"Currently restricting_mutation_reader restricts mutation_readears on a
count basis. This is inaccurate on multiple levels. The reader might be
a combined_mutation_reader, which might be composed of multiple
individual readers, whose number might change during the lifetime of the
reader. The memory consumption of the readers can vary and may change
during the lifetime of the reader as well.
To remedy this, make the restriction memory-consumption based. The
restricting semaphore is now configured with the amound of memory
(bytes) that its readers are allowed to consume in total. New readers
consume 128k units up-front to account for read-ahead buffers, and then
consume additional units for any buffer (returned
from input_stream<>::read()) they keep around.
Like before, readers already allowed to read will not be blocked,
instead new readers will be blocked on their first read if all the units
all consumed.
Fixes #2692."
* 'bdenes/restricting_mutation_reader-v5' of https://github.com/denesb/scylla:
Update reader restriction related metrics
Add restricted_reader_test unit test
restricted_mutation_reader: restrict based-on memory consumption
mutation_reader.hh: Move restricted_reader related code
bdbbfe9390