[tarantool-patches] Re: [PATCH 7/9] vinyl: randomize range compaction to avoid IO load spikes

[Konstantin Osipov]

* Vladimir Davydov <vdavydov.dev at gmail.com> [19/01/22 15:56]:
> I ran some tests and, surprisingly, it turned out that randomization
> didn't help at all: the compaction queue size jumped up to 30% and even
> 40% from time to time although there was plenty of compaction power -
> compaction threads were busy only half of the time. When I looked
> closer, I saw that the queue size behavior looked weird - it jumped
> after a dump for a very short period of time, until the next dump, which
> pushed it back to 10%. This made me wonder how it could happen at all -
> normally, compaction queue should only grow after a dump, not diminish.

> I think I've finally found the root cause of the problem. The code
> computing compaction priority (see vy_range_update_compaction_priority)
> is unstable meaning the size of the first level equals the size of the
> smallest run so if memory dumps produce runs of varying sizes, which is
> what happens in practice in contrast to simulation, the shape of the
> tree will vary as well, resulting in different compaction priority and
> unstable queue behavior.
> 
> We must fix this somehow. One way to do it is compute the first level
> size basing on the size of the last level run, which is constant most of
> the time, and indeed, when I did it and reran the tests, I found that
> the queue stayed below 10% all the time. Turning off randomization, made
> the queue jump up to 30%, which was expected. The diff is below. I'll
> spruce it up a little, wrap it nicely, and submit it separately later.

Why not use a simple weighted average? 


-- 
Konstantin Osipov, Moscow, Russia, +7 903 626 22 32
http://tarantool.io - www.twitter.com/kostja_osipov