"This patchset restricts background writers - such as compactions,
streaming flushes and memtable flushes to a maximum amount of CPU usage
through a seastar::thread_scheduling_group.
The said maximum is recommended to be set 50 % - it is default
disabled, but can be adjusted through a configuration option until we
are able to auto-tune this.
The second patch in this series provides a preview on how such auto-tune
would look like. By implementing a simple controller we automatically
adjust the quota for the memtable writer processes, so that the rate at
which bytes come in is equal to the rates at which bytes are flushed.
Tail latencies are greatly reduced by this series, and heavy spikes that
previously appeared on CPU-bound workloads are no more."
* 'memtable-controller-v5' of https://github.com/glommer/scylla:
simple controller for memtable/streaming writer shares.
restrict background writers to 50 % of CPU.
(cherry picked from commit c5ee62a6a4)
