netdev - Re: Supporting more devices with dev_alloc

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Date:	Thu, 25 Oct 2012 19:50:43 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Tom Parkin <tparkin@...alix.com>
Cc:	netdev <netdev@...r.kernel.org>
Subject: Re: Supporting more devices with dev_alloc_name()

On Thu, 2012-10-25 at 17:56 +0100, Tom Parkin wrote:
> Hi list,
> 
> I've recently been trying to create large-scale L2TPv3 configurations
> of up to 50,000 Ethernet pseudowires.
> 
> One of the limitations I've hit has been to do with dev_alloc_name().
> By default, l2tp_eth uses "l2tpeth%d" for device names, which is
> expanded by dev_alloc_name() into l2tpeth1, l2tpeth2, etc.  However,
> the algorithm dev_alloc_name() uses to derive the next free number for
> this scheme is bounded by the number of bits in a single page.  For
> kernels/platforms with a 4kB page, this limits these "autoderived"
> names to 32k.
> 
> In my testing I've been able to work around this by specifying
> interface names during the bringup of the l2tpeth interfaces, thereby
> bypassing dev_alloc_name() altogether.  Using this approach I am able
> to comfortably create 50k interfaces, even on fairly modest hardware.
> But it seems a shame to have to do this; it would be much nicer if
> the kernel were able to autogenerate names for more devices.
> 
> Is this something that would be worth my working on a patch for, or
> would the increased code complexity be too great an overhead to
> consider for such outlandish use-cases?

This issue was raised some ago, for the dummy device

modprobe dummy numdummies=33000

I guess each format (eg eth%d) could be attached/associated to an idr,
so that we can find the lowest available index very fast.



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