Re: [RFC] futex2: add NUMA awareness

On 7/14/22 18:00, André Almeida wrote:

Hi Andrey,

Thanks for the feedback.

Às 08:01 de 14/07/22, Andrey Semashev escreveu:

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On 7/14/22 06:18, André Almeida wrote:

[...]

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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.

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Is it possible to wait on a futex on different nodes?

Yes, given that you specify `.hint = id` with the proper node id.

So any given futex_wake(FUTEX_NUMA) operates only within its node, right?

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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.

In this example, T1, T2 and T3 are all blocking on node 2 (since all of
them presumably specify hint == 2), right? In this sense, it doesn't
matter which node they are running on, what matters is what node they
block on.

What I'm asking is can I wake all threads blocked on all nodes on the
same futex? That is, is the following possible?

  // I'm using hint == -1 to indicate the current node
  // of the calling thread for waiters and all nodes for notifiers
  struct futex32_numa f = {.value = 0, .hint = -1};

  Thread 1, running on node 3, blocks on node 3:

  futex_wait(&f, 0, FUTEX_NUMA | FUTEX_32, NULL);

  Thread 2, running on node 0, blocks on node 0:

  futex_wait(&f, 0, FUTEX_NUMA | FUTEX_32, NULL);

  Thread 3, running on node 2, blocks on node 2:

  futex_wait(&f, 0, FUTEX_NUMA | FUTEX_32, NULL);

  And then, Thread 4, running on whatever node:

  futex_wake(&f, -1, FUTEX_NUMA | FUTEX_32);

Here, futex_wake would wake T1, T2 and T3. Or:

  futex_wake(&f, 1, FUTEX_NUMA | FUTEX_32);

Here, futex_wake would wake any one of T1, T2 or T3.

There's no policy for fairness/starvation for futex_wake(). Do
you think this would be important for the NUMA case?

I'm not sure yet. If there isn't a cross-node behavior like in my
example above then, I suppose, it falls to the userspace to ensure fair
rotation of the wakeups on different nodes. If there is functionality
like this, I imagine, some sort of fairness would be desired.