Netmap mempool

[Luigi Rizzo]

On Thu, Jan 22, 2015 at 10:12 AM, Ciprian Barbu
<ciprian.barbu at linaro.org> wrote:
> Hello,
>
> I have some questions related to OpenDataPlane. I have implemented the
> I/O access using netmap for ODP but currently it's suffering from low
> performance due to packets being copied between the netmap slots and
> ODP buffers.
>
> I want to create ODP buffer pools on top of netmap memory but I'm
> concerned with three things:
> 1. how to keep a buffer for as long as needed without affecting the
> receive process; I guess I could use the NS_BUF_CHANGED flag in some
> way
> 2. the netmap memory may not be large enough to accommodate a complex
> ODP application
> 3. the ODP application may occasionally hold on to buffers for longer time
>

there is a way to hold buffers without returning them to the ring
using the 'cur' pointer in the ring, see the netmap manpage how this
is implemented and how it works on the tx and rx path.

However, before embarking in zero copy  i'd like you to elaborate on what
you mean for "low performance" (how many pps you see) and what are
your expectations. In my experience low performance almost always comes
from insufficient batching or other application bottlenecks.

Zero copy tricks come with great complications to track who owns what,
sometimes with additional hidden overhead (e.g. if you change buffers the driver
has to reprogram the IOMMU) and often have a very limited effect
in real world applications.

Sure the numbers do look good on benchmarks or academic papers, but there
people don't worry about robustness and code maintainability.

cheers
luigi