Re: tree rcu: call_rcu scalability problem?

[Paul E. McKenney]

On Thu, Sep 03, 2009 at 11:01:26AM +0200, Nick Piggin wrote:
> On Wed, Sep 02, 2009 at 10:14:27PM -0700, Paul E. McKenney wrote:
> > >From 0544d2da54bad95556a320e57658e244cb2ae8c6 Mon Sep 17 00:00:00 2001
> > From: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> > Date: Wed, 2 Sep 2009 22:01:50 -0700
> > Subject: [PATCH] Remove grace-period machinery from rcutree __call_rcu()
> > 
> > The grace-period machinery in __call_rcu() was a failed attempt to avoid
> > implementing synchronize_rcu_expedited().  But now that this attempt has
> > failed, try removing the machinery.
> 
> OK, the workload is parallel processes performing a close(open()) loop
> in a tmpfs filesystem within different cwds (to avoid contention on the
> cwd dentry). The kernel is first patched with my vfs scalability patches,
> so the comparison is with/without Paul's rcu patch.
> 
> System is 2s8c opteron, with processes bound to CPUs (first within the
> same socket, then over both sockets as count increases).
> 
> procs  tput-base          tput-rcu
> 1         595238 (x1.00)    645161 (x1.00)
> 2        1041666 (x1.75)   1136363 (x1.76)
> 4        1960784 (x3.29)   2298850 (x3.56)
> 8        3636363 (x6.11)   4545454 (x7.05)
> 
> Scalability is improved (from 2-8 way it is now actually linear), and
> single thread performance is significantly improved too.
> 
> oprofile results collecting clk unhalted samples shows the following
> results for __call_rcu symbol:
> 
> procs  samples  %        app name                 symbol name
> tput-base
> 1      12153     3.8122  vmlinux                  __call_rcu
> 2      29253     3.9899  vmlinux                  __call_rcu
> 4      84503     5.4667  vmlinux                  __call_rcu
> 8      312816    9.5287  vmlinux                  __call_rcu
> 
> tput-rcu
> 1      8722      2.8770  vmlinux                  __call_rcu
> 2      17275     2.5804  vmlinux                  __call_rcu
> 4      33848     2.6015  vmlinux                  __call_rcu
> 8      67158     2.5561  vmlinux                  __call_rcu
> 
> Scaling is cearly much better (it is more important to look at absolute
> samples because %age is dependent on other parts of the kernel too).
> 
> Feel free to add any of this to your changelog if you think it's important.

Very cool!!!

I got a dissenting view from the people trying to get rid of interrupts
in computational workloads.  But I believe that it is possible to
split the difference, getting you almost all the performance benefits
while still permitting them to turn off the scheduling-clock interrupt.
The reason that I believe it should get you the performance benefits is
that deleting the rcu_process_gp_end() and check_for_new_grace_period()
didn't do much for you.  Their overhead is quite small compared to
hammering the system with a full set of IPIs every ten microseconds
or so.  ;-)

So could you please give the following experimental patch a go?
If it works for you, I will put together a production-ready patch
along these lines.

							Thanx, Paul

------------------------------------------------------------------------