[PATCH 05/18] vinyl: wake up fibers waiting for quota one by one

[Vladimir Davydov]

On Mon, Aug 20, 2018 at 02:11:23PM +0300, Konstantin Osipov wrote:
> * Vladimir Davydov <vdavydov.dev at gmail.com> [18/08/16 23:03]:
> > Currently, we wake up all fibers whenever we free some memory. This
> > is inefficient, because it might occur that all available quota gets
> > consumed by a few fibers while the rest will have to go back to sleep.
> > This is also kinda unfair, because waking up all fibers breaks the order
> > in which the fibers were put to sleep. This works now, because we free
> > memory and wake up fibers infrequently (on dump) and there normally
> > shouldn't be any fibers waiting for quota (if there were, the latency
> > would rocket sky high because of absence of any kind of throttling).
> > However, once throttling is introduced, fibers waiting for quota will
> > become the norm. So let's wake up fibers one by one: whenever we free
> > memory we wake up the first fiber in the line, which will wake up the
> > next fiber on success and so forth.
> > +
> > +	/* Wake up the next fiber in the line waiting for quota. */
> > +	fiber_cond_signal(&q->cond);
> >  	return 0;
> 
> Please only wake up the next fiber if there is one, or, better
> yet,  if this fiber had been put to sleep and then was woken up. 

I don't quite understand what you mean - fiber_cond_signal() will do
nothing if there's no fiber waiting on it.