On Sunday, January 20, 2013 at 5:39 AM, Loic Dachary wrote:
> 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
>
>
> Attachments:
> - loic.vcf
>
Looking through the history of that test (in _reset_max), I think it's an accident and we actually want to be waking up the front if the maximum increases (or possibly in all cases, in case the front is a very large request we're going to let through anyway). Want to submit a patch? :)
The other possibility I was trying to investigate is that it had something to do with handling get() requests larger than the max correctly, but I can't find any evidence of that one...
-Greg
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