Re: RCU related performance regression in 3.3

From: Pascal Chapperon
Date: Wed Apr 18 2012 - 11:00:29 EST

Le 18/04/2012 16:01, Paul E. McKenney a Ãcrit :
On Wed, Apr 18, 2012 at 11:37:28AM +0200, Pascal Chapperon wrote:
Mount and umount operations are not slower with RCU_FAST_NO_HZ during
runtime; systemctl start and stop operations are also not slower. In
fact, i couldn't find a single operation slower during runtime with

Your boot-time setup is such that all CPUs are online before the
boot-time mount operations take place, right?
Yes :
[ 0.242697] Brought up 8 CPUs
[ 0.242699] Total of 8 processors activated (35118.33 BogoMIPS).

Struggling to understand
how RCU can tell the difference between post-CPU-bringup boot time
and run time...

systemd is controlling the whole boot process including mount
operation (apart root filesystem) and as I can see, uses heavily
sockets to do it (not to mention cpu-affinity). It controls also the
major part of umount operations. Is it possible that your patch hits
a systemd bug ?
What I don't understand is that systemd coexists well with
RCU_FAST_NO_HZ on a smaller laptop with older and much less powerful
I'll do further tests on another machine.


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