[Cerowrt-devel] Linux network is damn fast, need more use XDP (Was: [Bloat] DC behaviors today)

Jesper Dangaard Brouer brouer at redhat.com
Mon Dec 4 05:56:51 EST 2017

On Sun, 03 Dec 2017 20:19:33 -0800 Dave Taht <dave at taht.net> wrote:

> Changing the topic, adding bloat.

Adding netdev, and also adjust the topic to be a rant on that the Linux
kernel network stack is actually damn fast, and if you need something
faster then XDP can solved your needs...

> Joel Wirāmu Pauling <joel at aenertia.net> writes:
> > Just from a Telco/Industry perspective slant.
> >
> > Everything in DC has moved to SFP28 interfaces at 25Gbit as the server
> > port of interconnect. Everything TOR wise is now QSFP28 - 100Gbit.
> > Mellanox X5 cards are the current hotness, and their offload
> > enhancements (ASAP2 - which is sorta like DPDK on steroids) allows for
> > OVS flow rules programming into the card. We have a lot of customers
> > chomping at the bit for that feature (disclaimer I work for Nuage
> > Networks, and we are working on enhanced OVS to do just that) for NFV
> > workloads.  
> What Jesper's been working on for ages has been to try and get linux's
> PPS up for small packets, which last I heard was hovering at about
> 4Gbits.

I hope you made a typo here Dave, the normal Linux kernel is definitely
way beyond 4Gbit/s, you must have misunderstood something, maybe you
meant 40Gbit/s? (which is also too low)

Scaling up to more CPUs and TCP-stream, Tariq[1] and I have showed the
Linux kernel network stack scales to 94Gbit/s (linerate minus overhead).
But when the drivers page-recycler fails, we hit bottlenecks in the
page-allocator, that cause negative scaling to around 43Gbit/s.

[1] http://lkml.kernel.org/r/cef85936-10b2-5d76-9f97-cb03b418fd94@mellanox.com

Linux have for a _long_ time been doing 10Gbit/s TCP-stream easily, on
a SINGLE CPU.  This is mostly thanks to TSO/GRO aggregating packets,
but last couple of years the network stack have been optimized (with
UDP workloads), and as a result we can do 10G without TSO/GRO on a
single-CPU.  This is "only" 812Kpps with MTU size frames.

It is important to NOTICE that I'm mostly talking about SINGLE-CPU
performance.  But the Linux kernel scales very well to more CPUs, and
you can scale this up, although we are starting to hit scalability
issues in MM-land[1].

I've also demonstrated that netdev-community have optimized the kernels
per-CPU processing power to around 2Mpps.  What does this really
mean... well with MTU size packets 812Kpps was 10Gbit/s, thus 25Gbit/s
should be around 2Mpps.... That implies Linux can do 25Gbit/s on a
single CPU without GRO (MTU size frames).  Do you need more I ask?

> The route table lookup also really expensive on the main cpu.

Well, it used-to-be very expensive. Vincent Bernat wrote some excellent
blogposts[2][3] on the recent improvements over kernel versions, and
gave due credit to people involved.

[2] https://vincent.bernat.im/en/blog/2017-performance-progression-ipv4-route-lookup-linux
[3] https://vincent.bernat.im/en/blog/2017-performance-progression-ipv6-route-lookup-linux

He measured around 25 to 35 nanosec cost of route lookups.  My own
recent measurements were 36.9 ns cost of fib_table_lookup.

> Does this stuff offload the route table lookup also?

If you have not heard, the netdev-community have worked on something
called XDP (eXpress Data Path).  This is a new layer in the network
stack, that basically operates a the same "layer"/level as DPDK.
Thus, surprise we get the same performance numbers as DPDK. E.g. I can
do 13.4 Mpps forwarding with ixgbe on a single CPU (more CPUs=14.6Mps)

We can actually use XDP for (software) offloading the Linux routing
table.  There are two methods we are experimenting with:

(1) externally monitor route changes from userspace and update BPF-maps
to reflect this. That approach is already accepted upstream[4][5].  I'm
measuring 9,513,746 pps per CPU with that approach.

(2) add a bpf helper to simply call fib_table_lookup() from the XDP hook.
This is still experimental patches (credit to David Ahern), and I've
measured 9,350,160 pps with this approach in a single CPU.  Using more
CPUs we hit 14.6Mpps (only used 3 CPUs in that test)

[4] https://github.com/torvalds/linux/blob/master/samples/bpf/xdp_router_ipv4_user.c
[5] https://github.com/torvalds/linux/blob/master/samples/bpf/xdp_router_ipv4_kern.c

Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer

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