Re: [RFC PATCH] introduce sys_membarrier(): process-wide memory barrier

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

On Fri, Jan 08, 2010 at 05:21:28PM -0800, Paul E. McKenney wrote:
> On Fri, Jan 08, 2010 at 08:02:31PM -0500, Mathieu Desnoyers wrote:
> > * Paul E. McKenney (paulmck@linux.vnet.ibm.com) wrote:
> > > On Fri, Jan 08, 2010 at 06:53:38PM -0500, Mathieu Desnoyers wrote:
> > > > * Steven Rostedt (rostedt@goodmis.org) wrote:
> > > > > Well, if we just grab the task_rq(task)->lock here, then we should be
> > > > > OK? We would guarantee that curr is either the task we want or not.
> > > > 
> > > > Hrm, I just tested it, and there seems to be a significant performance
> > > > penality involved with taking these locks for each CPU, even with just 8
> > > > cores. So if we can do without the locks, that would be preferred.
> > > 
> > > How significant?  Factor of two?  Two orders of magnitude?
> > > 
> > 
> > On a 8-core Intel Xeon (T is the number of threads receiving the IPIs):
> > 
> > Without runqueue locks:
> > 
> > T=1: 0m13.911s
> > T=2: 0m20.730s
> > T=3: 0m21.474s
> > T=4: 0m27.952s
> > T=5: 0m26.286s
> > T=6: 0m27.855s
> > T=7: 0m29.695s
> > 
> > With runqueue locks:
> > 
> > T=1: 0m15.802s
> > T=2: 0m22.484s
> > T=3: 0m24.751s
> > T=4: 0m29.134s
> > T=5: 0m30.094s
> > T=6: 0m33.090s
> > T=7: 0m33.897s
> > 
> > So on 8 cores, taking spinlocks for each of the 8 runqueues adds about
> > 15% overhead when doing an IPI to 1 thread. Therefore, that won't be
> > pretty on 128+-core machines.
> 
> But isn't the bulk of the overhead the IPIs rather than the runqueue
> locks?
> 
>      W/out RQ       W/RQ   % degradation
> T=1:    13.91      15.8    1.14
> T=2:    20.73      22.48   1.08
> T=3:    21.47      24.75   1.15
> T=4:    27.95      29.13   1.04
> T=5:    26.29      30.09   1.14
> T=6:    27.86      33.09   1.19
> T=7:    29.7       33.9    1.14

Right...  s/% degradation/Ratio/  :-/

							Thanx, Paul

> So if we had lots of CPUs, we might want to fan the IPIs out through
> intermediate CPUs in a tree fashion, but the runqueue locks are not
> causing excessive pain.
> 
> How does this compare to use of POSIX signals?  Never mind, POSIX
> signals are arbitrarily bad if you have way more threads than are
> actually running at the time...
> 
> 							Thanx, Paul