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    SubjectRe: [RFC] NUMA futex hashing
    Date
    On Tuesday 08 August 2006 18:34, Nick Piggin wrote:
    > Eric Dumazet wrote:
    > > We certainly can. But if you insist of using mmap sem at all, then we
    > > have a problem.
    > >
    > > rbtree would not reduce cacheline bouncing, so :
    > >
    > > We could use a hashtable (allocated on demand) of size N, N depending on
    > > NR_CPUS for example. each chain protected by a private spinlock. If N is
    > > well chosen, we might reduce lock cacheline bouncing. (different threads
    > > fighting on different private futexes would have a good chance to get
    > > different cachelines in this hashtable)
    >
    > See other mail. We already have a hash table ;)

    Yes but still you want at FUTEX_WAIT time to tell the kernel the futex is
    private to this process.

    Giving the same info at FUTEX_WAKE time could avoid the kernel to make the
    second pass (using only a private futex lookup), avoiding again the mmap_sem
    touch in case no threads are waiting anymore on this futex.

    Eric
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