After recent changes to the memtable code, there is no reason for us to
uphold a maximum memtable size. Now that we only flush one memtable at a
time anyway, and also have soft limit notifications from the
region_group_reclaimer, we can just set the soft limit to the target
size and let all of that be handled by the dirty_memory_manager.
It does have the added property that we'll be flushing when we globally
reach the soft limit threshold. In conditions in which we have multiple
CF writes fighting for memory, that guarantees that we will start
flushing much earlier than the hard limit.
The threshold is set to 1/4 of dirty memory. While in theory we would
prefer the memtables to go as big as 1/2 of dirty memory, in my
experiments I have found 1/4 to be a better fit, at least for the
moment.
The reason for such behavior is that in situations where we have slow
disks, setting the soft limit to 1/2 of dirty will put us in a situation
in which we may not have finished writing down the memtable when we hit
the limit, and then throttle. When set the threshold to 1/4 of dirty, we
don't throttle at all.
This behavior could potentially be fixed by not doing the full
memtable-based throttling after we do the commitlog throttling, but that
is not something realistic for the moment.
Signed-off-by: Glauber Costa <glauber@scylladb.com>
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