netdev - Re: Scalability of interface creation and deletion

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Date:	Sun, 08 May 2011 14:06:07 +0100
From:	Alex Bligh <alex@...x.org.uk>
To:	paulmck@...ux.vnet.ibm.com
cc:	Eric Dumazet <eric.dumazet@...il.com>, netdev@...r.kernel.org,
	Alex Bligh <alex@...x.org.uk>
Subject: Re: Scalability of interface creation and deletion



--On 8 May 2011 05:44:28 -0700 "Paul E. McKenney" 
<paulmck@...ux.vnet.ibm.com> wrote:

> OK, let's break it out...
>
> 4.022 seconds for 100 interfaces means about 40 milliseconds per
> interface.

It's 200 interfaces as they are veth pairs, so 20ms.

> My guess is that you have CONFIG_NO_HZ=y, which means that RCU needs to
> figure out that various CPUs are in dyntick-idle state, which is a minimum
> of 6 jiffies.

That sounds like the main cause.

> It could be longer if a given CPU happens to be in IRQ
> when RCU checks, so call it 9 jiffies.

That sounds unlikely to happen much of the time. Looking at /proc/interrupts
not much is going on.

> If you are doing the interfaces
> synchronously, you will likely have to wait for a prior grace period (due
> to background activity).

Not sure I understand that (there's nothing else going on), but...

> So I can easily imagine 18 milliseconds for
> HZ=1000.  40 milliseconds sounds a bit high, but perhaps not impossible.

Add to that the fact that there are 2 x synchronize_sched() and one
rcu_barrier() per interface removal. If I ignore your IRQ idea, that's
6 x 3 = 18ms per interface removal at CONFIG_HZ=10, 180ms at
CONFIG_HZ=100.

-- 
Alex Bligh
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