pthread_lock

[Prabhu nath]

I guess we should not mix mutex and condition variable. Both have their own
respective semantics.
*mutex* is used to serialize access to a shared resource among competing
threads.
*condition variable* is used to notify a* state change* of a resource to
the interested thread.

In case of condition variable there is provision to explicitly notify a
single thread(pthread_cond_signal) or all the threads waiting on a
condition (or a state change) (pthread_cond_broadcast)

Question was on pthread_mutex_unlock() that whether this function
invocation will trigger the movement of all the threads in the wait queue
to the ready queue.
If all the threads are of equal priority, then the first thread waiting for
the lock will be put to READY queue.
If there are variable priority threads waiting for the lock, then the
thread with highest priority would be woken up

Regards,
Prabhunath G
Linux Trainer
Bangalore


On Tue, Mar 5, 2013 at 3:35 PM, ishare <june.tune.sea at gmail.com> wrote:

> On Tue, Mar 05, 2013 at 01:39:54PM +0530, Mandeep Sandhu wrote:
> > On Tue, Mar 5, 2013 at 11:32 AM,  <Valdis.Kletnieks at vt.edu> wrote:
> > > On Tue, 05 Mar 2013 11:02:45 +0530, Mandeep Sandhu said:
> > >
> > >> next schedule. I think the waiting threads (processes) will moved from
> > >> the wait queue to the run queue from where they will be scheduled to
> > >> run.
> > >
> > > For bonus points, read source code and/or comments and figure out what
> > > Linux does to prevent the 'thundering herd' problem (consider 100
> threads
> > > all waiting on the same mutex - if you blindly wake all 100 up, you'll
> schedule
> > > them all, the first will find the mutex available and then re-take it,
> and
> > > then the next 99 will get run only to find it contended and go back to
> > > sleep.  So figure out what Linux does in that case. :)
> >
> > Googling around, I found the 'thundering herd' being mentioned in
> > relation to threads waiting on sockets using the accept() sys call.
> > Are wait's on mutex's also plagued by the same issue? I guess it is,
> > though what sys call would be used in this case?
>
>  the threads waiting on sockets will be waked up by net event.
>  similarly,the waiters  on mutex's can be wake up by signal.I guess it is
> pthread_cont_signal
>
>
> >
> > Thanks,
> > -mandeep
>
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