Subj : Re: NPTL rwlock is really suboptimal
To   : comp.programming.threads
From : roger.faulkner
Date : Thu Jan 27 2005 06:08 pm


Joseph Seigh wrote:
> It appears to give reader preference so if you have a lot of reader
thread
> activity, the writer threads starve, i.e. hang indefinitely.  rwlocks
are
> a strategy to reduce contention but if you have any kind of
contention it
> starves the writers.  You're better off using a mutex.
>
> I'll have to see how Solaris rwlock handles this if I get around to
porting
> the atomic_ptr stuff to Solaris again.

In Solaris 9 and 10, preference is given to writers.
In Solaris 10, the priority of the threads is taken into account,
as specified for UNIX03:

The pthread_rwlock_rdlock() function applies a read lock  to
the  read-write  lock  referenced  by  rwlock.   The calling
thread acquires the read lock if a writer does not hold  the
lock and there are no writers blocked on the lock.

The calling thread does not acquire the  lock  if  a  writer
holds the lock or if writers of higher or equal priority are
blocked on the lock; otherwise, the calling thread  acquires
the  lock.  If  the  read  lock is not acquired, the calling
thread blocks until it can acquire the lock.
Roger Faulkner
roger.faulkner@sun.com

.