. F'ups set.