Basically +1 with what Daniel is saying (note that, as mentioned, a side effect of our effort to split out the scheduler will help but not solve this problem).
My only question is about the need to separate out each virt driver into a separate project, wouldn't you accomplish a lot of the benefit by creating a single virt project that includes all of the drivers? I wouldn't necessarily expect a VMware guy to understand the specifics of the HyperV implementation but both people should understand what a virt driver does, how it interfaces to Nova and they should be able to intelligently review each other's code. -- Don Dugger "Censeo Toto nos in Kansa esse decisse." - D. Gale Ph: 303/443-3786 -----Original Message----- From: Daniel P. Berrange [mailto:berra...@redhat.com] Sent: Thursday, September 4, 2014 4:24 AM To: OpenStack Development Subject: [openstack-dev] [nova] Averting the Nova crisis by splitting out virt drivers Position statement ================== Over the past year I've increasingly come to the conclusion that Nova is heading for (or probably already at) a major crisis. If steps are not taken to avert this, the project is likely to loose a non-trivial amount of talent, both regular code contributors and core team members. That includes myself. This is not good for Nova's long term health and so should be of concern to anyone involved in Nova and OpenStack. For those who don't want to read the whole mail, the executive summary is that the nova-core team is an unfixable bottleneck in our development process with our current project structure. The only way I see to remove the bottleneck is to split the virt drivers out of tree and let them all have their own core teams in their area of code, leaving current nova core to focus on all the common code outside the virt driver impls. I, now, none the less urge people to read the whole mail. Background information ====================== I see many factors coming together to form the crisis - Burn out of core team members from over work - Difficulty bringing new talent into the core team - Long delay in getting code reviewed & merged - Marginalization of code areas which aren't popular - Increasing size of nova code through new drivers - Exclusion of developers without corporate backing Each item on their own may not seem too bad, but combined they add up to a big problem. Core team burn out ------------------ Having been involved in Nova for several dev cycles now, it is clear that the backlog of code up for review never goes away. Even intensive code review efforts at various points in the dev cycle makes only a small impact on the backlog. This has a pretty significant impact on core team members, as their work is never done. At best, the dial is sometimes set to 10, instead of 11. Many people, myself included, have built tools to help deal with the reviews in a more efficient manner than plain gerrit allows for. These certainly help, but they can't ever solve the problem on their own - just make it slightly more bearable. And this is not even considering that core team members might have useful contributions to make in ways beyond just code review. Ultimately the workload is just too high to sustain the levels of review required, so core team members will eventually burn out (as they have done many times already). Even if one person attempts to take the initiative to heavily invest in review of certain features it is often to no avail. Unless a second dedicated core reviewer can be found to 'tag team' it is hard for one person to make a difference. The end result is that a patch is +2d and then sits idle for weeks or more until a merge conflict requires it to be reposted at which point even that one +2 is lost. This is a pretty demotivating outcome for both reviewers & the patch contributor. New core team talent -------------------- It can't escape attention that the Nova core team does not grow in size very often. When Nova was younger and its code base was smaller, it was easier for contributors to get onto core because the base level of knowledge required was that much smaller. To get onto core today requires a major investment in learning Nova over a year or more. Even people who potentially have the latent skills may not have the time available to invest in learning the entire of Nova. With the number of reviews proposed to Nova, the core team should probably be at least double its current size[1]. There is plenty of expertize in the project as a whole but it is typically focused into specific areas of the codebase. There is nowhere we can find 20 more people with broad knowledge of the codebase who could be promoted even over the next year, let alone today. This is ignoring that many existing members of core are relatively inactive due to burnout and so need replacing. That means we really need another 25-30 people for core. That's not going to happen. Code review delays ------------------ The obvious result of having too much work for too few reviewers is that code contributors face major delays in getting their work reviewed and merged. From personal experience, during Juno, I've probably spent 1 week in aggregate on actual code development vs 8 weeks on waiting on code review. You have to constantly be on alert for review comments because unless you can respond quickly (and repost) while you still have the attention of the reviewer, they may not be look again for days/weeks. The length of time to get work merged serves as a demotivator to actually do work in the first place. I've personally avoided doing alot of code refactoring & cleanup work that would improve the maintainability of the libvirt driver in the long term, because I can't face the battle to get it reviewed & merged. Other people have told me much the same. It is not uncommon to see changes that have been pending for 2 dev cycles, not because the code was bad but because they couldn't get people to review it. Contributors will simply walk away from nova if that happens too often. Even when fate is on your side and code is reviewed, the chances of it getting a success result from the CI systems first time around is slim due to false failures. This really compounds the already poor experiance of submitting code to Nova. Marginalization of areas ------------------------ Since the core team has far more work to do than it can manage, it has to prioritize what it looks at. The core team figures out what the overall project priorities are and will focus more effort in to those areas. Individual members will also focus their attention in areas where they have personal interest. Unfortunately the core team is not representative of the entire of Nova codebase. The inevitable result is that the HyperV and VMWare drivers can often loose out in the battle for attention. In the past we've said that it is the responsibility of people in those teams to invest in learning the entire of Nova so that they have the knowledge required to be promoted to core. I used to support that approach, but now consider to be flawed due to the increased difficulty of *anyone* getting onto core. The time investment required is simply too great to expect people to undertake it. The marginalized areas have no freedom to self-organize to solve their own problems because they are for ever dependant on the core team bottleneck. Increasing size --------------- There is a long standing policy that the Nova virt driver API is considered unstable and thus all virt driver implementations should ultimately be part of the Nova codebase. In Juno it is likely that the Ironic driver will be merged into Nova. In a future release we may yet see the Docker driver return to the Nova tree. The result of merging yet more drivers is that there will be yet more work for nova reviewers to do. It is far from obvious that merging new drivers will be accompanied by new members on the core team. So it is likely that the workload is going to get worse over future releases. Splitting out the scheduler will be beneficial in reducing the review backlog, but probably not enough to counter the growth from virt drivers. Killing of nova-network is unlikely to help at all, since that consumes little-to-no review time currently [2]. Exclusion of non-corporate devs ------------------------------- There is a strong push from nova core for everything that is merged into Nova to be accompanied by CI testing. This certainly makes sense from the POV of overall product quality and reducing the burden on the core reviewers to catch all mistakes through code review. What we don't take into account is that setting up and maintaining such testing infrastructure requires a major investment in terms of both hardware costs and man power. It has already been seen that this is too much to bear for some companies who contribute to Nova, eg with the Docker driver [3]. Developers who are not affiliated with any company do not stand any realistic chance of meeting the CI testing needs unless they're lucky that their feature can be covered by an existing running CI system. This looks like it could effectively prevent support for a community submitted FreeBSD BHyve driver from being merged, no matter how useful it might be to users who want it. NB, now a FreeBSD BHyve driver would probably be done as part of the libvirt driver, which complicates this particular point I'm trying to make, since I don't suggest reducing testing of the libvirt driver compared to what it has today. I don't want to get into a detailed testing discussion here really, since that's somewhat of a tangent to the question of our dev and review process. I am, however, concerned when our testing policy forces maintainers of some virt drivers into the position of being treated as second class citizens within the project as a whole, with a different development structure to the in-tree approved drivers. That said, Docker probably benefits from being out of tree, since it thus avoids the painful nova core bottleneck entirely. Problem summary --------------- The common thread through most of these problems is that the nova core team is a massive bottleneck in the development process. Processes adopted (or under discussion) by the core team are fundamentally not helping to remove the bottleneck. Rather they are introducing new layers of beaurocracy so that we can feel justified in telling contributors that we are going to ignore or reject their work. At best this is going to result in far less useful work taking place in Nova. At worst this is further reducing the ability of people to self organize to solve the problems, will cause our contribtors to leave the community and possibly even force some virt drivers to go out of tree to get their work done. Death by a thousand cuts. A sub-thread is around the idea that our current structure of one big repo also has other negative consequences for drivers who may not be able to meet the same high standards as the rest of the drivers. A driver is either in or out of the club, and if its out of the club life is made comparatively harder for its developers & users. By all means have rules around that requirements for a release to use the openstack trademarks based on CI testing coverage, but don't let that penalize the actual development process itself. Overall Nova is being increasingly hostile to its community of contributors. I don't mean this as a result of any sense of malice or ill-will. What we're seeing is merely a symptom of a hard worked team struggling to survive with a burden they can no longer be reasonably expected to cope with. Nova core has done an amazing job at surviving for so long as the project grew much larger & more quickly than anyone probably expected. The time has come for some radical changes to let nova adapt & evolve to the next level. This is a crisis. A large crisis. In fact, if you got a moment, it's a twelve-storey crisis with a magnificent entrance hall, carpeting throughout, 24-hour portage, and an enormous sign on the roof, saying 'This Is a Large Crisis'. A large crisis requires a large plan. Proposal / solution =================== In the past Nova has spun out its volume layer to form the cinder project. The Neutron project started as an attempt to solve the networking space, and ultimately replace the nova-network. It is likely that the schedular will be spun out to a separate project. Now Neutron itself has grown so large and successful that it is considering going one step further and spinning its actual drivers out of tree into standalone add-on projects [4]. I've heard on the grapevine that Ironic is considering similar steps for hardware drivers. The radical (?) solution to the nova core team bottleneck is thus to follow this lead and split the nova virt drivers out into separate projects and delegate their maintainence to new dedicated teams. - Nova becomes the home for the public APIs, RPC system, database persistent and the glue that ties all this together with the virt driver API. - Each virt driver project gets its own core team and is responsible for dealing with review, merge & release of their codebase. Note, I really do mean *all* virt drivers should be separate. I do not want to see some virt drivers split out and others remain in tree because I feel that signifies that the out of tree ones are second class citizens. It is important to set up our dev structure so that every virt driver is treated equally & so has equal chance to achieve success. As long as one driver remains in tree there will always be pressure for others to join it, which is exactly what we're trying to get away from here. By everyone being out of tree, drivers (like Docker) can take a decision about whether it is the right time for them to be investing in gating CI systems, without being penalized in their dev process if they make a decision to not have gate tests right now. This has quite a few implications for the way development would operate. - The Nova core team at least, would be voluntarily giving up a big amount of responsibility over the evolution of virt drivers. Due to human nature, people are not good at giving up power, so this may be painful to swallow. Realistically current nova core are not experts in most of the virt drivers to start with, and more important we clearly do not have sufficient time to do a good job of review with everything submitted. Much of the current need for core review of virt drivers is to prevent the mis-use of a poorly defined virt driver API...which can be mitigated - See later point(s) - Nova core would/should not have automatic +2 over the virt driver repositories since it is unreasonable to assume they have the suitable domain knowledge for all virt drivers out there. People would of course be able to be members of multiple core teams. For example John G would naturally be nova-core and nova-xen-core. I would aim for nova-core and nova-libvirt-core, and so on. I do not want any +2 responsibility over VMWare/HyperV/Docker drivers since they're not my area of expertize - I only look at them today because they have no other nova-core representation. - Not sure if it implies the Nova PTL would be solely focused on Nova common. eg would there continue to be one PTL over all virt driver implementation projects, or would each project have its own PTL. Maybe this is irrelevant if a Czars approach is chosen by virt driver projects for their work. I'd be inclined to say that a single PTL should stay as a figurehead to represent all the virt driver projects, acting as a point of contact to ensure we keep communication / co-operation between the drivers in sync. - A fairly significant amount of nova code would need to be considered semi-stable API. Certainly everything under nova/virt and any object which is passed in/out of the virt driver API. Changes to such APIs would have to be done in a backwards compatible manner, since it is no longer possible to lock-step change all the virt driver impls. In some ways I think this would be a good thing as it will encourage people to put more thought into the long term maintainability of nova internal code instead of relying on being able to rip it apart later, at will. - The nova/virt/driver.py class would need to be much better specified. All parameters / return values which are opaque dicts must be replaced with objects + attributes. Completion of the objectification work is mandatory, so there is cleaner separation between virt driver impls & the rest of Nova. - If changes are required to common code, the virt driver developer would first have to get the necccessary pieces merged into Nova common. Then the follow up virt driver specific changes could be proposed to their repo. This implies that some changes to virt drivers will still contend for resource in the common nova repo and team. This contention should be lower than it is today though since the current nova core team should have less code to look after per-person on aggregate. - Changes submitted to nova common code would trigger running of CI tests against the external virt drivers. Each virt driver core team would decide whether they want their driver to be tested upon Nova common changes. Expect that all would choose to be included to the same extent that they are today. So level of validation of nova code would remain at least at current level. I don't want to reduce the amount of code testing here since that's contrary to the direction we're taking wrt testing. - Changes submitted to virt drivers would trigger running CI tests that are applicable. eg changes to libvirt driver repo would not involve running database migration tests, since all database code is isolated in nova. libvirt changes would not trigger vmware, xenserver, ironic, etc CI systems. Virt driver changes should see fewer false positives in the tests as a result, and those that do occur should be more explicitly related to the code being proposed. eg a change to vmware is not going to trigger a tempest run that uses libvirt, so non-deterministic failures in libvirt will no longer plague vmware developers reviews. This would also make it possible for VMWare CI to be made gating for changes to the VMWare virt driver repository, without negatively impacting other virt drivers. So this change should increase testing quality for non-libvirt virt drivers and reduce pain of false failures for everyone. - Virt drivers shouldn't use oslo incubator code from nova, since that can be replaced any time and isn't upgrade safe. Ideally most of the incubator stuff virt drivers need should turn into stable oslo APIs. Failing that, virt drivers would need their own copy of the incubated code in their module namespace, to avoid clash or the need to lock-step upgrade code across separate git repos. Overall the outcome is that - Far larger pool of people able to approve changes for merge across nova core and the virt driver core teams. - Faster review & merge for virt driver patches that don't involve changes to common nova code, with less CI system testing pain. - Ability to set priority of work in virt drivers without a 3rd party being a bottleneck, where the work doesn't involve changes to common nova code. - Each virt driver team can accept as many features as they feel able to deal with, without it negatively impacting amount of features that other virt driver teams can accept. - Virt drivers have flexibility to set their own policies on testing without being penalized in the way they then develop their code. The migration ------------- Obviously a proposal such as this is a pretty major undertaking. It should be clear that it could not be done in a short amount of time. It is suggested that it be phased in over two dev cycles. In the Kilo release the focus would be on prep work: - Formalizing the separation between the virt driver impls and the rest of the nova codebase. Figure out exactly which areas of Nova internal code will need to be marked as 'semi-stable' for use by virt drivers, and ensure their APIs are sufficiently future proof. - Discussions with the infrastructure, docs, release, etc teams to identify impacts on them and do any required prep work. - Identify the teams which will lead the new virt driver projects. eg core reviewers, PTL or Czars for each job if applicable - Probably more things I can't think of right now Then at the start of the Lxxxx release, the virt drivers would actually be split out into separate git repos and start their dev process for the future. So for bulk of Lxxxx the drivers would be on their own. The two Lxxxx rc milestones would allow us to ensure our release processes were working well with the split drivers before the Lxxxx final release. Final thought ------------- Overall consider this a vote of no confidence in nova continuing to operate as it does today. As mentioned above this is not intended to be disrepectful to the effort every nova core member has put in, just a reflection on the changed environment we find ourselves in. Fiddling with our processes for the prioritization of work cannot fix the fundamental fact that nova core today is a massive single point of failure & bottleneck, increasingly crippling the project. The only way to address this is by a radical re-organization of our project to remove the bottlenecks by modularization of the project & leaders. Keeping a single team and adding more/changing process is simply akin to shifting deckchairs on the titanic and not a viable option to coninue with long term. Now, I'm realistic. Even with every driver separated out, I expect that each of them will individually still have more work proposed than their respective core teams have time to review. The new structure will, however, make it easier for the core individal teams to grow & adapt in ways that suit their specific needs. For self-contained virt driver changes it will mean that acceptance of work by one team will not take away capacity from another team. Further the burden of knowledge required to make it onto a virt driver core team would be greatly reduced due to the narrower focus of each core team, so we'll be able to promote good talent onto virt driver core teams more quickly. Thanks for reading so far. Now lets make some real change to prepare us for future sustainability & even growth. Regards, Daniel [1] http://lists.openstack.org/pipermail/openstack-dev/2014-August/044459.html [2] There was a ban on changes to nova-network for much of the past two cycles. It was relaxed primarily to allow full conversion of nova codebase to use objects, not for major new feature development. [3] http://lists.openstack.org/pipermail/openstack-dev/2014-July/040443.html [4] http://lists.openstack.org/pipermail/openstack-dev/2014-August/043036.html -- |: http://berrange.com -o- http://www.flickr.com/photos/dberrange/ :| |: http://libvirt.org -o- http://virt-manager.org :| |: http://autobuild.org -o- http://search.cpan.org/~danberr/ :| |: http://entangle-photo.org -o- http://live.gnome.org/gtk-vnc :| _______________________________________________ OpenStack-dev mailing list OpenStack-dev@lists.openstack.org http://lists.openstack.org/cgi-bin/mailman/listinfo/openstack-dev _______________________________________________ OpenStack-dev mailing list OpenStack-dev@lists.openstack.org http://lists.openstack.org/cgi-bin/mailman/listinfo/openstack-dev
