Re: [PATCH 17/18] vinyl: set quota timer period to 100 ms

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

On Thu, Aug 23, 2018 at 11:49:36PM +0300, Konstantin Osipov wrote:
> * Vladimir Davydov <vdavydov.dev@gmail.com> [18/08/16 23:03]:
> > Currently, it's 1 second, which is OK for calculating watermark, but
> > too long for throttling (think of latency of 1 seconds that would be
> > introduced by throttling if such timeout were used).
> 
> 1) I think the timer period should self-adjust based on the amount
>    of available quota.

Do we really need to bother? An execution of a timer callback is pretty
cheap - it's merely a few arithmetic calculations. I set it to 100 ms to
make sure the latency won't be too high when throttling is enabled.

> 
> 2) We narrowed down the problem to throttling only.
>    We never do anticipatory dump/compaction today.  Let's design a
>    system which swings the pendulum both ways - not only throttles
>    the client when the load is high but performs an anticipatory
>    dump/compaction when the load is low. See for example
>    https://github.com/tarantool/tarantool/issues/3225

It totally makes sense, of course, but it's a completely different
issue, which can be done separately on top of this patch set.

> 
>    In other words, I think we should not only enable "stalls" in
>    the timer callback when the write stream is strong, but
>    trigger compaction when the write stream is thin. Unfortunately
>    we do not collect read statistics per lsm as a secondary
>    indicator of compaction priority, so we may only trigger
>    compaction based simply on the number of levels, but never the
>    less.