(Or: knitting a delicate fabric, part II: sewing it all together)
(Or: Right, Joker, the underwear might be on the outside, but I get to drive the Batmobile!)
(Or: I just had to say something about Batman and the Batmobile. Couldn’t help it.)
Cool, now we have a space- and time-efficient data structure (which is nothing more than a nice structure to handle non-overlapping numeric ranges) we can use to implement the simplified stride scheduling thing I’ve been discussing ou the previous posts, which will remain small most of the time, and won’t change its shape like mad, so it’s also very gentle on the CPU cache. Not only that, but the same effects hold for any trees, shall we decide that red-black trees aren’t adequate and set ourselves to use splay trees, AA trees, AVL trees, Huffman trees, unbalanced binary trees (please: no.), whatever. So far we took care of the (not any more) skewed proportions due to randomness (by using strides), the mapping of “tickets” to tasks (by using trees to store the queues, “indexes” as the tickets, offsets to simulate “special” tickets, and lazy evaluation to efficiently implement offset propagation to upper queues), and the order of complexity (by using the queues as the node elements of the tree, where the key is base priority times number of threads in that queue).
(Or: knitting a delicate fabric, part I: the wool)
I sincerely hope you’ve read the disclaimer by now, but I guess I’d better link to it anyway :) Thanks.
I spent the better part of the last post explaining how simple strides would yield an approximation of the ideal shuffling of tickets. What I didn’t explain, however, was why the hell am I insisting on using tickets when strides/passes avoids those issues completely.
Well, the thing is, I didn’t ditch my previous attempt completely. It had flaws, but there were some gems there as well. I don’t know a single programmer who can’t recognise it’s possible to find sound ideas and really clever excerpts of code even when, on the whole, the code was crap.
(Yes, programmers and software architects are a proud bunch of people.)
And tickets are making a comeback, but in a very different context.
Anyway, suppose we’re starting with a clean slate, and all we have so far are the tasks and their respective priorities. Now let’s scribble a little in that slate :)
(Or: what you finally learn to do after a number of false starts)
But first, the obligatory disclaimer, in case you missed it. Thanks.
After I admitted the basic flaws of my previous attempts, I decided to go back to the basics and use a “bottom-up” approach to come up with a solution that suited the needs of the Haiku scheduler: fairness, responsiveness, responsive, scalability, succinctness and efficiency.
Right. So let’s keep in mind that the BeBook already hints at what the priority numbers should roughly mean: frequency of scheduling. We have 120 priority levels; the higher 20 are reserved to real-time threads.
This post is not going to address these directly. Instead, today I’ll be telling the background story of how quite a number of pieces are being woven together to fulfill those specific needs and constraints. OK, enough metablogging ;)
After letting go of the previous approach, I decided to study the subject on the very traditional literature about it. And while I was studying classic scheduler approaches on the Minix book, I noticed that the description of lottery-based scheduling was very compatible with that description of priorities meaning approximate frequency of being chosen by the scheduler. Lottery scheduling was proposed by Carl Waldspurger in 1994, and it works roughly like this:
(Or: rose-coloured glasses are both the blessing and the curse of being in love)
But first, the obligatory disclaimer, in case you missed it. Thanks.
Remember where I left on the previous post? Now, with greater confidence, I set myself to improve my original algorithm’s performance even further. But that’s just because I knew it sucked. It was extremely inefficient as far as implementation goes; it looked great in the benchmarks because it was being compared to O(n) (n being the number of threads in the system) algorithms, while it had O(1) complexity, so I already had a head-start, so to speak. Still, I knew the algorithm very well and understood that there were plenty of bottlenecks to fix. I wrote it, after all.
For those who don’t know me (or my GSoC assignment) already, I’m the one assigned to ticket #1069, namely:
Create an O(1) thread scheduler with CPU affinity and soft real-time support which targets desktop responsiveness.
According to current Haiku source ICMP implementation is only a … framework. Only ICMP echo request messages are processed.
There are essentially two RFCs that should be considered when implementing ICMP (for IPv4). RFC792 - ‘INTERNET CONTROL MESSAGE PROTCOL’ and RFC1122 - ‘Requirements for Internet Hosts – Communication Layers’.
According to RFC1122, paragraph 3.2.2. there are two types of ICMP messages a host must process: 1) ICMP Error Messages - Destination Unreachable - Redirect - Source Quench - Time Exceeded - Parameter Problem 2) ICMP query messages: - Echo - Information - Timestamp - Address Mask
Actually is more than half. This quick post is just to inform you that I wrote the part that schedule an isochronous request in the UHCI driver. I’ve already sent the patch to Michael for his review. The only part that is missing is the code that remove the request once it has been processed or canceled, which is not as trivial as I thought.
Personal rant: my university examination session draws near and with it all credit tests as well. I’m doing my best in time management not to put any of my current tasks and projects into starvation, but exactly as Ryan wrote to me - it’s not easy.
Going back to more Haiku-specific topics, last week I was mostly analyzing the .pkg format Be Inc. left us behind. After some tests, most crucial parts of it are clear to me now.
Haiku Admin Meeting 2007-06-04: There was some discussion about the status of several of the GSoC students/mentors now that GSoC has officially begun (May 28th). A couple of minor notes about some website changes were mentioned. Looks like June 4, 2007 was a short meeting :) I would like to remind readers that any official discussion about the decisions made by the Haiku Admins should be directed to the General Mailing List.
Haiku Admin Meeting 2007-05-28: Discussion about new “Admin organizer” internal-use website for managing tasks, admin votes, etc. Some discussion took place about possibly organizing a physical “admin gathering” somewhere. Michael Lotz has volunteered, and is currently a primary candidate for “Project management” type tasks. Mention of the need to assign/complete the Haiku, Inc. 2006 financial publication. Discussed Haiku Compatible Badges: It appears many are currently in favor of Stephan’s designs.