Subject: Re: iptables very slow after commit 784544739a25c30637397ace5489eeb6e15d7d49

From: Stephen Hemminger <[email-masked]>
Date: Sat, 11 Apr 2009 08:05:54 -0700

On Sat, 11 Apr 2009 09:08:54 +0200
Ingo Molnar <[email-masked]> wrote:

> 
> * Paul E. McKenney <[email-masked]> wrote:
> 
> > 	I will nevertheless suggest the following egregious hack to
> > 	get a consistent sample of one counter for some other CPU:
> > 
> > 	a.	Disable interrupts
> > 	b.	Atomically exchange the bottom 32 bits of the
> > 		counter with the value zero.
> > 	c.	Atomically exchange the top 32 bits of the counter
> > 		with the value zero.
> > 	d.	Concatenate the values obtained in (b) and (c), which
> > 		is the snapshot value.
> 
> Note, i have recently implemented full atomic64_t support on 32-bit 
> x86, for the perfcounters code, based on the CMPXCHG8B instruction.
> 
> Which, while not the lightest of instructions, is still much better 
> than the sequence above.
> 
> So i think a better approach would be to also add a dumb generic 
> implementation for atomic64_t (using a global lock or so), and then 
> generic code could just assume that atomic64_t always exists.
> 
> It is far nicer - and faster as well - as the hack above, even on 
> 32-bit x86.
> 
> 	Ingo

The iptables counters are write mostly, read rarely so they don't
fit the seq counter or atomic use case. Also, it is important
to get a consistent snapshot of the whole set not just each
individual counter.