Date: Thu, 11 Feb 2010 08:54:59 -0500 From: John Baldwin <jhb@freebsd.org> To: freebsd-threads@freebsd.org, Daniel Eischen <deischen@freebsd.org> Subject: Re: Thinking about kqueue's and pthread_cond_wait Message-ID: <201002110854.59265.jhb@freebsd.org> In-Reply-To: <Pine.GSO.4.64.1002101232240.13876@sea.ntplx.net> References: <3581A86D-9C9C-4E08-9AD3-CD550B180CED@lakerest.net> <3CF3033E-FD13-405B-9DC6-DDE9DF4FBF37@lakerest.net> <Pine.GSO.4.64.1002101232240.13876@sea.ntplx.net>
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On Wednesday 10 February 2010 12:46:46 pm Daniel Eischen wrote:
> On Wed, 10 Feb 2010, Randall Stewart wrote:
>
> >
> > On Feb 10, 2010, at 9:04 AM, Daniel Eischen wrote:
> >
> >> On Wed, 10 Feb 2010, Randall Stewart wrote:
> >>
> >>> All:
> >>>
> >>> I have once again come around to thinking about joining pthread cond
waits
> >>> and
> >>> kqueue's.
> >>>
> >>> After thinking about it, I think its doable.. with something like a:
> >>>
> >>> pthread_cond_wait_kqueue_np(kev, cond, mtx, ucontext)
> >>>
> >>> Then you can use kev inside a kqueue i.e.
> >>> ret = kevent(kq, kev, 1, outkev, 1, NULL);
> >>>
> >>> Now when you saw the event:
> >>> if (kev.filter == EVFILT_UMTX){ /* not sure about the name here */
> >>> pthread_kqueue_cond_wait_ret_np(kev, cond, mtx, ucontext)
> >>> do_user_action(cond,mtx, ucontext);
> >>> }
> >>>
> >>> Which would fill in the cond/mtx and ucontext for the user.
> >>>
> >>> Now does this sound useful to anyone.. i.e. should I spend the time
> >>> making it work?
> >>>
> >>> The only down side to this is that it would have to allocate memory so
> >>> one would need to do a:
> >>>
> >>> pthread_kqueue_cond_wait_free_np(kev)
> >>>
> >>> After you were done.. and I think it would be best for this to
> >>> be a ONE_SHOT.. i.e. you have to re-arm it if the event happens...
> >>> Of course until you free it that can be as simple as passing the kev
> >>> back down again (i.e. no pthread_cond_wait_kqueue_np() needed).
> >>>
> >>> Comments? Thoughts? i.e. especially is it worthwhile doing?
> >>
> >> Please don't mess with the pthread_ API like that :-) If you
> >> really want to munge them together, see my email to you a few
> >> weeks ago last time you brought it up.'
> >
> > If I remember right your email was basically don't do it... I will
> > go dig through the archives and re-read it all.
>
> No, it was to add an interface or two to the kqueue/kevent API, not
> to modify the pthread_ API (which shouldn't know anything about
> kqueues).
>
> I really think the OS is already given us the tools we need to
> do the job simply enough. You can easily use a pipe, socketpair,
> or EVFILT_SIGNAL to wakeup a thread stuck in kevent(). You can
> additionally use a mutex to protect data shared between thread
> waiting in kevent() and other threads.
>
> I don't see what problem this is trying to solve and I think
> whatever solution you come up with involving mutexes/CVs is
> not going to be any simpler and may even be more complex and
> messy. Mutexes and CVs are userland library thingies, not
> kernel entities. Yes, the umtx is a kernel entity, but it
> alone does not give you mutexes and CVs. So when you want
> to mix kqueues and mutexes/CVs, you are involving another
> userland library and this is what makes it messy.
He wants something like 'WaitForMultipleObjects()' in NT. The fact that these
are userland primitives is _precisely_ why I think it belongs in the pthread
library. What I think is that you want a pthread primitive that is a wrapper
around a kqueue. Something like:
/* A lot like a 'struct event' */
struct pthread_event {
};
/* Is actually an int to hold a kqueue fd. */
pthread_event_queue_t;
/* Calls kqueue() to create the event. */
pthread_event_queue_init();
/* Similar to the kevent() call, but works with 'struct pthread_event' arrays
instead of 'struct event' arrays. Internally, it maps 'struct pthread_event'
objects to 'struct event' objects to pass to an internal call to kevent().
This code can take care of mapping requests to wait for a cv or mutex to the
appropriate EVFILT_UMTX w/o exposing the EVFILT_UMTX implementation details to
userland. */
pthread_event_queue_wait();
The user code model would look something like this:
pthread_event_queue_t queue;
struct pthread_event event;
pthread_cond_t cv /* some condition variable */
int fd; /* some socket */
pthread_event_queue_init(&queue);
/* This is like EV_SET, maybe you would have a EVQ_SET? */
event.filter = READ;
event.fd = fd;
event.flags = EV_ADD;
/* Register socket. */
pthread_event_queue_wait(queue, NULL, 0, &event, 1, NULL);
event.filter = CONDVAR;
event.cv = cv;
event.flags = EV_ADD;
/* Register condvar. */
pthread_event_queue_wait(queue, NULL, 0, &event, 1, NULL);
/* Wait for something to happen. */
for (;;) {
pthread_event_queue_wait(queue, &event, 1, NULL, 0, NULL);
switch (event.filter) {
case READ:
/* socket is ready to read */
case CV:
/* cv is signalled */
}
}
To be honest, I find semaphores and mutexes more compelling than condvars for
this sort of thing (you would do a try-lock to acquire the mutex or semaphore
perhaps when it is signalled). If you supported thread objects you could do a
pthread_join() after a thread exits.
--
John Baldwin
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