My Faults My Own

One's ponens is another's tollens.

IN WHICH Ross Rheingans-Yoo, a sometimes-poet and else­wise a recently-graduated student of Computer Science and Math, oc­cas­ion­al­ly writes on things of int­erest.

Reading Feed (last update: April 2)

A collection of things that I was happy I read. Views expressed by linked authors are chosen because I think they're interesting, not because I think they're correct, unless indicated otherwise.


(19)

Blog: Don't Worry About the Vase | On Automoderation -- Zvi concretizes much the the vague disease I was feeling around Automoderation, despite it being an eminently plausible approach to its design specification.


(17)

Blog: JeffTK | Slack tool: predict -- Note that Jeff's implementation is of a market mechanism that's not budget-balanced, and rewards marginal improvements of the "last price", rather than marginal improvements of the "current best price". I suspect that these design decisions have the net effect of denoising the signal of predicter quality.

Blog: Schneier on Security | New Gmail Phishing Scam -- "The article is right; this is frighteningly good."

Blog: Marginal Revolution | The Baffling Politics

READ MORE

[CS161] The Classic CV Error

This is a very technical post, largely for the benefit of the students of CS161: Operating Systems, for which I am a Teaching Fellow this semester. It may be useful to you if you're interested in operating systems for some reason, but if you're not in a CS mood today, maybe just move along.

From what I've seen as a TF for this course, it is very, very normal to write condition-variables code that looks like this:
struct cv {  
    struct semaphore *sem;
    volatile int waiters;
}

void cv_wait(struct cv *cv, struct lock *lock) {  
    KASSERT(lk_do_i_hold(lock));

    cv->waiters++;
    lk_release(lock);
    P(cv->sem);
    lk_acquire(lock);
}

void cv_broadcast(struct cv *cy struct lock *lock) {  
    KASSERT(lk_do_i_hold(lock));

    for (; cv->waiters > 0; cv->waiters--)
        V(cv->sem);
}
This code is wrong (or, more specifically, badly synchronized). And it is such a common
READ MORE

[CS161] On Scheduling

This is a very technical post, largely for the benefit of the students of CS161: Operating Systems, for which I am a Teaching Fellow this semester. It may be useful to you if you're interested in operating systems for some reason, but if you're not in a CS mood today, maybe just move along.

Why Do We Schedule, Master Bruce?

A scheduler, as you know, is responsible for determining which threads run, for how long, and in what order. As much as possible, it should give the shared illusion that each process is running constantly to completion, using the entire processor. To this end, there are three major desiderata:

  • That interactive threads (in particular, user-interactive threads) are responsive.
  • That no process starves.
  • That the system, on average, runs quickly.

These high-level desiderata factor into the low-level conditions that:

  • Threads which block expecting a response are rescheduled promptly after waking.
  • Time
READ MORE
1 / 1