My Faults My Own

Any human’s death diminishes me,

because I am involved in humankind.

IN  WHICH Ross Rheingans-Yoo—a sometime economist, artist, trader, expat, poet, EA, and programmer—writes on things of int­erest.

Reading Feed (last update: July 5)

A collection of things that I was glad 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.


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Blog: Don't Worry About the Vase | Spoiler-Free Review: Witcher 3: Wild Hunt (plus a Spoilerific section)

Blog: Popehat | The Fourth of July [rerun]

Blog: Tyler Cowen @ Bloomberg View | The NBA’s Reopening Is a Warning Sign for the U.S. Economy — "If so many NBA players are pondering non-participation, how keen do you think those workers — none of whom are millionaire professional athletes — are about returning to the office?"

Comic: SMBC | Saturday Morning Breakfast Cereal - Holism


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Blog: Market Design | Job market technology is diffusing slowly through the armed forces

Blog: Marginal Revolution | Tales from Trinidad barter

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[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 error that I'm choosing to dub it The Classic CV Error. It's subject to a race condition in e.g.

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[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 is allocated more-or-less fairly, subject to:
  • Processes closer to completion are prioritized (recall that shortest-time-to-completion-first is provably optimal in total
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