Re: [dpdk-dev] ABI and inline functions

[Ray Kinsella <mdr@ashroe.eu>]

On 24/04/2019 12:08, Burakov, Anatoly wrote:
> On 23-Apr-19 3:12 PM, Ray Kinsella wrote:
>>
>>
>> On 18/04/2019 11:28, Bruce Richardson wrote:
>>> On Thu, Apr 18, 2019 at 04:34:53AM +0000, Honnappa Nagarahalli wrote:
>>>>>
>>>>> On Wed, Apr 17, 2019 at 05:12:43AM +0000, Honnappa Nagarahalli wrote:
>>>>>> Hello,
>>>>>>     There was a conversation [1] in the context of RCU library. I
>>>>>> thought
>>>>>> it needs participation from broader audience. Summary for the context
>>>>>> (please look at [1] for full details)
>>>>>>
>>>>>
>>>>> Thanks for kicking off this discussion
>>>>>
>>>>>> 1) How do we provide ABI compatibility when the code base contains
>>>>> inline functions? Unless we freeze development in these inline
>>>>> functions and
>>>>> corresponding structures, it might not be possible to claim ABI
>>>>> compatibility.
>>>>> Application has to be recompiled.
>>>>>
>>>>> I agree that in some cases the application "might" need to be
>>>>> recompiled,
>>>>> but on the other hand I also think that there are many cases where ABI
>>>>> function versioning can still be used to mitigate things. For
>>>>> example, if we
>>>>> think of a couple of various scenarios:
>>>>>
>>>>> 1. If everything is inline and variables are allocated by app, e.g.
>>>>> spinlock on stack, then there is no issue as everything is application
>>>>> contained.
>>>> If there is a bug fix which requires the structure to change, the
>>>> application would need to recompile. I guess you are talking about a
>>>> scenario when nothing changed in the inline function/variables.
>>>>
>>>
>>> If the application wants the bugfix, then yes. However, if the app is
>>> unaffected by the bug, then it should have the option of updating DPDK
>>> without a recompile.
>>
>> I would also imagine that should be an extremely rare case, that a
>> bugfix would require a structure change ... perhaps for an alignment
>> issues?
> 
> Multiprocess threading issues is one case i've had to do that more than
> once.

Another reason to dislike multi process I guess.

> 
> <snip>
> 
> 
>>
>> The reality is that most other system libraries provide strong
>> guarantees ... to date we have provided very little.
>>
> 
> To our credit, the libraries you're likely referring to aren't trying to
> reimplement the Linux kernel :) I don't think we do these API/ABI breaks
> just because we like doing them - DPDK is complex, and getting
> everything right the first time *and* allowing for future evolution is
> not a trivial undertaking.
>
> 
> To me, part of the problem is that DPDK is an "everything and the
> kitchen sink" kind of library where there is a bunch of drivers, a whole
> quasi-OS layer of dealing with hardware in a cross-platform manner, a
> separate memory management system, a bunch of libraries such as hash/lpm
> tables, plus there's QOS, IP Pipeline, flow stuff, etc. - normally, "a
> library" would concentrate on doing one thing well. DPDK, on the other
> hand, tries to do *everything* well. The sheer breadth of DPDK's scope
> is, i think, contributing to the breakages. If you keep 99% of your
> libraries stable between version, but there's a small ABI tweak in an
> LPM library, the entire DPDK stability gets invalidated.

Well I guess DPDK is no more complex than Java or .NET Framework in that
respect, as these also feature OS-layers, memory management systems,
application frameworks, primitives etc but do manage to give their
consumers strong guarantee's on API stability. Clearly ABI stability has
a no meaning when you always being JIT compiled.

I understand that doing everything right the first time, allowing for
future evolution, while keep ABI/APIs relatively stable is a tough ask.

I would propose that API's marked as "experimental" be given greater
freedom to change to give time of API's to bake, so they don't need to
be always "right first time", that there is wriggle room for these.

I would also propose more use of ABI Versioning to enable evolution of
DPDK while preserving backward compatibility.

> 
> Perhaps limiting DPDK's scope would help with this as well.

I agree.

> 
>>
>>>
>>> Regards,
>>> /Bruce
>>>
>>
> 
>