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    SubjectRe: New filesystem for Linux
    Mikulas Patocka a écrit :
    > Hi
    >
    > As my PhD thesis, I am designing and writing a filesystem, and it's now
    > in a state that it can be released. You can download it from
    > http://artax.karlin.mff.cuni.cz/~mikulas/spadfs/
    >
    > It has some new features, such as keeping inode information directly in
    > directory (until you create hardlink) so that ls -la doesn't seek much,
    > new method to keep data consistent in case of crashes (instead of
    > journaling), free space is organized in lists of free runs and converted
    > to bitmap only in case of extreme fragmentation.
    >
    > It is not very widely tested, so if you want, test it.
    >
    > I have these questions:
    >
    > * There is a rw semaphore that is locked for read for nearly all
    > operations and locked for write only rarely. However locking for read
    > causes cache line pingpong on SMP systems. Do you have an idea how to
    > make it better?
    >
    > It could be improved by making a semaphore for each CPU and locking for
    > read only the CPU's semaphore and for write all semaphores. Or is there
    > a better method?
    >

    If you believe you need a semaphore for protecting a mostly read structure,
    then RCU is certainly a good candidate. (ie no locked operation at all)

    The problem with a per_cpu biglock is that you may consume a lot of RAM for
    big NR_CPUS. Count 32 KB per 'biglock' if NR_CPUS=1024

    > * This leads to another observation --- on i386 locking a semaphore is 2
    > instructions, on x86_64 it is a call to two nested functions. Has it
    > some reason or was it just implementator's laziness? Given the fact that
    > locked instruction takes 16 ticks on Opteron (and can overlap about 2
    > ticks with other instructions), it would make sense to have optimized
    > semaphores too.

    Hum, please dont use *lazy*, this could make Andi unhappy :)

    What are you calling semaphore exactly ?
    Did you read Documentation/mutex-design.txt ?



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