Messages in this thread | | | Date | Thu, 02 Nov 2006 23:53:00 +0100 | From | Eric Dumazet <> | Subject | Re: 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 ?
- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
| |