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    SubjectRe: New filesystem for Linux
    > Mikulas Patocka <mikulas@artax.karlin.mff.cuni.cz> writes:
    >
    >> new method to keep data consistent in case of crashes (instead
    >> of journaling),
    >
    > What is that method?

    Some tricks to avoid journal --- see
    http://artax.karlin.mff.cuni.cz/~mikulas/spadfs/download/INTERNALS

    --- unlike journaling it survives only 65536 crashes :)

    >> * There is a rw semaphore that is locked for read for nearly all
    >
    > Depending on the length of the critical section rw locks are often
    > not faster than non rw locks because the read case has to bounce
    > around the cache line of the lock anyways and they're actually
    > a little more expensive.

    This critical section is long --- i.e. any reads/writes to disk. Making it
    simple semaphore would effectively serialize all operations.

    >> * 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
    >
    > The second call should be a tail call, i.e. just a jump

    It is down_write -> (tailcall) down_write_nested -> (normal call)
    spin_lock_irq and spin_unlock_irq.

    > The first call isn't needed on a non debug kernel, but doing the
    > two unconditional jumps should be basically free on a modern OOO CPU.

    But it kills one cacheline.

    > The actual implementation is spinlock based vs atomic based for i386.
    > This was because at some point nobody could benchmark a difference
    > between the two and the spinlock based version is much easier
    > to verify and to understand. If you can demonstrate a difference
    > that could be reevaluated.

    Maybe one day I'll try 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.
    >
    > In the last time we're going more for saved icache and better
    > use of branch predictors (who are more happy with less branch locations
    > to cache) I would expect the call/ret to be executed
    > mostly in parallel with the other code anyways.

    I see, but pushf, cli and popf in that spinlock hurt too (especially on
    Intel, it has them completely microcoded --- pushf/popf pair is 100
    ticks on Intel P4E and 12 ticks on Opteron).

    Mikulas

    > -Andi
    >
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