Paul Rubin <http> wrote:
> Antoon Pardon <[EMAIL PROTECTED]> writes:
> > I'm not sure that this would be an acceptable approach. I did the man
> > semop and it indicates this is part of system V IPC. This makes me
> > fear that semaphores will use file descriptors or other resources
> > that are only available in a limited amount. Not usefull if you are
> > talking about thousands of threads.
>
> That would be terrible, if semaphores are as heavy as file descriptors.
> I'd like to hope the OS's are better designed than that.
I believe futex is the thing you want for a modern linux. Not
very portable though.
>From futex(4)
The Linux kernel provides futexes ('Fast Userspace muTexes') as a
building block for fast userspace locking and semaphores. Futexes are
very basic and lend themselves well for building higher level locking
abstractions such as POSIX mutexes.
This page does not set out to document all design decisions but
restricts itself to issues relevant for application and library devel-
opment. Most programmers will in fact not be using futexes directly but
instead rely on system libraries built on them, such as the NPTL
pthreads implementation.
A futex is identified by a piece of memory which can be shared between
different processes. In these different processes, it need not have
identical addresses. In its bare form, a futex has semaphore semantics;
it is a counter that can be incremented and decremented atomically;
processes can wait for the value to become positive.
Futex operation is entirely userspace for the non-contended case. The
kernel is only involved to arbitrate the contended case. As any sane
design will strive for non-contension, futexes are also optimised for
this situation.
In its bare form, a futex is an aligned integer which is only touched
by atomic assembler instructions. Processes can share this integer over
mmap, via shared segments or because they share memory space, in which
case the application is commonly called multithreaded.
--
Nick Craig-Wood <[EMAIL PROTECTED]> -- http://www.craig-wood.com/nick
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