By now everybody has now heard about
Go, Google's expressive, concurrent,
garbage collecting language. One big, glaring thing stuck out at me
when I was reading the documentation:
Do not communicate by sharing memory; instead, share memory by communicating.
One of the examples given is a semaphore using a channel, which
I'll copy here for posterity.
var sem = make(chan int, MaxOutstanding)
func handle(r *Request)
sem <- 1; // Wait for active queue to drain.
process(r); // May take a long time.
<-sem; // Done; enable next request to run.
func Serve(queue chan *Request)
for
req := <-queue;
go handle(req); // Don't wait for handle to finish.
Here is a little illustration of that in operation.
Serve creates goroutines via the
go
keyword; each of which tries to get a slot in the channel. In the
example there are only 3 slots, so it acts like a semaphore of count
3. When done, each thread returns its slot to the channel, which
allows anyone blocked to be woken and continued.
This instantly reminded me of the very first thing you need to do
if you ever want to pass
Advanced Operating
Systems -- write a semaphore server to provide synchronisation
within your OS.
In
L4, threads communicate
with each other via inter-process communication (IPC). IPC
messages have a fixed format - you specify a target thread, bundle
some data into the available slots in the IPC format and fire it off.
By default you block waiting for a reply -- this all happens within a
single call for efficiency. On the other side, you can write servers
who are listening for remote IPC connections, where everything happens
in reverse.
Here's another illustration the of the trivial semaphore server
concept
Shehjar and I
implemented.
Look familiar? Instead of a blocking push of a number into a slot
into a channel, you make a blocking IPC call to a remote server.
My point here is that both take the approach of sharing memory via
communication. When using IPC, you bundle up all your information
into the available slots in the IPC message and send it. When using a
channel, you bundle your information into an entry in the channel and
call your goroutine. Receiving the IPC is the same as draining a
channel - both result in you getting the information that was bundled
into it by the caller.
| IPC | Go |
|---|
| Start thread | Start goroutine |
| New thread blocks listening for IPC message | Goroutine blocks draining empty channel |
| Bundle information into IPC message | Bundle data into type of your channel |
| Send IPC to new thread | Push data into channel |
| Remote thread unbundles IPC | goroutine drains channel and gets data |
Whenever you mention the word "microkernel", people go off the
deep-end and one thing they seem to forget about is the inherent
advantages of sharing state only via communication. As soon as you do
that, you've broken open an amazing new tool for concurrency, which is
now implicitly implied. By communicating via messages/channels rather
than shared global state, it doesn't matter where you run! One of
those threads in the example could be running on another computer in
your cloud, marshalling up it's IPC messages/channel entries and
sending them over TCP/IP -- nobody would care!
At any rate, do not communicate by sharing memory; instead,
share memory by communicating is certainly an idea whose time has
come.
Since today for kfreebsd-amd64, and probably tomorrow for kfreebsd-i386 too, the gnome metapackage is installable on Debian GNU/kFreeBSD. In the end, this should hopefully give a fully functional desktop for these brand new architectures (to be included in the Squeeze release), with a few notable exceptions:
Revisiting two old ssh points:-
This is the gist of it:
$ cd /usr/src/
I've really enjoyed reading some of 
Such was the scene I arrived at on Wednesday last week at the municipal health center at Kyll , Jyv skyl , Finland. A queue extended a hundred meters beyond the door. It was not hard to guess what it was about, as it had been announced that the pandemic influenza A/H1N1 vaccine would be administered there to people in specific risk groups from 10 am to 3:30 pm.
I should explain here the Finnish health care setup. There are three parallel systems a comprehensive public health care system maintained by the municipalities to standards set by the state, a network of private for-profit health care providers, and a national foundation dedicated to university student health care. Employers are required by law to provide a minimal level of health care to their employees, and most of them also provide, free of charge to the employees, access to near-comprehensive general-practice-level care; most employers buy this service from the for-profit providers. The public system suffers from resource starvation at the general-practice level but provides excellent care in the central hospitals that handle the difficult cases.
Anyway, the H1N1 vaccine is only available through the public system and through the foundation free of charge if you qualify for the vaccine, and no amount of money buys it for you in this country if you don t. Thus, I and many others, normally cared by the employee services, found ourselves queuing up at a public health care institute. And clearly, the public health care system was overwhelmed on that first day.
When I entered the queue, its tail was a traffic hazard. Fortunately, the queue moved faster than new people arrived thereafter, and the hazard ended. The queue moved surprisingly fast it took me one hour to advance the 100 meters to the door. Even so, this was a failure in the system there is no good reason to have people with underlying illnesses (and we all had them, to qualify for the vaccine) stand around in freezing cold weather for an hour!
Once, a nurse came and asked us if any of us was 63 years old or older. Apparently no-one was, since no-one was asked to leave (they will be vaccinated later). Later, another nurse asked everyone in the queue outside to show their KELA cards KELA is the national health insurance system, and its card carries information about extra coverage one qualifies for due to underlying illnesses.
Eventually, I reached the door. Two guards stopped anyone who tried to enter directly, sidestepping the queue, and let in only those who had legitimate business other than vaccination. The main hall was full of people, and I quickly realized that the queue took a full circle inside the hall to reach the multiplexing point. It took me another hour to slowly advance my way though the hall. At the multiplexing point, I was asked to wait a bit, and then I was assigned a vaccination room and a waiting number.
Some twenty minutes later I was called in. The vaccination room I was assigned to was a nurse s office. Two nurses were there, one who would administer the vaccine, and another at the computer, to keep a record. I gave them my KELA card, and shed my coat and my outer shirt, and bared my left shoulder. I was quickly given the pandemic vaccine; there was no question I was qualified for it, not with my obesity being obvious.
Then they asked what my diagnosis was. Prmarily I m here because of my obesity, I said. But I also have paroxysmal atrial fibrillation.
That s not what your KELA card says, accused the nurse at the computer.
The diagnosis is so new, I countered. There has been no time to do the paperwork for KELA. (And indeed, I later learned, it would come up in my next checkup in the spring.)
They stared at each other.
I can show you my prescriptions, I said, making no move for them.
Stares.
I stared back.
Do you want the seasonal vaccine or not? asked the nurse with the injectors.
I lauged briefly. I might as well. It had, honestly, never crossed my mind that I might qualify.
She injected me on my right shoulder. You should stay in out there for ten minutes.
I picked up my clothes and found the cafeteria with coffee and pastry.
Then the real fun started. Next day, I woke with horrible upper back pain. The thermometer showed mild fever; but since i didn t have any respiratory symptoms, I decided to go to work. In the evening, turning in bed was excruciatingly painful. It took days for the pain to subside.
I left the building three hours after I arrived at the end of the queue. What? You think that s excessive? So do I and many others; queuing feels so Soviet Union! But honestly, while it did take time, it worked. I am vaccinated; are you?
Whew!
Is it karma or what? What makes me get involved in two horribly complex, two-week-long conferences, year after year? Of course, both (DebConf and EDUSOL) are great fun to be part of, and both have greatly influenced both my skills and interests.
Anyway, this is the fifth year we hold EDUSOL. Tomorrow we will bring the two weeks of activities to an end, hold the last two videoconferences, and finally declare it a done deal. I must anticipate the facts and call it a success, as it clearly will be recognized as such.
One of the most visible although we insist, not the core activities of the Encounter are the videoconferences. They are certainly among the most complex. And the videoconferences' value is greatly enhanced because, even if they are naturally a synchronous activity (it takes place at a given point in time), they live on after they are held: I do my best effort to publish them as soon as possible (less than one day off), and they are posted to their node, from where comments can continue. This was the reason, i.e., why we decided to move at the last minute tomorrow's conference: Due to a misunderstanding, Beatriz Busaniche (a good friend of ours and a very reknown Argentinian Free Software promotor, from 
[The following story is fictional and does not depict any actual person or event, just like in Law & Order.]
James filed an RC bug on a package, complaining that it attempted to execute an illegal instruction on a Netwinder,
running Linux 2.6.xx. This was sort of an ironic hipster tribute to the olden times when buildd admins would actually
do their jobs, keeping chroots clean and current, reviewing buildd logs, and filing FTBFS bugs where appropriate,
except that he didn't actually do any of those things.
18 minutes later, Aurelien replied, asking for more information in order to debug the problem.
Teh-oh said James, but nobody could hear him because he was talking to his Raggedy Ann doll.
Stephen suggested that Aurelien contact Vince or Steve to get access to different Netwinders, and Aurelien
tried to reproduce the problem on other ARM machines unsuccessfully.
Could you tell me the exact kernel version on this Netwinder, and the one on europa, which seems to have the
same problem? Aurelien asked James.
Aaagh, they're trying to steal my precious bodily fluids! hissed James, but nobody could hear him because he
was talking to his inflatable astronut.
Also, the contents of /proc/cpuinfo would be nice. Aurelien added.
We hates them, Clive Oven! James announced, but nobody could hear him because he
was talking to his EASY-BAKE REAL MEAL Oven.
The Netwinder you were talking about is europa, wasn't it? Could you tell us the kernel versions on europa and elara?
There's no problem on other Netwinders, so it's probably a kernel issue.
THE MARTIANS ARE COMING! James cried, but nobody could hear him because he was talking to a small band of subservient
deaf-mute milkmen.
It's been more than a week since this bug has been filed, and there has been no useful information provided at all.
Can anybody help? asked Peter.
HELLO. IS THIS THING ON? Could you have the elementary courtesy to follow up to the bugs you report? asked Pierre.
Bromeliads.. I'll buy you some bromeliads, James muttered, but no one could hear him because he was talking to his ficus plant.
Lo c looked around, slipped into a cave, sprinted through a maze of tunnels, and sped in a small nuclear-powered
submarine to James's secret undersea lair. Rapping on the bulkhead three times, he uttered the code phrase
and James let him in.
After a conversation that took place strictly through pantomime, Lo c departed and James said, For fuck's sake, but
no one could hear him because he was talking to a broken sonar antenna.
Yo, James is busy, but he said that it's okay to downgrade the bug since it's not a problem with
other Netwinders, reported Lo c.
I never said that! I don't even know who that guy is! I'm going to convert him into a kangaroo! James sang, but no one could
hear him since he was alone on a beach in Normandy.
A month later, Riku wrote to Woody, Hey, we have this bug. Could you take a look?
Sure, replied Woody, and he gave Aurelien access, and Aurelien debugged the problem in under an hour.
Now everyone knows that europa and elara were running Linux 2.6.11. This is a breach of national security!
declared James, but nobody could hear him because he was talking to the crushed skull of a kitten who had
recently been sunning himself.
Under UNIX, it is possible to get into a position where your shell's working directory is pointing to an inode that no longer exists, but a "newer" directory with the same name exists.
The easiest way of reproducing this scenario is as follows: