netdev - Re: [PATCH net-next] net: percpu net

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Message-ID: <1286605396.2692.10.camel@edumazet-laptop>
Date:	Sat, 09 Oct 2010 08:23:16 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	paulmck@...ux.vnet.ibm.com
Cc:	Stephen Hemminger <shemminger@...tta.com>,
	David Miller <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next] net: percpu net_device refcount

Le vendredi 08 octobre 2010 à 14:56 -0700, Paul E. McKenney a écrit :
> On Thu, Oct 07, 2010 at 10:30:51AM -0700, Stephen Hemminger wrote:
> > On Thu, 07 Oct 2010 19:12:35 +0200
> > Eric Dumazet <eric.dumazet@...il.com> wrote:
> > 
> > > We tried very hard to remove all possible dev_hold()/dev_put() pairs in
> > > network stack, using RCU conversions.
> > > 
> > > There is still an unavoidable device refcount change for every dst we
> > > create/destroy, and this can slow down some workloads (routers or some
> > > app servers)
> > > 
> > > We can switch to a percpu refcount implementation, now dynamic per_cpu
> > > infrastructure is mature. On a 64 cpus machine, this consumes 256 bytes
> > > per device.
> > 
> > It makes sense, but what about 256 cores and 1024 Vlans?
> > That adds up to 4M of memory which is might be noticeable.
> 
> I bet that systems that have 256 cores have >100GB of memory, at which
> point 4MB is way down in the noise.

Well, first its 1MB added, and secondly we added percpu stats for vlan
devices, and this consumed 8x more :

(struct vlan_rx_stats is 32 bytes per cpu and per vlan
32*256*1024  ->  8 Mbytes

Some strange machines have many cores sharing a small amount of memory,
but I am not sure they want to run many net devices ;)



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