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

[Thomas F Herbert]

On 6/19/15 1:02 PM, Thomas Monjalon wrote:
> 2015-06-19 12:13, Thomas F Herbert:
>>
>> On 6/19/15 9:16 AM, Thomas Monjalon wrote:
>>> 2015-06-19 09:02, Neil Horman:
>>>> On Fri, Jun 19, 2015 at 02:32:33PM +0200, Thomas Monjalon wrote:
>>>>> 2015-06-19 06:26, Neil Horman:
>>>>>> On Thu, Jun 18, 2015 at 04:55:45PM +0000, O'Driscoll, Tim wrote:
>>>>>>> For the 2.1 release, I think we should agree to make patches that change
>>>>>>> the ABI controllable via a compile-time option. I like Olivier's proposal
>>>>>>> on using a single option (CONFIG_RTE_NEXT_ABI) to control all of these
>>>>>>> changes instead of a separate option per patch set (see
>>>>>>> http://dpdk.org/ml/archives/dev/2015-June/019147.html), so I think we
>>>>>>> should rework the affected patch sets to use that approach for 2.1.
>>>>>>
>>>>>> This is a bad idea.  Making ABI dependent on compile time options isn't a
>>>>>> maintainable solution.  It breaks the notion of how LIBABIVER is supposed to
>>>>>> work (that is to say you make it impossible to really tell what ABI version you
>>>>>> are building).
>>>>>
>>>>> The idea was to make LIBABIVER increment dependent of CONFIG_RTE_NEXT_ABI.
>>>>> So one ABI version number refers always to the same ABI.
>>>>>
>>>>>> If you have two compile time options that modify the ABI, you
>>>>>> have to burn through 4 possible LIBABIVER version values to accomodate all
>>>>>> possible combinations, and then you need to remember that when you make them
>>>>>> statically applicable.
>>>>>
>>>>> The idea is to have only 1 compile-time option: CONFIG_RTE_NEXT_ABI.
>>>>>
>>>>> Your intent when introducing ABI policy was to allow smooth porting of
>>>>> applications from a DPDK version to another. Right?
>>>>> The adopted solution was to provide backward compatibility during 1 release.
>>>>> But there are cases where it's not possible. So the policy was to notice
>>>>> the future change and wait one release cycle to break the ABI (failing
>>>>> compatibility goals).
>>>>> The compile-time option may provide an alternative DPDK packaging when the
>>>>> ABI backward compatibility cannot be provided (case of mbuf changes).
>>>>> In such case, it's still possible to upgrade DPDK by providing 2 versions of
>>>>> DPDK libs. So the existing apps continue to link with the previous ABI and
>>>>> have the possibility of migrating to the new one.
>>>>> Another advantage of this approach is that we don't have to wait 1 release
>>>>> to integrate the changes.
>>>>> The last advantage is to benefit early of these changes with static libraries.
>>>>
>>>> Hm, ok, thats a bit more reasonable, but it still seems shaky to me.
>>>> Implementing an ABI preview option like this implies the notion that, after a
>>>> release, you have to remove all the ifdefs that you inserted to create the new
>>>> ABI.  That seems like an easy task, but it becomes a pain when the ABI delta is
>>>> large, and is predicated on the centralization of work effort (that is to say
>>>> you need to identify someone to submit the 'remove the NEXT_ABI config ifdefs
>>>> from the build' patch every release.
>>>
>>> It won't be so huge if we reserve the NEXT_ABI solution to changes which cannot
>>> have easy backward compatibility with the compat macros you introduced.
>>> I feel I can do the job of removing the ifdefs NEXT_ABI after each release.
>>> At the same time, the deprecated API, using the compat macros, will be removed.
>>>
>>>> What might be better would be a dpdk-next branch (or even a dpdk-next tree, of
>>>> the sort that Thomas Herbert proposed a few weeks ago).
>>>
>>> This tree was created after Thomas' request:
>>> 	http://dpdk.org/browse/next/dpdk-next/
>>
>> Thomas, I am sorry if I went quiet for awhile but I was on personal
>> travel with inconsistent access so I almost missed most of this
>> discussion about ABI changes.
>>
>> My understanding of the purpose of the dpdk-next tree is to validate
>> patches by applying and compiling against a "pull" from the main dpdk
>> tree. I think a good way to handle ABI change while effectively using
>> the dpdk-next might be to do as follows:
>>
>> Create a specific branch for the new ABI such as 2.X in the main dpdk
>> tree. Once that 2.X branch is created, dpdk-next would mirror the 2.X
>> branch along with master.
>>
>> Since, dpdk-next would also have the 2.X branch that is in the main dpdk
>> tree, submitted patches could be applied to either the main branch or
>> the new-ABI 2.X branch. Providing that patch submitters make it clear
>> whether a submitted patch is for the new ABI or the old ABI, dpdk-next
>> could continue to validate the patches for either the main branch or the
>> new ABI 2.X branch.
>
> What is the benefit of a new-ABI branch in the -next tree?
I don't think that there is any specific benefit to an new-ABI branch in 
the dpdk-next tree. I was responding to the suggestion above and perhaps 
I missread it. It sounded like what was being proposed was to use the 
dpdk-next tree specifically for pre-integration of new-ABI. I don't 
think this is of any benefit either.

However if it should be decided to integrate new-ABI patches in a branch 
of dpdk rather then in a separate new-ABI tree, then net-next can 
"mirror" that branch along with the master branch so patches can be 
smoke tested whether they are submitted to the master or to the new-ABI 
branch.
>
> The goal of this discussion is to find a consensus on ABI policy to
> smoothly integrate new features without forcing users of shared libraries
> to re-build their application when upgrading DPDK, and let them do the
> transition before the next upgrade.
I understand this and I think it is a good suggestion to have a 
mechanism to ease the transition.
>