Older blog entries for lukeg (starting at number 19)

23 Dec 2002 (updated 23 Dec 2002 at 17:02 UTC) »
Threads and state machines

So it looks like threads make an interesting topic! pphaneuf's comments and references make a good case that shared-everything threads are hard to use. It's a pity that MichaelCrawford brought up SMP though ;-)

Since consensus is no fun, let me suggest that both threads and state machines have advantages and disadvantages, and that there are other solutions that take the best of both worlds and more. So, bold-assertion-mode-on, and please forgive me my blunders :-)

The state of play, pros and cons

To put things in context, here's what I think are the main good and bad points of shared-memory threads and state machines.

The upside to state machines is that they can be implemented efficiently in languages like C using structs for states and select()-style scheduling, and they avoid micro-level synchronization problems by performing multiple concurrent tasks with a single thread of execution. The main downside is that they are more cumbersome to code because they have to explicitly represent states that would otherwise be implicit in the stack. I'll wave my arms a bit about this below.

The upside to threads is that they easily support blocking operations (like I/O and IPC), which makes a lot of things simpler, and they are easily implemented to take advantage of SMP. The downside of typical implementations is that they are heavy-weight (e.g. because they pre-allocate large private stacks), and that preemption in shared-memory environments makes every line of code susceptible to race conditions. This area is already very well covered by pphaneuf's last post so I'll leave it alone.

Both models still leave you to find a good way for your threads or state-machines to communicate, if they need to.

The good thing is that it's possible to implement light-weight threads with much the same efficiency (and even implementation strategy) as state machines, while still supporting blocking operations and being free of fine-grained race conditions. There are also some elegant and practical high-level models of concurrency and communication (IPC) which make it much easier to write concurrent programs, and which can even take advantage of SMP without the problems of shared-memory threads. These approaches are used in practice to write very nice programs. To avoid lengthiness, I'll do another post later about some types of concurrency and IPC that I really like (unless someone beats me to it), and say why I reckon Erlang is a masterpiece. For now I just want to make an unoriginal observation.

State machines and Continuations

I think that state machine programming has a lot in common with Continuation Passing Style, and that the concept of continuations would be very interesting for people writing event-based programs. The best description of CPS that I know of is in Essentials of Programming Languages by Friedman, Wand, and Haynes. I have the first edition, which I'm told is better than the second.

Continuations are interesting if we think about how to convert an imaginary C program that runs as a thread and uses blocking I/O into an event-driven state machine that only uses non-blocking I/O. (Let's suppose it doesn't interact with other threads, for the moment.) The basic algorithm is that each blocking call in the original program becomes a state in the state machine. Each time the new program reaches its next state, it makes a non-blocking call for I/O and then saves "where I'm up to" in a state structure to pass on for the next event. What goes into the state structure is some explicit representation of the current execution context (continuation) - i.e. the information that the original program has in the stack and program counter, saying where the program is up to and what it has to do next.

After we've done that for each blocking operation, the new program is an asynchronous state machine. Unfortunately, the new program takes more coding because of the explicit representation our execution state. This, I think, is the fundamental downside to C-style state machine programming. For some programs I think explicit state machines are nice and clear, but most of the time I think it's awkward.

The interesting thing is that the above conversion from blocking to non-blocking is really a case of continuation-passing-style conversion (or "CPS-conversion") - maintaining the execution state explicitly and passing it around instead of using the stack. They've figured out a lot of useful things about continuations, like how compilers can do CPS-conversion automatically, and how to represent "first-class" continuations (which let you capture the execution state automatically) efficiently. This is great stuff for concurrent programming.

Fancy languages that support first class continuations, like Scheme and Ruby, don't need to put their state in a struct, they can just use call-with-current-continuation to capture the whole execution context automatically. In fact, in those languages the read() and write() functions could do this themselves and invoke the scheduler (or select()-loop) to convert the whole program from blocking to non-blocking automagically.

Running away, eh?

I'll put my money where my mouth is with some of the things that I think are really great when the festive season allows :-)

New email address:


22 Dec 2002 (updated 22 Dec 2002 at 03:28 UTC) »
    mbp: I really agree on threads vs. state machines. I'm curious about where the idea that shared-everything threading is a Good Thing came from, and particularly why it keeps getting adopted for high-level applications like user interfaces. In particular, I've been digging into state-machine based concurrent programming in Emacs Lisp in the past year and finding it very pleasant - now my mind boggles at the idea of adding traditional threads to an Emacs-like editor (which is often seen on wishlists.)

    I also think that some of the work on building new abstractions for non-sharing explicitly-communicating state machines is really wonderful. I bet you'd enjoy the book Communicating Sequential Processes by C.A.R. Hoare, which starts from bare bones state machines and works out some very elegant (and practical) ways to make them communicate and synchronize.

New Computer

    I got a new laptop yesterday, quite an upgrade from my 3 year old thinkpad: 5x CPU clock speed, 5x memory, 10x disk space at 3x speed, 1600x1200 vs. 1024x768, etc.

    But where's the fun in new computers these days? The hardware all works fine in Linux without trouble shooting, and is even copiously documented; being used to distcc means "make" output doesn't scroll any faster than on the old machine; and with Debian it only takes 10 seconds to install j-random-program when you notice you haven't got it. What used to be a month's distraction only took one evening this time. Maybe the time is ripe for another operating system to take off with a similar rallying cry to the original Linux announcement :-)


raph: I'm very happy with how my meagre google juice has worked for me. Now if you search for "oktoberfest" on images.google.com, the first page of results shows me enjoying a stein there last year. Fame beyond my wildest dreams :-)

(Thanks again purcell for the photo!)

More help!

robocoder: Thanks for having a look, but I don't think the problem in my iptables target is the search for the "fatso" - that part looks correct to me.

Here's the snippet again. (Note that tasklist_lock does get unlocked in the code that follows this snippet!):

        struct task_struct *p, *fatso = NULL;
        int fatso_rss;
        /* Find the "fatso" process -- the one with the most RSS */
        for_each_task(p) {
                if (fatso == NULL || (p->mm->rss > fatso_rss)) {
                        fatso = p;
                        fatso_rss = p->mm->rss;

The "post condition" is that fatso should always be set to the process with the most RSS, and it looks correct to me:

In the first iteration, fatso is NULL so the if will be entered, causing fatso and fatso_rss to be initialized based on the first process. The uninitialized value of fatso_rss wasn't used because fatso == NULL is true in the condition.

On each other iteration (one for each other process), if that process has more RSS then it will replace fatso. Eventually the process with the most RSS will be considered and will replace the current fatso, and then won't be replaced by another process because none has more RSS. So after all processes are considered the fatso should be the process with the most RSS.

Or am I missing something trivial?

I've been expecting the problem to be some subtle kernel issue, like perhaps it being illegal to take the process list lock from the context where iptables rules are invoked. It's certainly true that suspicions like that tend to blind one to his own embarrassing programming errors though :-). I have tried moving the "real work" into a kernel thread, but that didn't help.


I've done a small kernel hack, and it doesn't work. I suspect the problem is some deep kernel magic ("blame the compiler" :-)), so I'm hoping that posting it here will lead to some kindly kernel hacker spotting the trouble.

The idea is to make an "emergency rescue" feature for a thrasing system. Recently my desktop machine was thrashing so badly that I couldn't rsh into it, couldn't kill the X server with C-M-Backspace, and basically just couldn't get it working again. It did however have perfectly good pings times during the thrasing. I assume that the kernel was healthy but all userspace processes were swapped into oblivion, and I ended up cycling the power to recover.

The attempted solution is a kernel-space program that simply kills the process using the most physical memory. It is implemented as an iptables target, so that it can be triggered remotely by sending a magic packet that matches some special iptables rule. Fun, huh?

Here's the code for the iptables target, which freezes the whole machine when I try to use it. See the problem?

/*-*- linux-c -*-
 * This is a module for recovering a system that is thrashing. When
 * trigged by a matched packet, it will kill the task that is using
 * the most physical memory (RSS). Not too subtle, but hopefully it
 * beats hitting the power button when userspace is totally thrashed
 * out of operation.
 * -- luke@bluetail.com
#include <linux/sched.h>
#include <linux/module.h>
#include <linux/skbuff.h>
#include <linux/ip.h>
#include <linux/spinlock.h>
#include <net/icmp.h>
#include <net/udp.h>
#include <net/tcp.h>
#include <linux/netfilter_ipv4/ip_tables.h>

struct in_device; #include <net/route.h>

static unsigned int ipt_killfatso_target(struct sk_buff **pskb, unsigned int hooknum, const struct net_device *in, const struct net_device *out, const void *targinfo, void *userinfo) { struct task_struct *p, *fatso = NULL; int fatso_rss; /* Find the "fatso" process -- the one with the most RSS */ read_lock(&tasklist_lock); for_each_task(p) { spin_lock(&p->mm->page_table_lock); if (fatso == NULL || (p->mm->rss > fatso_rss)) { fatso = p; fatso_rss = p->mm->rss; } spin_unlock(&p->mm->page_table_lock); } /* And kill him.. */ if (fatso != NULL) { /* presumably there is some process.. */ printk(KERN_NOTICE "killing fatso: %d", fatso->pid); force_sig(SIGKILL, fatso); } /* Unlock the task list, *after* sending the signal - this seems to be important. */ read_unlock(&tasklist_lock); return IPT_CONTINUE; }

static int ipt_killfatso_checkentry(const char *tablename, const struct ipt_entry *e, void *targinfo, unsigned int targinfosize, unsigned int hook_mask) { return 1; }

static struct ipt_target ipt_killfatso_reg = { { NULL, NULL }, "KILLFATSO", ipt_killfatso_target, ipt_killfatso_checkentry, NULL, THIS_MODULE };

static int __init init(void) { if (ipt_register_target(&ipt_killfatso_reg)) { return -EINVAL; }

return 0; }

static void __exit fini(void) { ipt_unregister_target(&ipt_killfatso_reg); }

module_init(init); module_exit(fini); /* MODULE_LICENSE("GPL"); */

24 Nov 2002 (updated 24 Nov 2002 at 23:54 UTC) »

Presented my paper about Distel at the Erlang conference, where my prerelease Emacs slideware package (shot1 shot2) went over really well - nothin' like executing code from inside the slides to distract people from your blathering :-). Also implemented some really nice new Distel features, like completion of Erlang modules and functions in Emacs based on the lisp-complete-symbol command for Emacs Lisp.

Great fun, the Erlang hackers are a fun bunch :-)

wlach: as an Emacs tutorial I would highly recommend Keywiz!

Did some fun stuff:

  • Released version 3.1 of Distel, my system for Erlang-compatible concurrent and distributed Emacs Lisp programming. Also wrote a paper about it for the Erlang conference. This is my first real paper, and I must say it's an awful lot of work :-). I'm greatly indebted (once again) to demoncrat for his help.

    I hope the paper is interesting to Emacs hackers, though it'll take an afternoon's perusal of Concurrent Programming in Erlang to see the programming model that's being implemented.

  • Did a teeny-tiny bit more hacking on my Lisp network stack. Now if you telnet to its IP address, it prints "Hello, world!" over TCP.

And otherwise occupied with Erlang hacking at work. Also visited Canada, braved the first real snow of the winter in Stockholm, and some other real world things.

More hacking on my Lisp IP stack. Just got it to answer pings! Very exciting :-)

The code's a little untidy at the moment, but it's still down at 823 lines of hand-written code, and I'm pretty happy with that. I've left out everything not absolutely essential sofar, like routing, fragmentation, etc.

It also seems to me that the low-level internet protocols are a lot more straightforward than CORBA/HTTP/XML/Javathings/etc, and much better specified too. Maybe toy IP stacks could become suitable weekend-hacks like toy webservers are today - that would be pretty interesting!

Long time no diary! Many random hacks:

And, learned to use Word and Powerpoint at work - obviously signaling a great disturbance in the Force.

(I also crashed Netscape right before posting this article, and am delighted to find that Advogato preserved it.)

17 Aug 2002 (updated 17 Aug 2002 at 09:36 UTC) »

Of Roshambo, Bram writes:

    A tempting strategy is to make your bot 'wimp out' and start playing randomly if it isn't doing well. Tournaments play two programs against each other many times with no persistent information between runs to keep this strategy from being effective.

Going random when you're down sounds like a recipe for staying down. How about going random any time you get a modest lead, in the hope of keeping it?

Me, demoncrat, and another friend have had some ultra-simple Roshambo tornaments recently, with robots written in idel. The game is included in the Idel distro if you're interested in having a crack.

You can see demoncrat's first-generation champion to get the flavour.

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