From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from dpdk.org (dpdk.org [92.243.14.124]) by dpdk.space (Postfix) with ESMTP id 22AA1A0679 for ; Thu, 4 Apr 2019 12:54:55 +0200 (CEST) Received: from [92.243.14.124] (localhost [127.0.0.1]) by dpdk.org (Postfix) with ESMTP id 9C4FC1B140; Thu, 4 Apr 2019 12:54:53 +0200 (CEST) Received: from mga03.intel.com (mga03.intel.com [134.134.136.65]) by dpdk.org (Postfix) with ESMTP id 101F61B12D; Thu, 4 Apr 2019 12:54:51 +0200 (CEST) X-Amp-Result: UNSCANNABLE X-Amp-File-Uploaded: False Received: from fmsmga008.fm.intel.com ([10.253.24.58]) by orsmga103.jf.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 04 Apr 2019 03:54:50 -0700 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.60,308,1549958400"; d="scan'208";a="137532576" Received: from bricha3-mobl.ger.corp.intel.com ([10.237.221.35]) by fmsmga008.fm.intel.com with SMTP; 04 Apr 2019 03:54:47 -0700 Received: by (sSMTP sendmail emulation); Thu, 04 Apr 2019 11:54:47 +0100 Date: Thu, 4 Apr 2019 11:54:47 +0100 From: Bruce Richardson To: "Burakov, Anatoly" Cc: Ray Kinsella , dev@dpdk.org, Kevin Traynor , "techboard@dpdk.org" Message-ID: <20190404105447.GA1351@bricha3-MOBL.ger.corp.intel.com> References: <94df3cc4-de54-72d6-84c6-81bebd209a81@intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <94df3cc4-de54-72d6-84c6-81bebd209a81@intel.com> User-Agent: Mutt/1.11.4 (2019-03-13) Subject: Re: [dpdk-dev] [dpdk-techboard] DPDK ABI/API Stability X-BeenThere: dev@dpdk.org X-Mailman-Version: 2.1.15 Precedence: list List-Id: DPDK patches and discussions List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: dev-bounces@dpdk.org Sender: "dev" Message-ID: <20190404105447.ykX-3e-HgbhonFH86R39ky-m_-Ue71TOTyxdUPYOGSo@z> On Thu, Apr 04, 2019 at 10:29:19AM +0100, Burakov, Anatoly wrote: > On 03-Apr-19 4:42 PM, Ray Kinsella wrote: > > Hi folks, > > > > Recently I started a discussion with the DPDK Technical Board on DPDK > > ABI/API stability. This was born out informal feedback I had received > > from a number of users of DPDK about ABI churn. In turn this feedback > > then prompted an ABI analysis of DPDK using tools from abi-laboratory. > > > > https://abi-laboratory.pro/index.php?view=timeline&l=dpdk > > > > I guess the short story is that DPDK ABI hasn't really settled down as > > the project has matured. If you take a look at the “Backward Compat.” > > column which measures ABI compatibility compared to the previous > > releases, you will see significant churn in the ABI over successive > > releases since v16.04. > > > > Now compare DPDK to GStreamer as an example of a very mature project > > with a similar intent, a framework for building applications, and which > > enjoys a very stable API. > > > > https://abi-laboratory.pro/index.php?view=timeline&l=gstreamer > > > > The DPDK ABI churn has the following affects for users:- > > > > 1. The churn obliges users of DPDK to commit to a constant > > re-integration and re-validation effort for new versions of DPDK. This > > effort from their perspective may not add value to their consuming > > project, particular if they are only updating to "stay current". > > 2. The churn encourages users of DPDK to slip versions, putting off > > reintegration to later, building up technical debt and causing their > > projects to miss support for new hardware or features. > > 3. It makes DPDK different to almost every other system library and > > framework that an operating systems might ship. This makes DPDK trickier > > to dynamically link against, package and maintain for OS maintainers. > > > > In order to address this issue, I have put together the minimal set of > > concrete proposals below for discussion at the Technical Board next > > Wednesday. > > > > I wanted to share this, as these might not yet be the right proposals, > > however I am putting them out there for feedback to start the discussion. > > > > Thanks, > > > > Ray K > > > > > > Experimental API > > 1. APIs designated as experimental are not considered part of the ABI > > and may change without warning at any time. > > 2. APIs designated as experimental must be marked depreciated for a > > least one quarterly release before removal. > > 3. APIs designated as experimental will no longer automatically graduate > > to core after one release, they may stay experimental until their author > > and the maintainer agree that graduation is appropriate. > > > > Core API (non-experimental API) > > 4. APIs designated as core must be depreciated for a least two years > > before removal, to facilitate the continued compatibility with LTS > > releases. A final removal notice will be published to the DPDK Mailing > > List, and if there are no strong objections only then an API may be > > removed. > > 5. APIs designated as core may be changed as follows:- > > 5.a The change proposer must demonstrated that the change has a > > supporting use case and could not be achieved in any other way. > > 5.b ABI version compatibility must be retained, as described below. > > Hi Ray, > > My somewhat rambly 2 cents :) > > While i think some solution has to be found for the situation, we also have > to balance this against speed of development and new features rollout. > > For example, let's consider what i am intimately familiar with - the memory > rework. I have made enormous efforts to ensure that pre-18.05 and post-18.05 > remain as ABI/API compatible as possible, but there were a couple of API > calls that were removed, and there couldn't have been any replacements > (these API's were exposing internal structures that shouldn't have been > exposed in the first place), and 18.05 also broke the ABI compatibility, > because there was no way to do it without it (shared internal structures > needed to change in part to support multiprocess). > > So, if i understand your proposal correctly, assuming a 2-year waiting > period for the deprecation of core API's, you would essentially still be > waiting for the memory rework to land for a year more. Moreover, even > *after* it has landed, there was a continuous stream of improvements and > bugfixes, some of which has broke ABI compatibility as well. Some of them > were my fault (as in, i could've foreseen the need for those changes, but > didn't), but others came as a result of people using these new features in > the wild and reporting issues/problems/suggestions - i am but one man, after > all. Plus, you know, there's only 24 hours in a day, and some stuff takes > time to implement :) > > Since this rework goes right at the heart of DPDK (arguably there isn't a > more "core" API than memory!), there is no (sane) way in the universe to 1) > keep backwards compatibility for this, or 2) keep two parallel versions of > it. We also need to test all that, and, to be honest, one validation cycle > for a release wouldn't be enough to figure out all of the kinks and > implications of such a case. It was really great that memory rework has > landed in 18.05 and we had time to improve and prepare it for an 18.11 LTS - > i think everyone can say that it's in much better shape in 18.11 than it was > in 18.05, but if we couldn't do an ABI break here or there, this rate of > improvements would have slowed down significantly. > > Now, i understand that this is probably a highly exceptional case, but i'm > sure that maintainers of other parts of DPDK will have their own examples of > similar things happening. > > I have no idea what a proper solution would look like. Any "splitting" of > the trees into "experimental" vs. "stable" will end up causing the same > issue - people choose to use stable over experimental because, well, it's > more stable, and new/experimental features don't get tested as much because > no one runs the thing in the first place. > > TL;DR we have to be careful not to constrain the pace of > development/bugfixing just for the sake of having a stable API/ABI :) > Actually, I think we *do* need to constrain the pace of development for the sake of ABI stability. At this stage DPDK has been around for quite a number of years and so should be considered a fairly mature project - it should just start acting like it. Now, in terms of features like the memory rework, that is indeed a case that there was no alternative other than a massive ABI break. However, for that rework there was a strong need for improvement in that area that we can make the case for an ABI break to support it - and it is of a scale that nothing other than an ABI change would do. For other areas and examples, I doubt there are many in the last couple of years that are of that scale. My thoughts on the matter are: 1. I think we really need to do work to start hiding more of our data structures - like what Stephen's latest RFC does. This hiding should reduce the scope for ABI breaks. 2. Once done, I think we should commit to having an ABI break only in the rarest of circumstances, and only with very large justification. I want us to get to the point where DPDK releases can immediately be picked up by all linux distros and rolled out because they are ABI compatible. I'm not sure I like the idea of planned ABI break releases - that strikes me as a plan where we end up with the same number of ABI breaks as before, just balled into one release. Question for Kevin, Luca and others who look at distro-packaging: is it the case that each distro will only ship one version of DPDK, or is it possible that if we have ABI breaks, a distro will provide two copies of DPDK simultaneously, e.g. a 19.11 ABI version and a 20.11 ABI version? So, in short, I'm generally in favour of a zero-tolerance approach for DPDK ABI breaks, and having ABI breaks as a major event reserved only for massive rework changes, such as major mbuf changes, or new memory layout or similar. Regards, /Bruce