netdev - Re: What's the right way to use a *large* number of source addresses?

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Message-ID: <CACP96tQMEQcTPZaCT7mehis-H5Sw_-74F6fSvZxB7Lr3Hqo0kQ@mail.gmail.com>
Date:	Fri, 23 May 2014 08:53:18 -0400
From:	sowmini varadhan <sowmini05@...il.com>
To:	Niels Möller <nisse@...thpole.se>
Cc:	netdev <netdev@...r.kernel.org>, Jonas Bonn <jonas@...thpole.se>
Subject: Re: What's the right way to use a *large* number of source addresses?

On Fri, May 23, 2014 at 5:38 AM, Niels Möller <nisse@...thpole.se> wrote:

> 1. Simply assign all addresses to be used to the interface, fixing any
>    remaining performance problems.
>
>    I've done a simple benchmark with a script assigning n addresses
>    using "ip address add", and this seems to have O(n^2) complexity.
>    E.g, assigning n=25500 addresses took 26 s, and doubling n, assigning
>    51000 addresses, took 122 s, 4.6 times longer. Which isn't
>    necessarily a problems once all the addresses are assigned, but it
>    sounds a bit like there's a linear datastructure in there, not
>    intended for a large number of addresses.

I think the issue here is the netlink API: if you try to do the same thing
with ifconfig (instead of /sbin/ip), you'll find things are much faster,
which seems paradoxical because ip(7) and netlink sockets are the
recommended config paths. When I ran into this, it turned out that the
difference was due to the implementation- the ioctl path was able to
use the ifr_name efficiently to identify the target interface, unlike
the netlink config path.

But to solve  your specific problem, since you are implementing
a packet generator, wouldnt it be easier to craft the packet and feed
it over RAW or PF_PACKET sockets, allowing you more flexibility
(if you need it down the road) to set all fields, not just the
IP source address? Might mean a bit more work in your user-space,
to compute checksums etc. though.

--Sowmini
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