netdev - Re: Scalability of interface creation and deletion

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Date:	Sun, 08 May 2011 09:12:22 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Alex Bligh <alex@...x.org.uk>
Cc:	netdev@...r.kernel.org,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: Scalability of interface creation and deletion

Le samedi 07 mai 2011 à 20:42 +0200, Eric Dumazet a écrit :
> Here is my trace here for one device deletion on one 8 core machine
> 
> [  800.447012] synchronize_rcu() in 15787 us
> [  800.455013] synchronize_rcu() in 7682 us
> [  800.464019] rcu_barrier() in 8487 us
> 
> Not that bad.
> 
> $ grep RCU .config
> # RCU Subsystem
> CONFIG_TREE_RCU=y
> # CONFIG_PREEMPT_RCU is not set
> CONFIG_RCU_TRACE=y
> CONFIG_RCU_FANOUT=32
> # CONFIG_RCU_FANOUT_EXACT is not set
> # CONFIG_RCU_FAST_NO_HZ is not set
> CONFIG_TREE_RCU_TRACE=y
> 

By the way, if I change HZ from 1000 to 100 I now have ten times slower
result :

# ip link add link eth0 eth0.103 type vlan id 103
# time ip link del eth0.103

real	0m0.430s
user	0m0.000s
sys	0m0.000s

So all this is related to your HZ value, even in a CONFIG_NO_HZ=y
kernel. Alex, I guess you have HZ=250 ?

# uname -a
Linux svivoipvnx021 2.6.39-rc6-00214-g5511a34-dirty #574 SMP Sun May 8
08:44:14 CEST 2011 x86_64 x86_64 x86_64 GNU/Linux
# cat /proc/cmdline

I enabled CONFIG_RCU_FAST_NO_HZ and got worse results (but not
alsways... its very variable)

# time ip link del eth0.103

real	0m0.544s
user	0m0.000s
sys	0m0.000s


# time ip link del eth0.103

real	0m0.414s
user	0m0.000s
sys	0m0.000s


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