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    SubjectRe: [RFC][PATCH RT 0/2] futex priority based wakeup
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    On Wed, 2006-05-10 at 12:08 +0200, Ingo Molnar wrote:
    > * Sébastien Dugué <sebastien.dugue@bull.net> wrote:
    >
    > > in the current futex implementation, tasks are woken up in FIFO
    > > order, (i.e. in the order they were put to sleep). For realtime
    > > systems needing system wide strict realtime priority scheduling, tasks
    > > should be woken up in priority order.
    > >
    > > This patchset achieves this by changing the futex hash bucket list
    > > into a plist. Tasks waiting on a futex are enqueued in this plist
    > > based on their priority so that they can be woken up in priority
    > > order.
    >
    > hm, i dont think this is enough. Basically, waking up in priority order
    > is just the (easier) half of the story - what you want is to also
    > propagate priorities when you block. We provided a complete solution via
    > the PI-futex patchset (currently included in -mm).
    >
    > In other words: as long as locking primitives go, i dont think real-time
    > applications should use wakeup-priority-ordered futexes, they should use
    > the real thing, PI futexes.

    Yeah, that's right as long as userland pthread_mutexes are used, but
    currently, pthread_condvars do not use PI-futexes. Therefore when we
    have some threads sleeping on a condvar and the condvar is broadcasted,
    those blocked threads are woken up through futex_requeue() in the order
    they were put to sleep and not in their priority order.

    Maybe the pthread_cond_*() function should be made to use the
    PI-futexes support in glibc.

    >
    > There is one exception: when a normal futex is used as a waitqueue
    > without any contention properties. (for example a waitqueue for worker
    > threads) But those are both rare, and typically dont muster tasks with
    > different priorities - i.e. FIFO is good enough.
    >
    > Also, there's a performance cost to this. Could you try to measure the
    > impact to SCHED_OTHER tasks via some pthread locking benchmark?

    Right, I will try to quantify the impact for SCHED_OTHER tasks. Any
    pointers to such a benchmark?

    Thanks,

    Sébastien.


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