Netmap mempool

[Ciprian Barbu]

On Thu, Jan 22, 2015 at 9:03 PM, Luigi Rizzo <rizzo at iet.unipi.it> wrote:
> 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.

I didn't do exact measurements, but there seemed to be a bottleneck
somewhere, I will have another look, maybe I missed something.

>
> 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