Hi Andrey,
Thanks for the feedback.
Às 08:01 de 14/07/22, Andrey Semashev escreveu:
> On 7/14/22 06:18, André Almeida wrote:
[...]
>>
>> Feedback? Who else should I CC?
>
> Just a few questions:
>
> Do I understand correctly that notifiers won't be able to wake up
> waiters unless they know on which node they are waiting?
>
If userspace is using NUMA_FLAG, yes. Otherwise all futexes would be
located in the default node, and userspace doesn't need to know which
one is the default.
> Is it possible to wait on a futex on different nodes?
Yes, given that you specify `.hint = id` with the proper node id.
>
> Is it possible to wake waiters on a futex on all nodes? When a single
> (or N, where N is not "all") waiter is woken, which node is selected? Is
> there a rotation of nodes, so that nodes are not skewed in terms of
> notified waiters?
Regardless of which node the waiter process is running, what matter is
in which node the futex hash table is. So for instance if we have:
struct futex32_numa f = {.value = 0, hint = 2};
And now we add some waiters for this futex:
Thread 1, running on node 3:
futex_wait(&f, 0, FUTEX_NUMA | FUTEX_32, NULL);
Thread 2, running on node 0:
futex_wait(&f, 0, FUTEX_NUMA | FUTEX_32, NULL);
Thread 3, running on node 2:
futex_wait(&f, 0, FUTEX_NUMA | FUTEX_32, NULL);
And then, Thread 4, running on node 3:
futex_wake(&f, 2, FUTEX_NUMA | FUTEX_32);
Now, two waiter would wake up (e.g. T1 and T3, node 3 and 2) and they
are from different nodes. futex_wake() doesn't provide guarantees of
which waiter will be selected, so I can't say which node would be
selected. There's no policy for fairness/starvation for futex_wake(). Do
you think this would be important for the NUMA case?
Let me know if this clarifies your questions.
