Throttle::wait use case clarification

[Loic Dachary <loic@dachary.org>]

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Hi,

While working on unit tests for Throttle.{cc,h} I tried to figure out a use case related to the Throttle::wait method but couldn't

https://github.com/ceph/ceph/pull/34/files#L3R258

Although it was not a blocker and I managed to reach 100% coverage anyway, it got me curious and I would very much appreciate pointers to understand the rationale.

wait() can be called to set a new maximum before waiting for all pending threads to get get what they asked for. Since the maximum has changed, wait() wakes up the first thread : the conditions under which it decided to go to sleep have changed and the conclusion may be different.

However, it only does so when the new maximum is less than current one. For instance

A) decision does not change

  max = 10, current 9
  thread 1 tries to get 5 but only 1 is available, it goes to sleep
  wait(8)
  max = 8, current 9
  wakes up thread 1
  thread 1 tries to get 5 but current is already beyond the maximum, it goes to sleep

B) decision changes

  max = 10, current 1
  thread 1 tries to get 10 but only 9 is available, it goes to sleep
  wait(9)
  max = 9, current 1
  wakes up thread 1
  thread 1 tries to get 10 which is above the maximum : it succeeds because current is below the new maximum

It will not wake up a thread if the maximum increases, for instance:

  max = 10, current 9
  thread 1 tries to get 5 but only 1 is available, it goes to sleep
  wait(20)
  max = 20, current 9
  does *not* wake up thread 1
  keeps waiting until another thread put(N) with N >= 0 although there now is 11 available and it would allow it to get 5 out of it

Why is it not desirable for thread 1 to wake up in this case ? When debugging a real world situation, I think it would show as a thread blocked although the throttle it is waiting on has enough to satisfy its request. What am I missing ?

Cheers


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