[PATCH 8/8] vinyl: keep track of thread pool idle ratio

[Vladimir Davydov]

On Thu, Sep 06, 2018 at 01:26:31PM +0300, Konstantin Osipov wrote:
> * Vladimir Davydov <vdavydov.dev at gmail.com> [18/09/06 11:19]:
> 
> > > > To understand whether the disk is fully utilized or can still handle
> > > > more compaction load and make right decisions regarding transaction
> > > > throttling, we need a metric that would report how much time worker
> > > > threads spent being idle. So this patch adds two new metrics to global
> > > > statistics, disk.dump_idle_ratio and compact_idle_ratio, which show how
> > > > much time dump threads and compaction threads were idle, respectively.
> > > > The metrics are updated using the following formula:
> > > > 
> > > >                        idle_time
> > > >   idle_ratio = --------------------------
> > > >                dump_period * worker_count
> > > 
> > > I don't understand the formula. There can be many workers.
> > > Is idle time measured per worker or per entire pool? 
> > > 
> > > If it is measured per entire pool, how is idle time calculated if
> > > some workers are busy and some not?
> > 
> > It is measured for entire pool - note that I divide the result by
> > worker_count. E.g. if there were two workers and one of them were
> > busy all the time between two last dumps while another were idle,
> > idle_ratio would be 0.5.
> 
> This looks imprecise. Why not measure idle time of each worker and
> then even it out over the total number of workers?

That's exactly what I do. I maintain the idle time for each worker
thread and then divide it by the total number of workers.

> Besides, once again, how do you define the window over which you
> measure?

I use dump period for the window, i.e. time between two subsequent
memory dumps. Respectively, I update the idle_ratio after each memory
dump. I'm planning to update throttle rate limit there too.