On Sun, Nov 2, 2014 at 6:53 PM, Attilio Rao <atti...@freebsd.org> wrote: > On Sun, Nov 2, 2014 at 6:49 PM, Konstantin Belousov <kostik...@gmail.com> > wrote: >> On Sun, Nov 02, 2014 at 06:07:20PM +0100, Attilio Rao wrote: >>> On Sun, Nov 2, 2014 at 5:59 PM, Konstantin Belousov <kostik...@gmail.com> >>> wrote: >>> > It is easy and cheap to record the set of the owned lockmgr locks for >>> > current thread. I do not believe that we have a situation where more >>> > than 3 locks are held in one time. To give it some slack, we can record >>> > 8 locks shared-owned and do the check for recursion in LK_CAN_SHARE(). >>> > If the thread-locks table overflows, we could either panic() or fall >>> > back to indiscriminative deadlock avoidance (i.e. relying on the sole >>> > td_lk_slocks value). >>> >>> I don't think it is going to be cheap (and certainly it is not viable >>> for things like sx locks and rwlocks). >> sx and rw do not implement exclusive starvation avoidance. > > rw do. > It is believed for sx it should not be too helpful, on FreeBSD I've > never seen an sx that is particulary congested. > It can be added, however and the algorithm they would use is the same than rw. > >>> Checking for the owner chain anytime you acquire the lock is certainly >>> not I would call cheap. >> >> I did not proposed to verify owner chain. I said that it is easy to >> record the locks owned by current thread, only for current thread >> consumption. Below is the prototype. > > I think it is too expensive, think that this must happen for every shared > lock. > I know we may not be using too many shared locks on lockmgr right now, > but it is not a good reason to make shared lock bloated and more > expensive on lockmgr.
And to be honest, I find already wrong the fact that we don't have a faster path for lockmgr() (ie. inlined atomics). I think this sloppiness (at least on my side) cames on the fact that locking overhead is not a major factor for the performance of current lockmgr users. That doesn't mean, however, that we should have a slower primitive. Attilio -- Peace can only be achieved by understanding - A. Einstein _______________________________________________ svn-src-head@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/svn-src-head To unsubscribe, send any mail to "svn-src-head-unsubscr...@freebsd.org"
