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Schrödinger's Cat and RCU (Well, Structured Procrastination, Actually) 43

Posted by Soulskill
from the everything-on-the-internet-must-involve-cats dept.
davecb writes "Paul E. McKenney, one of the Linux RCU implementors, addresses the problem of synchronization using structured deferral on, what else, Mr Schrödinger's famous cat. Courtesy of deferral/procrastination, the cat can be both alive and dead at the same time. 'In this example, Schrödinger would like to construct an in-memory database to keep track of the animals in his zoo. Births would of course result in insertions into this database, while deaths would result in deletions. The database is also queried by those interested in the health and welfare of Schrödinger's animals. Schrödinger has numerous short-lived animals such as mice, resulting in high update rates. In addition, there is a surprising level of interest in the health of Schrödinger's cat, so much so that Schrödinger sometimes wonders whether his mice are responsible for most of these queries. Regardless of their source, the database must handle the large volume of cat-related queries without suffering from excessive levels of contention. Both accesses and updates are typically quite short, involving accessing or mutating an in-memory data structure, and therefore synchronization overhead cannot be ignored.'"
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Schrödinger's Cat and RCU (Well, Structured Procrastination, Actually)

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  • Huh? (Score:5, Insightful)

    by cultiv8 (1660093) on Saturday May 25, 2013 @06:03PM (#43823885) Homepage
    I feel like this submission was generated by SCIgen [slashdot.org].
    • by dmbasso (1052166)

      Thanx, I feel relieved now... I thought I was the only one that felt like on drugs while reading the summary.

      • Re:Huh? (Score:5, Informative)

        by Dahamma (304068) on Sunday May 26, 2013 @02:53AM (#43825869)

        The summary is completely unintelligible because it does an absolutely ridiculous job of quoting TFA. TFA, on the other hand, is actually a fairly interesting read... IMO go read the article and skip any slashdot summary or comments and you will be wiser for it.

        • ... IMO go read the article and skip any slashdot summary or comments and you will be wiser for it.

          Oh, for mod points!

          • by davecb (6526)
            For some reason it got longer than what I had tried to post... --dave
  • by GlobalEcho (26240) on Saturday May 25, 2013 @06:27PM (#43823997)

    I know! Let's explain the workings of a deterministic computational system using analogies to quantum mechanics! Everyone find quantum mechanics clear and intuitive.

    // Advanced degrees in physics and math
    /// Still find article unclear
    //// Reference counting makes sense already

    • You can take a MOOC on quantum physics. No need to remain ignorant...

      • by GlobalEcho (26240)

        You can take a MOOC on quantum physics. No need to remain ignorant...

        In my opinion, the only people who can learn much from a MOOC are N00Bs.

        (I'm being flippant, but give me a break. A MOOC in quantum physics, without underlying groundwork sufficient to understand, say, what a Hamiltonian is, couldn't possibly yield more than a superficial picture.)

        • I took the Berkeley Quantum Computation MOOC through Coursera, taught by Umesh Vazirani, twice. First time I maybe got half of what was in it (though I passed, barely); second time maybe 70% (again barely passing; homeworks and tests had mostly different problems). I learned to manipulate Hamiltonians in Octave to find their eigenvectors and eigenvalues, for example. So now I have a better sense of the math used to describe a superposition state, and I feel better informed about current models and ways of t

          • by Visserau (2433592)

            Do these courses have some references/a question forum/anything helpful other than a recorded lecture? Are any of these still active?

            I've done the first few lectures of a series from Stanford, and am having a bit of trouble. There is no supporting material, but the lecturer appears to be relying on tutorials/etc to fill in a lot of the detail. It doesn't help much that he often makes mistakes which aren't pointed out by the students until later. I have been filling in gaps with google (and a lot of pausing)

            • The forums for the class are an excellent resource. Often other students can explain things better than the instructors, because the instructors are so familiar with the material they forget what is confusing about it to someone learning it.

              The edx quantum computation class will be offered again, not sure when. The Coursera "Exploring Quantum Physics" class is finishing this week, but will likely be offered again.

              There may be others, you can check coursera's and edx's sites...

              • by Visserau (2433592)

                Yeah I had a look through the Coursera web site and was actually able to sign up to the class, since the exam isn't actually finally due until tomorrow morning. So I guess I'll at least download the reference materials and see if they help any :)

                The course I've mentioned is here: http://theoreticalminimum.com/courses/quantum-mechanics/2012/winter [theoreticalminimum.com] . I've heard good things about it and I suspect it is probably one of the more in depth classes, but it is not for the feint of heart. I'm probably doing myself a

    • Oh, I did worse than that! I tried to prop up an entire cosmology with the same metaphor [briandonohue.org].
  • by VortexCortex (1117377) <VortexCortexNO@S ... t-retrograde.com> on Saturday May 25, 2013 @06:40PM (#43824087)

    Slashdot Translation: It's similar to how an article exists in a superposition of both interesting and uninteresting at the same time, so long as no one reads it. Rather than RTFA and collapse the superposition, we can simply read the quantumly entangled comments to determine the degree of interest in accordance with our groupthink.

    • When I read the summary my cognitive wave function collapsed into "bafflement". I dare not RTFA, lest I lose the tenuous grasp of reality I have left.
  • ... would feel about the fact that his cat became a household name to illustrate quantum weirdness.

    After all he created that thought experiment to mock the Copenhagen Interpretation. He was severely unhappy with the latter, and argued against it in his article on the meaning of wave mechanics [wp.me].

    • by Livius (318358)

      Schrödinger's whole point was that for a macroscopic system as complex as a cat to be simultaneously alive and dead was obvious nonsense, and therefore there was something still missing in our understanding of quantum mechanics.

      Ironically, the outcome was people agreeing that quantum weirdness is, well, weird, but simultaneously believing he was talking about a cat actually being both alive and dead.

      • by quax (19371)

        So probably he turning in his grave with a spin 1/2 :-)

      • And still have not determined it's state.
      • Schroedinger was a mystic. Read some of his quotations about Nirvana and Hinduism at http://en.wikiquote.org/wiki/Erwin_Schr%C3%B6dinger [wikiquote.org]

        Here's a sample:

        The observing mind is not a physical system, it cannot interact with any physical system. And it might be better to reserve the term "subject" for the observing mind. ... For the subject, if anything, is the thing that senses and thinks. Sensations and thoughts do not belong to the "world of energy."

        Another sample:

        The world is given to me only once, not one

  • Key of the problem is that we can not know whether the cat is dead or alive without looking at it. Poor cat. But could we maybe determine the dead/alive state of the cat indirectly? For example by looking at the death rates of the mice? After all, when the cat is alive mice will die. So by just looking at the death rate of mice we can tell the cat is alive or dead, without looking at the cat directly. And as we never look at the cat directly it will remain dead and alive forever, without being forced in one

    • by ultranova (717540)

      Key of the problem is that we can not know whether the cat is dead or alive without looking at it. Poor cat. But could we maybe determine the dead/alive state of the cat indirectly? For example by looking at the death rates of the mice?

      How is this any more indirect than looking at the photons reflecting from the cat? Which you also don't observe directly, BTW: you observe the chemical reactions they cause in your eyes. Or, if we want to get really specific: photons reflect from the cat and cause chemical r

  • by mcmonkey (96054)

    Just no.

    While births indeed should mean insertions in to the database, deaths should not be deletions. They should be updating the Living flag to 'N' and populating the Date of Death and Cause of Death fields.

    They're going to want reports and stats not just on the current living animals. I didn't RTFA ('natch) but if they missed that requirement, what else have they overlooked?

    • by Anonymous Coward

      I think you took the metaphor too seriously. The whole point is he wants to free memory while maintaining coherent state across threads. He proposes three solutions of increasing "subtlety", though none of them appear novel.

    • by Jeremi (14640)

      They're going to want reports and stats not just on the current living animals. I didn't RTFA ('natch) but if they missed that requirement, what else have they overlooked?

      I think what you overlooked was that the "database" in the article was just a colorful contrivance to illustrate common usages of an in-memory data structure (e.g. a page-table in the Linux kernel) and not a True Database in the SQL/web-server/generate-me-a-report sense.

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