Subject: Re: Designing a safe RX-zero-copy Memory Model for Networking

From: Jesper Dangaard Brouer <[email-masked]>
Date: Tue, 13 Dec 2016 17:10:28 +0100

On Mon, 12 Dec 2016 12:06:59 -0600 (CST) Christoph Lameter <[email-masked]> wrote:
> On Mon, 12 Dec 2016, Jesper Dangaard Brouer wrote:
> 
> > Hmmm. If you can rely on hardware setup to give you steering and
> > dedicated access to the RX rings.  In those cases, I guess, the "push"
> > model could be a more direct API approach.  
> 
> If the hardware does not support steering then one should be able to
> provide those services in software.

This is the early demux problem.  With the push-mode of registering
memory, you need hardware steering support, for zero-copy support, as
the software step happens after DMA engine have written into the memory.

My model pre-VMA map all the pages in the RX ring (if zero-copy gets
enabled, by a single user).  The software step can filter and zero-copy
send packet-pages to the application/socket that requested this. The
disadvantage is all zero-copy application need to share this VMA
mapping.  This is solved by configuring HW filters into a RX-queue, and
then only attach your zero-copy application to that queue.

> > I was shooting for a model that worked without hardware support.
> > And then transparently benefit from HW support by configuring a HW
> > filter into a specific RX queue and attaching/using to that queue.  
> 
> The discussion here is a bit amusing since these issues have been
> resolved a long time ago with the design of the RDMA subsystem. Zero
> copy is already in wide use. Memory registration is used to pin down
> memory areas. Work requests can be filed with the RDMA subsystem that
> then send and receive packets from the registered memory regions.
> This is not strictly remote memory access but this is a basic mode of
> operations supported  by the RDMA subsystem. The mlx5 driver quoted
> here supports all of that.

I hear what you are saying.  I will look into a push-model, as it might
be a better solution.
 I will read up on RDMA + verbs and learn more about their API model.  I
even plan to write a small sample program to get a feeling for the API,
and maybe we can use that as a baseline for the performance target we
can obtain on the same HW. (Thanks to Björn for already giving me some
pointer here)

> What is bad about RDMA is that it is a separate kernel subsystem.
> What I would like to see is a deeper integration with the network
> stack so that memory regions can be registred with a network socket
> and work requests then can be submitted and processed that directly
> read and write in these regions. The network stack should provide the
> services that the hardware of the NIC does not suppport as usual.

Interesting.  So you even imagine sockets registering memory regions
with the NIC.  If we had a proper NIC HW filter API across the drivers,
to register the steering rule (like ibv_create_flow), this would be
doable, but we don't (DPDK actually have an interesting proposal[1])

> The RX/TX ring in user space should be an additional mode of
> operation of the socket layer. Once that is in place the "Remote
> memory acces" can be trivially implemented on top of that and the
> ugly RDMA sidecar subsystem can go away.
 
I cannot follow that 100%, but I guess you are saying we also need a
more efficient mode of handing over pages/packet to userspace (than
going through the normal socket API calls).

Appreciate your input, it challenged my thinking.
-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer

[1] https://rawgit.com/6WIND/rte_flow/master/rte_flow.html

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