On Fri, Jan 31, 2014 at 01:59:02PM -0500, Waiman Long wrote: > On 01/31/2014 04:26 AM, Peter Zijlstra wrote: > >On Thu, Jan 30, 2014 at 04:17:15PM +0100, Peter Zijlstra wrote: > >>The below is still small and actually works. > >OK, so having actually worked through the thing; I realized we can > >actually do a version without MCS lock and instead use a ticket lock for > >the waitqueue. > > > >This is both smaller (back to 8 bytes for the rwlock_t), and should be > >faster under moderate contention for not having to touch extra > >cachelines. > > > >Completely untested and with a rather crude generic ticket lock > >implementation to illustrate the concept: > > > > Using a ticket lock instead will have the same scalability problem as the > ticket spinlock as all the waiting threads will spin on the lock cacheline > causing a lot of cache bouncing traffic. A much more important point for me is that a fair rwlock has a _much_ better worst case behaviour than the current mess. That's the reason I was interested in the qrwlock thing. Not because it can run contended on a 128 CPU system and be faster at being contended. If you contend a lock with 128 CPUs you need to go fix that code that causes this abysmal behaviour in the first place. -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
