Date: Tue, 13 Sep 2005 09:46:12 -0700
From: Stephen Hemminger <shemminger@osdl.org>
To: Ben Greear <greearb@candelatech.com>
Cc: Ravikiran G Thirumalai <kiran@scalex86.org>, Andrew Morton <akpm@osdl.org>,
       linux-kernel@vger.kernel.org, dipankar@in.ibm.com, bharata@in.ibm.com,
       shai@scalex86.org, Rusty Russell <rusty@rustcorp.com.au>,
       netdev@vger.kernel.org, davem@davemloft.net
Subject: Re: [patch 7/11] net: Use bigrefs for net_device.refcount
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On Tue, 13 Sep 2005 09:35:14 -0700
Ben Greear <greearb@candelatech.com> wrote:

> Stephen Hemminger wrote:
> > On Tue, 13 Sep 2005 09:10:12 -0700
> > Ravikiran G Thirumalai <kiran@scalex86.org> wrote:
> > 
> > 
> >>The net_device has a refcnt used to keep track of it's uses.
> >>This is used at the time of unregistering the network device
> >>(module unloading ..) (see netdev_wait_allrefs) .
> >>For loopback_dev , this refcnt increment/decrement  is causing
> >>unnecessary traffic on the interlink for NUMA system
> >>affecting it's performance.  This patch improves tbench numbers by 6% on a
> >>8way x86 Xeon (x445).
> >>
> > 
> > 
> > Since when is bringing a network device up/down performance critical?
> 
> We grab and drop a reference for each poll of a device, roughly.
> 
> See dev_hold in _netif_rx_schedule(struct net_device *dev)
> in include/netdevice.h, for instance.

Yeah, that would be an issue, especially since the rest of that
path is nicely per-cpu
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