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SubjectRe: tree rcu: call_rcu scalability problem?
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@linux.vnet.ibm.com>
> > 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

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

From 57b7f98303a5c5aa50648c71758760006af49bab Mon Sep 17 00:00:00 2001
From: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Date: Thu, 3 Sep 2009 06:19:45 -0700
Subject: [PATCH] Reduce grace-period-encouragement impact on rcutree __call_rcu()

Remove only the emergency force_quiescent_state() from __call_rcu(),
which should get most of the reduction in overhead while still
allowing the tick to be turned off when non-idle, as proposed in
http://lkml.org/lkml/2009/9/1/229, and which reduced interrupts to
one per ten seconds in a CPU-bound computational workload according to
http://lkml.org/lkml/2009/9/3/7.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
kernel/rcutree.c | 1 -
1 files changed, 0 insertions(+), 1 deletions(-)

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index d2a372f..4c8e0d2 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -1220,7 +1220,6 @@ __call_rcu(struct rcu_head *head, void (*func)(struct rcu_head *rcu),
/* Force the grace period if too many callbacks or too long waiting. */
if (unlikely(++rdp->qlen > qhimark)) {
rdp->blimit = LONG_MAX;
- force_quiescent_state(rsp, 0);
} else if ((long)(ACCESS_ONCE(rsp->jiffies_force_qs) - jiffies) < 0)
force_quiescent_state(rsp, 1);
local_irq_restore(flags);
--
1.5.2.5


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