When row cache is disabled, update_cache() will do nothing to the
memtable. Active readers may keep the memtable alive for unbounded
amount of time, preventing it from going away. This doesn't play well
with virtual dirty accounting. Soon before calling update_cache(), the
memory which was subtracted during flush is added back to the amount
of virtual dirty memory. If there was write pressure all along, we
will be at the dirty memory limit. When we give back subtracted memory
this will put virtual dirty way above the limit. This will stall all
writes until another memtable flush drags virtual dirty down or
readers finally release the memtable. We want to prevent upward
jumps of virtual dirty.
First part of the fix is to ensure that as long as the memtable's
region is in the dirty group, we will not revert flushed memory. This
must happen synchronously from region's memory being removed from the
group in order to prevent upward virtual dirty jumps. To make this
easier, tracking of flushed memory was moved to the memtable object.
Another part of the fix is to gradually clear the memtable when cache
is disabled in a similar fashion as when it's moved to cache. This
ensures that the actual memory held by memtable's region is released
sooner than it dies.
Refs #1879
3511bf4a81