A resource manager for semaphore management.

QNX DEVELOPERS Realtime and Embedded
1

Is it possible that semaphores, mutexes etc, could be implemented outside
the kernel as services of a resource manager, where message passing and
signals be part of the semaphore infrastructure. I believe QNX’s use of
Priority Inheritence upon a message-receivce could help in this aspect.

Thanks In Advance,
LeMerc.

2

Not if you want to use the same API sem_* etc. But if you want to use your own API I guess it’s possible but I don’t see the point. Maybe you could explain what exactly you need to do?

3

Mario,

Maybe he wants to do synchronization across a network?
4

Maschoen is right. Synchronization across the network would be the
advantage. But say, if I want to write my own QNX-like OS but do not want
to include semaphores etc in the kernel (primarily because semaphores are
boring to me, heh heh), what would be the disadvantages of having a
Semaphore Resource Manager.

5

Performance

6

This may sound silly, but in a architecture where resource sharing - except
shared-memory - is inherently synchronised / serialised with also the
provison of priority-inheritance, does one really requires kernel-semaphores.
Fine, with SMP one does require spinlocks but otherwise what else can qualify
semaphores?.

7

The main reason for semaphore is shared memory between processes, or local variable shared between threads.

8

Sorry to keep this topic burning Mario… but IMHO even if there’s a slight
performance penalty by keeping semaphore-management outside the kernel,
one gains advantage like netwide shared-memory synchronisation etc. So, a
more sophisticated semaphore manager I suppose. Of course, I do not
presently know of all the issues involved with thread synchronisation in the
embedded environment.

9

Networked shared-memory is an oxymoron. Memory cannot be shared via network. I guess I shouldn’t say cannot, I guess it could be it would be a nightware to implement and have lots of caveats I think.

You mentioned thread synchronisation but semaphore are typical not about synchronisation they are about protecting shared data (like CriticalSection under windows).

.

10

Thank You all. Though I would still vote for spinlocks in kernel, and semaphores out. Thanks again.

11

Hi all,

Following is not for QNX but some might find it interesting, so I’ll post it anyway.

At: lsub.org/who/nemo/9.intro.pdf

Is an first draft of: Introduction to Operating Systems Abstractions, Using Plan 9 from Bell Labs

At page 320 is an example, how to implement semaphores as a 9P fileserver. Example provides semaphores as if they were files.

–jalih