netdev - Re: [PATCH 2/2] net: dummy: allocate devices with alloc_netdev

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Date:	Tue, 07 Jun 2011 09:59:52 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Lucian Adrian Grijincu <lucian.grijincu@...il.com>
Cc:	netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH 2/2] net: dummy: allocate devices with alloc_netdev_id

Le mardi 07 juin 2011 à 10:49 +0300, Lucian Adrian Grijincu a écrit :
> On Tue, Jun 7, 2011 at 6:19 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> > Le mardi 07 juin 2011 à 04:39 +0300, Lucian Adrian Grijincu a écrit :
> >> The most like case is that no one else is registering devices with a
> >> name like "dummy%d".
> >>
> >> We can bring the complexity down by replacing:
> >> - alloc_netdev_id which is O(N) with
> >> - alloc_netdev_id which, on the average case, is O(1).
> >>
> >> $ time modprobe dummy numdummies=5000
> >> - with alloc_netdev   : 9.50s
> >> - with alloc_netdev_id: 3.50s
> >>
> >> NOTE: Stats generated on a heavily patched 3.0-rc1 which replaces the
> >>       current O(N^2) sysctl algorithm with a better one.
> >
> > Yes, and disabled hotplug I guess.
> 
> 

Some distros have :

$ cat /proc/sys/kernel/hotplug
/sbin/hotplug

http://linux.die.net/man/8/hotplug

Basically this starts a lot of process when a new device is created.

modprobe dummy numdummies=5000

This previous line ask 5000 asynchronous hotplug start, so it launches
thousands of processes, all fighting to get RTNL because they access
network configuration data.


Please note I was not commenting your patch (it seems fine at a first
glance), only warning people not doing "modprobe dummy numdummies=5000"
without thinking a bit if their machine wont crash or freeze :)



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