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

[Thomas F Herbert <therbert@redhat.com>]

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.

>
>> Patches that aren't ABI stable can be put on the next-branch/tree in thier
>> final format.  You can delcare the branch unstable (thereby reserving your
>> right to rebase it).  People can use that to preview the next ABI version
>> (complete with the update LIBABIVER bump), and when you release dpdk-X,
>> the new ABI for dpdk-X+1 is achieved by simply merging.
>
> Having this tree living would be a nice improvement but it won't provide any
> stable (and enough validated) releases to rely on.
>