[cfe-dev] LLVM Releases: Upstream vs. Downstream / Distros

[Kristof Beyls via cfe-dev]

FWIW, for our ARM Compiler product, we follow top-of-trunk, not the releases.
Next to picking up new functionality quicker, it also allows us to detect regressions
in LLVM against our in-house testing quickly, not 6 months later. We find that when
we find a regression within 24 to 48 hours of the commit introducing it, it's much
cheaper to get it fixed.

In my opinion, it would be better overall for the LLVM project if top-of-trunk is
tested as much as possible, if testing resources are so scarce that a choice
has to be made between testing top-of-trunk or testing a release branch.

Thanks,

Kristof

On 12 May 2016, at 02:10, Robinson, Paul <paul.robinson at sony.com<mailto:paul.robinson at sony.com>> wrote:

-----Original Message-----
From: Renato Golin [mailto:renato.golin at linaro.org]
Sent: Wednesday, May 11, 2016 11:11 AM
To: Hans Wennborg
Cc: LLVM Dev; Clang Dev; Quentin Colombet; Tom Stellard; Robinson, Paul;
Jim Grosbach; Kristof Beyls; Frédéric Richez; Reid Kleckner; Philip
Reames; Matthias Braun; Bernhard Rosenkränzer; Sylvestre Ledru; Matthias
Klose; Stephen Hines; Jeff Law; Ed Maste; Behan Webster
Subject: Re: LLVM Releases: Upstream vs. Downstream / Distros

On 11 May 2016 at 17:16, Hans Wennborg <hans at chromium.org<mailto:hans at chromium.org>> wrote:
This is a long email :-) I've made some comments inline, but I'll
summarize my thoughts here:

Thanks Hans!

I'll respond them inline, below.


- I think we should use the bug tracker to capture issues that affect
releases. It would be cool if a commit hook could update bugzilla
entries that refer to it.

That seems like a simple hook.


At least for the major releases, I think we're doing pretty well on
timing in terms of predictability: since 3.6, we have release every
six months: first week of March and first week of September (+- a few
days). Branching has been similarly predictive: mid-January and
mid-July.

Indeed, we got a lot better more recently (last 2y), and mostly thanks
to you. :)

Absolutely.  It's enough of a track record to allow reasonable planning.


We used to vary 3 months +-, and now we're down to a few days.
Whatever we decide, I think we should make it official but putting it
out somewhere, so people can rely on that.

A public commitment to the future release schedule would be that much
more justification for planning to participate.


Right now, even if you're extra awesome, there's nothing telling the
distros and LLVM-based products that it will be so if someone else
takes over the responsibility, so they can't adapt.

That's what I meant by "quasi-chaotic".


If there are many downstream releases for which shifting this schedule
would be useful, I suppose we could do that, but it seems unlikely
that there would be agreement on this, and changing the schedule is
disruptive for those who depend on it.

That's the catch. If we want them to participate, the process has to
have some meaning to them. The fact that not many people do, is clear
to me what it means.

We also need to know better how many other releases already depend on
the upstream process (not just Chromium, for obvious reasons), to be
able to do an informed choice of dates and frequency.

The more well positioned and frequent we are, the more people will
help, but there's a point where the curve bends down, and the cost is
just too great. We need to find the inflection point, and that will
require some initial investigations and guesses, and a lot of fine
tuning later. But if we're all on the same page, I think we can do
that, even if it takes time.

I'm particularly concerned with Android, because they not only have
their own tree with heavily modified LLVM components (ex.
Compiler-RT), but they also build differently and so their process are
completely alien to ours. One of the key reasons why these things
happened is because:

* They couldn't rely on our releases, as fixing bugs and back-porting
wasn't a thing back then
* They already had their own release schedule, so aligning with ours
brought no extra benefit
* We always expected people to work off trunk, and everyone had to
create their own process

I don't want to change how people work, just to add one more valid way
of working, which is most stable for upstream releases. :)



The only reasonable way I see of aligning upstream releases with
downstream schedules would be to release much more often. This works
well in Chromium where there's a 6-week staged release schedule. This
would mean there's always a branch going for the next release, and
important bug fixes would get merged to that.

Full validation every 6 weeks is just not possible. But a multiple of
that, say every 3~4 months, could be much easier to work around.



In Chromium we drive
this from the bug tracker -- it would be very hard to scan each commit
for things to cherry-pick. This kind of process has a high cost
though, there has to be good infrastructure for it (buildbots on the
branch for all targets, for example), developers have to be aware, and
even then it's a lot of work for those doing the releases. I'm not
sure we'd want to take this on. I'm also not sure it would be suitable
for a compiler, where we want the releases to have long life-time.

This works because you have a closed system. As you say, Chromium is
mostly final product, not a tool to develop other products, and the
validation is a lot simpler.

With Clang, we'd want to involve external releases into it, and it
simply wouldn't scale.



For the major releases, I've tried to do this. We could certainly
formalize it by posting it on the web page though.

I think that'd be the first step, yes. But I wanted to start with a
good number. 2 times a year? Would 3 times improve things that much
for the outsiders? Or just moving the dates would be enough for most
people?

That's why I copied so many outsiders, so they can chime in and let us
know what would be good for *them*.

Data point: At Sony we ship our stuff every 6 months and that isn't
going to change.  Our policy has been to base our releases on upstream
releases, and given our current lead times, the current upstream release
schedule is actually not bad, especially if it has stopped drifting.
Having four releases pretty consistently follow the current schedule is
extremely positive, thanks!

It would help our internal planning to publish the schedule for future
releases.  You never know, we might even be able to fork an internal
branch in time to help with the release testing, although that is not
in any way a lightweight process (so no promises).

There has been some talk about moving toward releasing from upstream
trunk, or more precisely to streamlining our internal testing process
in order to allow us to seriously contemplate releasing from upstream
trunk.  I can't argue with the streamlining part, for sure.  Whether
the rest of it works out must remain to be seen.
--paulr



Most importantly, those folks should get involved :-)

Indeed!


In practice, we kind of have this for at least some of the targets.
Maybe we should write this down somewhere instead of me asking for
(the same) volunteers each time the release process starts?

I give consent to mark me as the ARM/AArch64 release tester for the
foreseeable future. :)

I can also help Sylvestre, Doko, Ed, Jeff, Bero etc. to test on their
system running on ARM/AArch64 hardware.

cheers,
--renato

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