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

[Thomas Monjalon <thomas.monjalon@6wind.com>]

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?

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.