On Thu, Nov 02, 2006 at 10:52:47PM +0100, Mikulas Patocka wrote:
> Hi
Hi Mikulas
>
> 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?
I am currently experimenting with a light-weight reader writer semaphore
with an objective to do away what you call a reader side cache line
"ping pong". It achieves this by using a per-cpu refcount.
A drawback of this approach, as Eric Dumazet mentioned elsewhere in this
thread, would be that each instance of the rw_semaphore would require
(NR_CPUS * size_of(int)) bytes worth of memory in order to keep track of
the per-cpu refcount, which can prove to be pretty costly if this
rw_semaphore is for something like inode->i_alloc_sem.
So the question I am interested in is, how many *live* instances of this
rw_semaphore are you expecting to have at any given time?
If this number is a constant (and/or not very big!), the light-weight
reader writer semaphore might be useful.
Regards
Gautham.
--
Gautham R Shenoy
Linux Technology Center
IBM India.
"Freedom comes with a price tag of responsibility, which is still a bargain,
because Freedom is priceless!"
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