Re: [tarantool-patches] Re: [PATCH v2 05/11] vinyl: implement quota wait queue without fiber_cond

[Vladimir Davydov <vdavydov.dev@gmail.com>]

On Sat, Sep 29, 2018 at 08:05:31AM +0300, Konstantin Osipov wrote:
> * Vladimir Davydov <vdavydov.dev@gmail.com> [18/09/28 21:00]:
> > Using fiber_cond as a wait queue isn't very convenient, because:
> >  - It doesn't allow us to put a spuriously woken up fiber back to the
> >    same position in the queue where it was, thus violating fairness.
> >  - It doesn't allow us to check whether we actually need to wake up a
> >    fiber or it will have to go back to sleep anyway as it needs more
> >    memory than currently available.
> >  - It doesn't allow us to implement a multi-queue approach where fibers
> >    that have different priorities are put to different queues.
> > 
> > So let's rewrite the wait queue with plain rlist and fiber_yield.
> 
> Maybe you could factor out this change into an abstraction in fiber.[hc]?
> Or fix fiber_cond to be fair?

I don't see any point in abstracting such a tiny and simple thing as a
wait queue, at least not right now, when we need it only in vy_quota.
Note, what we need in vy_quota isn't as simple as waking up fibers in
the order they were put to sleep, because we don't want to wake up a
fiber if it will have to go back to sleep immediately. Besides, take a
look at the last patch in the series where I turn the queue into a
multi-queue in order to introduce quota consumer priorities. I don't
rule out that it may be possible to make this code abstract, but IMO it
isn't worth the effort right now.