Date: Wed, 10 Feb 2010 11:17:09 -0800 From: Alfred Perlstein <alfred@freebsd.org> To: Daniel Eischen <deischen@freebsd.org> Cc: threads@freebsd.org Subject: Re: Thinking about kqueue's and pthread_cond_wait Message-ID: <20100210191709.GD71374@elvis.mu.org> In-Reply-To: <Pine.GSO.4.64.1002101401460.14317@sea.ntplx.net> References: <3581A86D-9C9C-4E08-9AD3-CD550B180CED@lakerest.net> <Pine.GSO.4.64.1002101202060.13656@sea.ntplx.net> <3CF3033E-FD13-405B-9DC6-DDE9DF4FBF37@lakerest.net> <Pine.GSO.4.64.1002101232240.13876@sea.ntplx.net> <07AA24BB-DA26-406A-B24F-59E0CB36FEBE@lakerest.net> <Pine.GSO.4.64.1002101331240.14115@sea.ntplx.net> <20100210185746.GC71374@elvis.mu.org> <Pine.GSO.4.64.1002101401460.14317@sea.ntplx.net>
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* Daniel Eischen <deischen@freebsd.org> [100210 11:06] wrote:
> On Wed, 10 Feb 2010, Alfred Perlstein wrote:
>
> >* Daniel Eischen <deischen@freebsd.org> [100210 10:50] wrote:
> >>On Wed, 10 Feb 2010, Randall Stewart wrote:
> >>
> >>>
> >>>
> >>>while (notdone) {
> >>> nev = kevent(kq, , ev);
> >>> if (ev.fitler == EVFILTER_READ) {
> >>> handle_the_read_thingy(ev);
> >>> } else if (ev.filter == EVFILTER_COND) {
> >>> lock_mutex(if needed)
> >>> handle_condition_event();
> >>> }
> >>>}
> >>>
> >>>
> >>>One of the things I will note about a condition variable is that the
> >>>downside is
> >>>you ALWAYS have to have a mutex.. and not always do you need one... I
> >>>have
> >>>found
> >>>multiple times in user apps where i am creating a mutex only for the
> >>>benefit of
> >>>the pthread_cond() api... sometimes just being woken up is enough ;-)
> >>
> >>[ I didn't see that you were waiting on multiple CVs... ]
> >>
> >>I don't understand why you need to wait on multiple
> >>condition variables. Either way, you have to maintain
> >>a queue of them along with their associated mutexes and
> >>then take some action unique to each one of them. What
> >>is the difference between that and maintaining a queue of
> >>some other thingies that maintain similar state data?
> >
> >Developer convenience.
> >
> >If we offer a stable API and way of doing it right, then we offer
> >a solid base for programs. By making users roll their own we have
> >them duplicate code and introduce errors, in fact the idea of how
> >to get this working (using a pipe(2) loop back) is so esoteric to
> >likely block a significant portion of users from solving this problem
> >at all.
>
> See the proposed solution hacking the pthread API and think twice
> about that.
>
> If you need a generic way of waiting and waking threads in kevent,
> then extend the kqueue/kevent interface to allow it. Add a userland
> struct kq_wait object along with EVFILT_KQ_WAIT, and a syscall to
> send wake up that event.
Again, this is not convenient. It is more complex and error prone
for users.
I will review the _additions_ to the pthreads API to see if they
cause any performance issues for the "non-use" case.
--
- Alfred Perlstein
.- AMA, VMOA #5191, 03 vmax, 92 gs500, 85 ch250
.- FreeBSD committer
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