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SubjectRe: Linus on Linux, Apache and Threads
Date
> > On Fri, 23 Apr 1999, Ian D Romanick wrote:
> > > Huh? It's not that hard of a problem. You have one (or several) thread
> > > that just reads from network sockets. They package up the requests and put
> > > them on the end of a queue. The other threads just pull requests off the
> > > head of the queue.
> >
> > i wasn't trying to suggest it was hard to code or understand. my question
> > is how to do this efficiently. has anyone compared the performance of
> > this model with the performance of the same application implemented
> > using NT's completion ports?
> >
> > every Unix model i've seen that uses the "single thread waits for event
> > and wakes up worker threads" has suffered from a variety of problems that
> > can cripple its performance, like:
> >
> > how does the event waiter thread determine whether there is a
> > suitable/idle worker thread to awaken?
> >
> > > The trick is all in waking up the sleeping threads when
> > > the queue becomes non-empty without having the "thundering herd" problem.
> >
> > agreed! any ideas?
>
> It could be done several ways. Does Linux have Tanenbaum style up/down
> semaphores? If not, it shouldn't be too hard to do using pthread_mutex.
> Each time an element is put on the event queue, you up the semaphore.
> Before getting the dequeue lock on the queue, the thread would down the
> semaphore.
>

Yes, pthreads, including the Linux implementation thereof, has POSIX
semaphores, which are "up/down" in style. Create with sem_init(),
down with sem_wait(), up with sem_post(). So when there is nothing in
the queue, all worker threads will be blocked in sem_wait(), and when
a single element is put on the event queue, you call sem_post(), which
should wake up exactly one worker thread.

--
Darius S. Naqvi email(home): dsn@scrap.myrias.com
email(work): dsn@myrias.com

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