Date: Wed, 10 Feb 2010 12:18:23 -0800 From: Randall Stewart <rrs@lakerest.net> To: Alfred Perlstein <alfred@freebsd.org> Cc: Daniel Eischen <deischen@freebsd.org>, threads@freebsd.org Subject: Re: Thinking about kqueue's and pthread_cond_wait Message-ID: <AC28C918-B60A-46E2-95A6-02D4B132DFAB@lakerest.net> In-Reply-To: <20100210200631.GE71374@elvis.mu.org> 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> <20100210191709.GD71374@elvis.mu.org> <Pine.GSO.4.64.1002101500570.14544@sea.ntplx.net> <20100210200631.GE71374@elvis.mu.org>
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On Feb 10, 2010, at 12:06 PM, Alfred Perlstein wrote:
> * Daniel Eischen <deischen@freebsd.org> [100210 12:01] wrote:
>> On Wed, 10 Feb 2010, Alfred Perlstein wrote:
>>
>>> * 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 strongly disagree. Using mutexes and condition variables in the
>> proper way is not as easy as it sounds, let alone trying to mix
>> them as userland thingies into kqueue.
>>
>> I will strongly oppose this...
>
> Well then you "win". I have no desire to engage in such discussion.
>
> I do hope that when you see this facility leveraged elsewhere for
> an application that you reflect on this conversation and think back
> on it the next time an opportunity presents itself to lead in
> functionality.
>
Alfred:
I have to say lead would only be in the *nix domain.. the windows
world (dare I say that)
has had the ability to mix things like condition variables and fd's
for ages.. or at
least so I have been told (I have never coded in windows)...
But it would be nice to have a similar ability in FreeBSD.. but with
DE so against it
I will just find another way ..
I guess what will happen is I will end up creating my own condition
variable. I have
never been a fan of the mutex tied to it... so I may not build that
unless folks
ask for it ;-) (I usually tend to be receptive to new features)....
R
------------------------------
Randall Stewart
803-317-4952 (cell)
803-345-0391(direct)
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