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    SubjectRe: New filesystem for Linux
    >> 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)

    RCU is for non-blocking operation only.

    Maybe it could be use for maintaining an unlocked variable that prevents
    readers from entering a critical section --- but waiting in a write for a
    quiscent state would hurt.

    > 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 ?

    I see --- I could replace that semaphore with mutex. But rw-semaphores
    can't be replaces with them.

    Mikulas
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