netdev - Re: [PATCH net-next-2.6] ipv4: speedup inet_dump

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Message-ID: <20091112111218.0a561166@nehalam>
Date:	Thu, 12 Nov 2009 11:12:18 -0800
From:	Stephen Hemminger <shemminger@...tta.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	"David S. Miller" <davem@...emloft.net>,
	Linux Netdev List <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next-2.6] ipv4: speedup inet_dump_ifaddr()

On Thu, 12 Nov 2009 18:44:25 +0100
Eric Dumazet <eric.dumazet@...il.com> wrote:

> Stephen Hemminger a écrit :
> > On Thu, 12 Nov 2009 15:11:36 +0100
> > Eric Dumazet <eric.dumazet@...il.com> wrote:
> > 
> >> When handling large number of netdevices, inet_dump_ifaddr()
> >> is very slow because it has O(N^2) complexity.
> >>
> >> Instead of scanning one single list, we can use the NETDEV_HASHENTRIES
> >> sub lists of the dev_index hash table, and RCU lookups.
> >>
> >> Signed-off-by: Eric Dumazet <eric.dumazet@...il.com>
> > 
> > You might be able to make RCU critical section smaller by moving
> > it into loop.
> > 
> 
> Indeed. But we dump at most one skb (<= 8192 bytes ?), so rcu_read_lock
> holding time is small, unless we meet many netdevices without
> addresses. I wonder if its really common...
> 
> Thanks

One case where that might happen is:
  modprobe dummy numdummies=10000

But dummy device should really be added with netlink, not at boot time.

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