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

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

2015-06-16 22:28, Stephen Hemminger:
> On Tue, Jun 16, 2015 at 9:36 PM, Matthew Hall <mhall@mhcomputing.net> wrote:
> > On Wed, Jun 17, 2015 at 01:29:47AM +0200, Thomas Monjalon wrote:
> > > 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
> >
> > This goes against the idea I have seen before that we should be moving
> > toward a distro-friendly approach where one copy of DPDK can be used by
> > multiple apps without having to rebuild it. It seems like it is also a
> > bit ABI hostile according to the below goals / discussions.
> >
> > Jemalloc is also very high-performance code and still manages to allow
> > enabling and disabling statistics at runtime. Are we sure it's impossible
> > for DPDK or just theorizing?

Please Matthew, it is better to comment in the thread dedicated to statistics.

[...]
> > Personally to me it seems more important to preserve the ABI on patch
> > releases, like 2.X.Y going to 2.X.Z. But maybe I missed something?

The goal of the ABI deprecation process was to provide a smooth integration
of the release 2.X+1.0. There are 4 months between releases 2.X.0 and 2.X+1.0.

[...]
> > However new-style libraries such as libcurl usually just have init
> > functions which initialize all the secret structs based on some defaults
> > and some user parameters and hide the actual structs from the user.
> > If you want to adjust some you call an adjuster function that modifies
> > the actual secret struct contents, with some enum saying what field to
> > adjust, and the new value you want it to have.
> >
> > If you want to keep a stable ABI for a non-stable library like DPDK,
> > there's a good chance you must begin hiding all these weird device
> > specific structs all over the DPDK from the user needing to directly
> > allocate and modify them.
> > Otherwise the ABI breaks everytime you have to add adjustments,
> > extensions, modifications to all these obscure special features.
> 
> The DPDK makes extensive use of inline functions which prevents data hiding
> necessary for ABI stablility. This a fundamental tradeoff, and since the
> whole reason for DPDK is performance; the ABI is going to be a moving target.
> 
> It would make more sense to provide a higher level API which was abstracted,
> slower, but stable for applications. But in doing so it would mean giving
> up things like inline lockless rings. Just don't go as far as the Open (not)
> dataplane API;
> which is just an excuse for closed source.

I don't understand what you mean.