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  • How to diagnose computer freezing problem

    - by reinierpost
    I have a laptop (a Medion from Aldi) that tends to hang quite often - so often, in fact, that several attempts to install Windows XP or Ubuntu on it have all failed. However, I am able to boot and run Ubuntu as found on the standard Ubuntu 10.10 installation image. I have done this two times thus far. The first time everything was running smoothly, until at some point the GUI (i.e. X) became unresponsive. The cursor kept moving with the mouse, but menus would no longer show and clicking things no longer produced any response. So I switched to the consoles (Ctrl-F1, Ctrl-F2, etc., which in this setup automatically run shells. The shells were still responsive, and the cd command would still work, but any command that invoked an executable (e.g. /bin/ls or cd /bin; ./find caused the shell to hang up uninterruptibly. My hypothesis was that all attempts at disk access were hanging up, but I didn't actually try a command like echo /proc/$$ or while read line; do echo $line; done < /var/log/syslog to verify this. Another possibility is that an essential system library is cached in memory and somehow failing to function properly. The second time I left the system running overnight and it didn't hang itself spontaneously. I'm not sure I have the patience to just twiddle with the running system until the condition reappears, and I'm, not sure what to do once it does. Clearly we can rule out a software cause. It seems disk access related, but clearly it's not permanent hard disk failure because the system will reboot just fine. What kind of hardware problem might produce these symptoms? Can it be a memory problem?

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  • fast similarity detection

    - by reinierpost
    I have a large collection of objects and I need to figure out the similarities between them. To be exact: given two objects I can compute their dissimilarity as a number, a metric - higher values mean less similarity and 0 means the objects have identical contents. The cost of computing this number is proportional to the size of the smaller object (each object has a given size). I need the ability to quickly find, given an object, the set of objects similar to it. To be exact: I need to produce a data structure that maps any object o to the set of objects no more dissimilar to o than d, for some dissimilarity value d, such that listing the objects in the set takes no more time than if they were in an array or linked list (and perhaps they actually are). Typically, the set will be very much smaller than the total number of objects, so it is really worthwhile to perform this computation. It's good enough if the data structure assumes a fixed d, but if it works for an arbitrary d, even better. Have you seen this problem before, or something similar to it? What is a good solution? To be exact: a straightforward solution involves computing the dissimilarities between all pairs of objects, but this is slow - O(n2) where n is the number of objects. Is there a general solution with lower complexity?

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