Re: [PATCH RFC ticketlock] Auto-queued ticketlock

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Tue, Jun 11, 2013 at 02:10:31PM -0400, Steven Rostedt wrote:
> On Tue, 2013-06-11 at 10:53 -0700, Davidlohr Bueso wrote:
> 
> > I hate to be the bearer of bad news but I got some pretty bad aim7
> > performance numbers with this patch on an 8-socket (80 core) 256 Gb
> > memory DL980 box against a vanilla 3.10-rc4 kernel:
> 
> This doesn't surprise me as the spin lock now contains a function call
> on any contention. Not to mention the added i$ pressure on the embedded
> spinlock code having to setup a function call.
> 
> Even if the queues are not used, it adds a slight overhead to all
> spinlocks, due to the code size increase as well as a function call on
> all contention, which will also have an impact on i$ and branch
> prediction.

Was this system hyperthreaded?  If so, it might be suffering from the
misplaced cpu_relax(), which would mean that hardware threads spinning
on the lock would fail to inform the CPU that it was not doing anything
useful.

							Thanx, Paul

> > * shared workload: 
> > 10-100 users is in the noise area.
> > 100-2000 users: -13% throughput.
> > 
> > * high_systime workload: 
> > 10-700 users is in the noise area.
> > 700-2000 users: -55% throughput.
> > 
> > * disk:
> > 10-100 users -57% throughput.
> > 100-1000 users: -25% throughput
> > 1000-2000 users: +8% throughput (this patch only benefits when we have a
> 
> Perhaps this actually started using the queues?
> 
> > lot of concurrency).
> > 
> > * custom:
> > 10-100 users: -33% throughput.
> > 100-2000 users: -46% throughput.
> > 
> > * alltests:
> > 10-1000 users is in the noise area.
> > 1000-2000 users: -10% throughput.
> > 
> > One notable exception is the short workload where we actually see
> > positive numbers:
> > 10-100 users: +40% throughput.
> > 100-2000 users: +69% throughput.
> 
> Perhaps short work loads have a cold cache, and the impact on cache is
> not as drastic?
> 
> It would be interesting to see what perf reports on these runs.
> 
> -- Steve
> 
>