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	by chubot 2012 days ago
	I think dtrace has the same problem, i.e. it's pretty tightly coupled to the exact functions / trace points in the kernel. A different kernel can break a dtrace script, although I think their code changes a lot less than Linux does. It seems somewhat unavoidable, if the goal is to introspect the kernel at a very intimate level ...

2 comments

bcantrill 2012 days ago

No, DTrace does not have this problem, though our solution to it is one of the least well known aspects of DTrace: we have a notion of explicit stability that allows for stable scripts to be built on top of very low level implementation details that themselves might change. See the chapter on "Stability" in the Dynamic Tracing Guide[1] for details.

[1] http://dtrace.org/guide/chp-stab.html

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btown 2011 days ago

> Stability attributes are computed for most D language statements by taking the minimum stability and class of the entities in the statement.

That's a fascinating read and an amazing idea. To your knowledge are there any other software ecosystems that track stability in nearly as formalized a way? Has there been investigation into bringing these ideas into other modern languages? (I don't believe Rust has a concept like this, for instance, though it would even further strengthen the language's concept of correctness if it did!)

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bcantrill 2011 days ago

Well, we certainly thought it was a big deal! We were really trying to address this issue of writing stable scripts -- allowing for stable, powerful tooling without ossifying the underlying system. I'm really pleased with the work we did there -- but it's unquestionably one of the more esoteric aspects of DTrace.

Something of a funny story that this brings to mind: the taxonomy we have here is actually the interface taxonomy from what was Sun's Platform Software Architecture Review Committee (PSARC), which itself borrowed it from Sun's larger Software Development Framework (SDF). We had to get DTrace reviewed by PSARC, which we weren't necessarily looking forward to -- in part because of big developments like this one. To get past our PSARC review, we adopted several strategies, one of which was to separate out DTrace from its instrumentation providers as separate cases before the committee. When we first presented DTrace to PSARC, committee members wanted to fixate on instrumentation methodology -- and it was very helpful to be able to defer these fixations to later cases (after having let members pontificate and chew up some of the clock, of course). The other technique that we developed (which was devastatingly effective) was to distract the committee with issues that were irrelevant but amenable to debate. When a debate emerged among the committee members (and PSARC being more or less a debating society, this was practically guaranteed), we would effectively feed both sides of the debate -- and in the end, run out the clock on something we didn't care about. All of this worked exceedingly well -- and DTrace itself (one of the largest cases that had ever appeared before PSARC) was approved with essentially no modifications.

Shortly after the DTrace case was approved, we started bringing forward cases on instrumentation providers. With each case, we presented the stability matrix of that particular provider; on the first such case, I remember vividly one committee member asking: "what the hell is this and when do we review it?!" We explained that it was the stability matrix -- as explained at length in the case that they had in fact already approved. They realized in an instant that they had fixated on a dinghy of nomenclature while we had slipped behind them an ocean liner of semantics -- and it was glorious.

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rrdharan 2011 days ago

> The other technique that we developed (which was devastatingly effective) was to distract the committee with issues that were irrelevant but amenable to debate.

It's not exactly the same but this reminds me of the way Matt Stone described his interactions with the MPAA board in This Film Is Not Yet Rated (https://en.wikipedia.org/wiki/This_Film_Is_Not_Yet_Rated).

i.e. they went into the Team America rating negotiation with aggressive material they were prepared to cut, and probably wanted to cut anyway, and let the committee spend all their time on that.