Re: [dpdk-dev] [ovs-dev] [PATCH RFC] dpif-netdev: Add support Intel DPDK based ports.

["David Nyström" <david.c.nystrom@gmail.com>]

On 2014-01-29 21:47, François-Frédéric Ozog wrote:
>>> First and easy answer: it is open source, so anyone can recompile. So,
>>> what's the issue?
>>
>> I'm talking from a pure distribution perspective here: Requiring to
>> recompile all DPDK based applications to distribute a bugfix or to add
>> support for a new PMD is not ideal.
>
>>
>> So ideally OVS would have the possibility to link against the shared
>> library long term.
>
> I agree that distribution of DPDK apps is not covered properly at present.
> Identifying the proper scheme requires a specific analysis based on the
> constraints of the Telecom/Cloud/Networking markets.
>
> In the telecom world, if you fix the underlying framework of an app, you
> will still have to validate the solution, ie app/framework. In addition, the
> idea of shared libraries introduces the implied requirement to validate apps
> against diverse versions of DPDK shared libraries. This translates into
> development and support costs.
>
> I also expect many DPDK applications to tackle core networking features,
> with sub micro second packet handling delays  and even lower than 200ns
> (NAT64...). The lazy binding based on ELF PLT represent quite a cost, not
> mentioning that optimization stops are shared libraries boundaries (gcc
> whole program optimization can be very effective...). Microsoft DLL linkage
> are an order of magnitude faster. If Linux was to provide that, I would
> probably revise my judgment. (I haven't checked Linux dynamic linking
> implementation for some time so my understanding of Linux dynamic linking
> may be outdated).
>
>
>>
>>> I get lost: do you mean ABI + API toward the PMDs or towards the
>>> applications using the librte ?
>>
>> Towards the PMDs is more straight forward at first so it seems logical to
>> focus on that first.
>
> I don't think it is so straight forward. Many recent cards such as Chelsio
> and Myricom have a very different "packet memory layout" that does not fit
> so easily into actual DPDK architecture.
>
> 1) "traditional" architecture: the driver reserves X buffers and provide the
> card with descriptors of those buffers. Each packet is DMA'ed into exactly
> one buffer. Typically you have 2K buffers, a 64 byte packet consumes exactly
> one buffer
>
> 2) "alternative" new architecture: the driver reserves a memory zone, say
> 4MB, without any structure, and provide a a single zone description and a
> ring buffer to the card. (there no individual buffer descriptors any more).
> The card fills the memory zone with packets, one next to the other and
> specifies where the packets are by updating the supplied ring. Out of the
> many issues fitting this scheme into DPDK, you cannot free a single mbuf:
> you have to maintain a ref count to the memory zone so that, when all mbufs
> have been "released", the memory zone can be freed.
> That's quite a stretch from actual paradigm.
>
> Apart from this aspect, managing RSS is two tied to Intel's flow director
> concepts and cannot accommodate directly smarter or dumber RSS mechanisms.
>
> That said, I fully agree PMD API should be revisited.

Hi,

Sorry for jumping in late.
Perhaps you are already aware of OpenDataPlane, which can use DPDK as 
its south bound NIC interface.

>
> Cordially,
>
> François-Frédéric
>