Date: Fri, 28 Jul 2006 09:52:33 +0100 (BST) From: Robert Watson <rwatson@FreeBSD.org> To: Joel Dahl <joel@FreeBSD.org> Cc: doc-committers@FreeBSD.org, cvs-doc@FreeBSD.org, cvs-all@FreeBSD.org Subject: Re: cvs commit: www/en/projects/ideas index.sgml Message-ID: <20060728092503.U4612@fledge.watson.org> In-Reply-To: <200607280705.k6S7585g094248@repoman.freebsd.org> References: <200607280705.k6S7585g094248@repoman.freebsd.org>
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On Fri, 28 Jul 2006, Joel Dahl wrote: > Modified files: > en/projects/ideas index.sgml > Log: > - Extend the ktrace project with a new task. [1] I think I might be somewhat cautious with this one -- right now, ktrace has a fairly clean and simple design, and trying to turn it into a general purpose tracing facility talking to additional IPC classes, etc, risks adding significant complexity. One of the nice simplifying assumptions of ktrace as it stands today is that writing directly to disk files limits the opportunity for deadlock despite synchronous/reliable logging. ptrace() and related facilities include some basic deadlock prevention code to help avoid debugging cycles; IPC types lend themselves less well to this, so if ktrace is going to support synchronous (blocking, reliable) IPC, then caution will need to be exercised. The result of moving in the proposed direction may be that we want to look at reintroducing asynchrony for ktrace over IPC, which I only just eliminated in ktrace following wide-spread complaints of unreliability and record loss after asynchrony was added in 5.x. I think we should add a note to this idea that while the idea is simple, the correct and safe implementation is quite tricky, and that this is a useful idea to explore but not necessarily something we will adopt. BTW, a problem that has occurred a number of times in the past is that people have approached us with implementations of ideas in the idea list that it has later transpired we aren't actually interested in (sometimes at all). I think it might not be a bad idea to sprinkle the idea list with some additional cautionary language -- often ideas listed there are things to explore, not to adopt without very careful consideration. For example, the "FPU subsystem overhaul", "Process checkpointing", "Pluggable disk shceduler", "Magic Symlinks", "NFS Lockd (kernel implementation)", and several others -- the task here often isn't to port/write the code, the task is to port/write and then perform a detailed and careful evaluation of the changes to decide whether they are a good idea, and then consider adopting the code only if the evaluation suggests it is a good idea and after significant refinment. Some of the ideas on this list are distinctly "explore this direction as a computer scientist, not a code hacker" sorts of problems -- for example, the "Process checkpointing" task seems to suggest that if you can read the DFBSD repository and write some C code, you're set. In fact, this is not remotely the case. Checkpointing is a very difficult problem in computer science, with little consensus on how it should be done (and indeed whether it should be done at all) by general purpose operating systems. Not only that, but we would not adopt the DFBSD implementation as-is, as it solves a few of the easy problems, and none of the hard ones (i.e., security). The requirements here aren't just the ability to write code, but an understanding of distributed systems, our application/execution model, a strong understanding of the performance and security requirements, and willingness to not just look at code but the extensive research literature on this topic. I think people often grab ideas from the list thinking that if implemented as described, they will get committed, and this is not the case. In many of the sorts of "scientific" cases it's likely we'll look at the results and say, "Oh, that was a bad idea", or maybe slightly more likely, "Oh, hmm, not so sure about that". The existing cautionary language captures that there might be disagreements on the specifics, but fails to capture that there may be disagreements on the fundamental ideas themselves. I like the ideas list idea a lot, and don't want to see it removed, but I also don't want people getting the false impression that this is a "todo" list. Some items are todo items and obvious short-order commit candidates, others are out-there ideas that have potential and should be characterized as "high risk" when it comes to the results actually being used. Maybe what we should be thinking about is classifying the todo list items into rote items (things where the chances of adoption of a decent implementation are high, subject to review) and researchy things (where the chances of adoption are low, not just because the chances of a good implementation are low, but because there are lots of open and very difficult questions involved). This would help prevent misunderstandings, if nothing else. Robert N M Watson Computer Laboratory University of Cambridge
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