Re: [dpdk-dev] [dpdk-announce] important design choices - statistics - ABI

[Vincent JARDIN <vincent.jardin@6wind.com>]

On 17/06/2015 14:14, Panu Matilainen wrote:
> (initially accidentally sent to announce, resending to dev)
>
> On 06/17/2015 01:35 PM, Neil Horman wrote:
>> On Wed, Jun 17, 2015 at 01:29:47AM +0200, Thomas Monjalon wrote:
>>> Hi all,
>>>
>>> Sometimes there are some important discussions about architecture or
>>> design
>>> which require opinions from several developers. Unfortunately, we cannot
>>> read every threads. Maybe that using the announce mailing list will help
>>> to bring more audience to these discussions.
>>> Please note that
>>>     - the announce@ ML is moderated to keep a low traffic,
>>>     - every announce email is forwarded to dev@ ML.
>>> In case you want to reply to this email, please use dev@dpdk.org
>>> address.
>>>
>>> There were some debates about software statistics disabling.
>>> Should they be always on or possibly disabled when compiled?
>>> We need to take a decision shortly and discuss (or agree) this proposal:
>>>     http://dpdk.org/ml/archives/dev/2015-June/019461.html
>>>
>>> During the development of the release 2.0, there was an agreement to
>>> keep
>>> ABI compatibility or to bring new ABI while keeping old one during
>>> one release.
>>> In case it's not possible to have this transition, the (exceptional)
>>> break
>>> should be acknowledged by several developers.
>>>     http://dpdk.org/doc/guides-2.0/rel_notes/abi.html
>>> There were some interesting discussions but not a lot of participants:
>>>     http://thread.gmane.org/gmane.comp.networking.dpdk.devel/8367/focus=8461
>>>
>>>
>>> During the current development cycle for the release 2.1, the ABI
>>> question
>>> arises many times in different threads.
>>> To add the hash key size field, it is proposed to use a struct
>>> padding gap:
>>>     http://dpdk.org/ml/archives/dev/2015-June/019386.html
>>> To support the flow director for VF, there is no proposal yet:
>>>     http://dpdk.org/ml/archives/dev/2015-June/019343.html
>>> To add the speed capability, it is proposed to break ABI in the
>>> release 2.2:
>>>     http://dpdk.org/ml/archives/dev/2015-June/019225.html
>>> To support vhost-user multiqueues, it is proposed to break ABI in 2.2:
>>>     http://dpdk.org/ml/archives/dev/2015-June/019443.html
>>> To add the interrupt mode, it is proposed to add a build-time option
>>> CONFIG_RTE_EAL_RX_INTR to switch between compatible and ABI breaking
>>> binary:
>>>     http://dpdk.org/ml/archives/dev/2015-June/018947.html
>>> To add the packet type, there is a proposal to add a build-time option
>>> CONFIG_RTE_NEXT_ABI common to every ABI breaking features:
>>>     http://dpdk.org/ml/archives/dev/2015-June/019172.html
>>> We must also better document how to remove a deprecated ABI:
>>>     http://dpdk.org/ml/archives/dev/2015-June/019465.html
>>> The ABI compatibility is a new constraint and we need to better
>>> understand
>>> what it means and how to proceed. Even the macros are not yet well
>>> documented:
>>>     http://dpdk.org/ml/archives/dev/2015-June/019357.html
>>>
>>> Thanks for your attention and your participation in these important
>>> choices.
>>>
>>
>> Thomas-
>>     Just to re-iterate what you said earlier, and what was discussed
>> in the
>> previous ABI discussions
>>
>> 1) ABI stability was introduced to promote DPDK's ability to be
>> included with
>> various linux and BSD distributions.  Distributions, by and large, favor
>> building libraries as DSO's, favoring security and updatability in
>> favor of all
>> out performance.
>>
>> 2) The desire was to put DPDK developers in a mindset whereby ABI
>> stability was
>> something they needed to think about during development, as the DPDK
>> exposes
>> many data structures and instances that cannot be changed without
>> breaking ABI
>>
>> 3) The versioning mechanism was introduced to allow for backward
>> compatibility
>> during periods in which we needed to support both an old an new ABI
>>
>> 4) As Stephan and others point out, its not expected that we will
>> always be able
>> to maintain ABI, and as such an easy library versioning mechanism was
>> introduced
>> to prevent the loading of an incompatible library with an older
>> application
>>
>> 5) The ABI policy was introduced to create a method by which new ABI
>> facets
>> could be scheduled while allowing distros to prepare their downstream
>> users for
>> the upcomming changes.
>>
>>
>> It seems to me, looking back over these last few months, that we're
>> falling down
>> a bit on our use of (3).  I've seen several people take advantage of
>> the ABI
>> scheduled updates, but no one has tried the versioning interface, and
>> as a
>> result patches are getting delayed, which was never my intent.  Not
>> sure whats
>> to be done about that, but we should probably address it.  Is use of the
>> versionnig interface just too hard or convoluted?
>
> To me it seems that by far the biggest problem with ABI stability in
> DPDK is features requiring changes to public structs (often directly
> allocated and accessed by apps), which is something the symbol
> versioning doesn't directly help with, you'd need to version the structs
> too.
>
> One only needs to glance at the glibc documentation on how to accomplish
> it [1] to see it gets rather involved. Glibc promises backwards
> compatibility for life, so the effort is justified. However in where
> DPDK we're talking about extending compatibility for a few months by
> minimum requirement, people are unlikely to bother.
>
> [1] https://sourceware.org/glibc/wiki/Development/Versioning_A_Structure

Does it means that it requires a specific engineering so we are back to 
the needs of using two layers:
   a- the direct calls to "non ABI stable layers" (current librte*) for 
those we can/are fine to use it,
   b- the calls thru such layers that guarantee the ABI stabilities (a 
librte_compat?) for those who needs it.

Could both be exposed by the distributions so they can be consumed?

Thank you,
   Vincent