"The current criteria for memtable flush is not being respected. The
problem is demonstrated to happen when the dirty memory group is over
limit, and so is the system table extra allowance. In that situation,
both the normal region and the system table region will be under
pressure and try to flush.
More specifically, because the normal region inherits from the system
region, if the normal region is under pressure (over the soft limit
threshold), the system region will certainly be as well, even though it
has an extra allowance. This is because after virtual dirty, we start
blocking when we reach half the region, but memory itself can grow up to
100 % of the region. So the total amount of memory used will be
certainly bigger than the system pressure threshold, which is now 50 %
plus the allowance.
To fix that, this patch reworks the flush logic so that the regions are
not dependent on each other.
Fixes#1918"
* 'flush-criteria-v6' of github.com:glommer/scylla:
config: get rid of memtable_total_space
database: rework dirty memory hierarchy
system keyspace: write batchlog mutation in user memory
database: remove flush_token
database: abstract pressure condition notification
database: encapsulate semaphore_units into a flush_permit
database: remove friendship declaration
database: simplify flush_one
database: make memtable_list aware in cases it can't flush
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