I am the blog of Hal Fire, and I bring you…

… interesting tidbits of release engineering.

Tuning Legacy vcs-sync for 2x profit!

One of the challenges of maintaining a legacy system is deciding how much effort should be invested in improvements. Since modern vcs-sync is “right around the corner”, I have been avoiding looking at improvements to legacy (which is still the production version for all build farm use cases).

While adding another gaia branch, I noticed that the conversion path for active branches was both highly variable and frustratingly long. It usually took 40 minutes for a commit to an active branch to trigger a build farm build. And worse, that time could easily be 60 minutes if the stars didn’t align properly. (Actually, that’s the conversion time for git -> hg. There’s an additional 5-7 minutes, worst case, for b2g_bumper to generate the trigger.)

The full details are in bug 1226805, but a simple rearrangement of the jobs removed the 50% variability in the times and cut the average time by 50% as well. That’s a savings of 20-40 minutes per gaia push!

Moral: don’t take your eye off the legacy systems – there still can be some gold waiting to be found!

Complexity & * Practices

I was fortunate enough to be able to attend Dev Ops Days Silicon Valley this year. One of the main talks was given by Jason Hand, and he made some great points. I wanted to highlight two of them in this post:

  1. Post Mortems are really learning events, so you should hold them when things go right, right? RIGHT!! (Seriously, why wouldn’t you want to spot your best ideas and repeat them?)
  2. Systems are hard – if you’re pushing the envelope, you’re teetering on the line between complexity and chaos. And we’re all pushing the envelope these days - either by getting fancy or getting lean.

Post Mortems as Learning Events

Our industry has talked a lot about “Blameless Post Mortems”, and techniques for holding them. Well, we can call them “blameless” all we want, but if we only hold them when things go wrong, folks will get the message loud and clear.

If they are truly blameless learning events, then you would also hold them when things go right. And go meh. Radical idea? Not really - why else would sports teams study game films when they win? (This point was also made in a great Ignite by Katie Rose: GridIronOps - go read her slides.)

My $0.02 is - this would also give us a chance to celebrate success. That is something we do not do enough, and we all know the dedication and hard work it takes to not have things go sideways.

And, by the way, terminology matters during the learning event. The person who is accountable for an operation is just that: capable of giving an account of the operation. Accountability is not responsibility.

Terminology and Systems – Setting the right expectations

Part way through Jason’s talk, he has this awesome slide about how system complexity relates to monitoring which relates to problem resolution. Go look at slide 19 - here’s some of what I find amazing in that slide:

  • It is not a straight line with a destination. Your most stable system can suddenly display inexplicable behavior due to any number of environmental reasons. And you’re back in the chaotic world with all that implies.
  • Systems can progress out of chaos, but that is an uphill battle. Knowing which stage a system is in (roughly) informs the approach to problem resolution.
  • Note the wording choices: “known” vs “unknowable” – for all but the “obvious” case, it will be confusing. That is a property of the system, not a matter of staff competency.

While not in his slide, Jason spoke to how each level really has different expectations. Or should have, but often the appropriate expectation is not set. Here’s how he related each level to industry terms.

Best Practices:

The only level with enough certainty to be able to expect the “best” is the known and familiar one. This is the “obvious” one, because we’ve all done exactly this before over a long enough time period to fully characterize the system, its boundaries, and abnormal behavior.

Here, cause and effect are tightly linked. Automation (in real time) is possible.

Good Practices:

Once we back away from such certainty, it is only realistic to have less certainty in our responses. With the increased uncertainty, the linkage of cause and effect is more tenuous.

Even if we have all the event history and logs in front of us, more analysis is needed before appropriate corrective action can be determined. Even with automation, there is a latency to the response.

Emergent Practices:

Okay, now we are pushing the envelope. The system is complex, and we are still learning. We may not have all the data at hand, and may need to poke the system to see what parts are stuck.

Cause and effect should be related, but how will not be visible until afterwards. There is much to learn.

Novel Practices:
For chaotic systems, everything is new. A lot is truly unknowable because that situation has never occurred before. Many parts of the system are effectively black boxes. Thus resolution will often be a process of trying something, waiting to see the results, and responding to the new conditions.

Next Steps

There is so much more in that diagram I want to explore. The connecting of problem resolution behavior to complexity level feels very powerful.

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My experience tells me that many of these subjective terms are highly context sensitive, and in no way absolute. Problem resolution at 0300 local with a bad case of the flu just has a way of making “obvious” systems appear quite complex or even chaotic.

By observing the behavior of someone trying to resolve a problem, you may be able to get a sense of how that person views that system at that time. If that isn’t the consensus view, then there is a gap. And gaps can be bridged with training or documentation or experience.

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