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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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