[lkml]   [2006]   [Nov]   [4]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
    SubjectRe: New filesystem for Linux
    Gautham R Shenoy a écrit :
    > 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
    >> 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.

    We might use an hybrid approach : Use a percpu counter if NR_CPUS <= 8

    #define refcount_addr(zone, cpu) zone[cpu]

    For larger setups, have a fixed limit of 8 counters, and use a modulo

    #define refcount_addr(zone, cpu) zone[cpu & 7]

    In order not use too much memory, we could use kind of vmalloc() space, using
    one PAGE per cpu, so that addr(cpu) = base + (cpu)*PAGE_SIZE;
    (vmalloc space allows a NUMA allocation if possible)

    So instead of storing in an object a table of 8 pointers, we store only the
    address for cpu0.

    > 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.

    To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
    the body of a message to
    More majordomo info at
    Please read the FAQ at

     \ /
      Last update: 2006-11-04 19:31    [W:0.034 / U:119.364 seconds]
    ©2003-2016 Jasper Spaans. hosted at Digital OceanAdvertise on this site