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" The last major untracked area of the reader pipeline is the reader buffers. These scale with the number of readers as well as with the size and shape of data, so their memory consumption is unpredictable varies wildly. For example many small rows will trigger larger buffers allocated within the `circular_buffer<mutation_fragment>`, while few larger rows will consume a lot of external memory. This series covers this area by tracking the memory consumption of both the buffer and its content. This is achieved by passing a tracking allocator to `circular_buffer<mutation_fragment>` so that each allocation it makes is tracked. Additionally, we now track the memory consumption of each and every mutation fragment through its whole lifetime. Initially I contemplated just tracking the `_buffer_size` of `flat_mutation_reader::impl`, but concluded that as our reader trees are typically quite deep, this would result in a lot of unnecessary `signal()`/`consume()` calls, that scales with the number of mutation fragments and hence adds to the already considerable per mutation fragment overhead. The solution chosen in this series is to instead track the memory consumption of the individual mutation fragments, with the observation that these are typically always moved and very rarely copied, so the number of `signal()`/`consume()` calls will be minimal. This additional tracking introduces an interesting dilemma however: readers will now have significant memory on their account even before being admitted. So it may happen that they can prevent their own admission via this memory consumption. To prevent this, memory consumption is only forwarded to the semaphore upon admission. This might be solved when the semaphore is moved to the front -- before the cache. Another consequence of this additional, more complete tracking is that evictable readers now consume memory even when the underlying reader is evicted. So it may happen that even though no reader is currently admitted, all memory is consumed from the semaphore. To prevent any such deadlocks, the semaphore now admits a reader unconditionally if no reader is admitted -- that is if all count resources all available. Refs: #4176 Tests: unit(dev, debug, release) " * 'track-reader-buffers/v2' of https://github.com/denesb/scylla: (37 commits) test/manual/sstable_scan_footprint_test: run test body in statement sched group test/manual/sstable_scan_footprint_test: move test main code into separate function test/manual/sstable_scan_footprint_test: sprinkle some thread::maybe_yield():s test/manual/sstable_scan_footprint_test: make clustering row size configurable test/manual/sstable_scan_footprint_test: document sstable related command line arguments mutation_fragment_test: add exception safety test for mutation_fragment::mutate_as_*() test: simple_schema: add make_static_row() reader_permit: reader_resources: add operator== mutation_fragment: memory_usage(): remove unused schema parameter mutation_fragment: track memory usage through the reader_permit reader_permit: resource_units: add permit() and resources() accessors mutation_fragment: add schema and permit partition_snapshot_row_cursor: row(): return clustering_row instead of mutation_fragment mutation_fragment: remove as_mutable_end_of_partition() mutation_fragment: s/as_mutable_partition_start/mutate_as_partition_start/ mutation_fragment: s/as_mutable_range_tombstone/mutate_as_range_tombstone/ mutation_fragment: s/as_mutable_clustering_row/mutate_as_clustering_row/ mutation_fragment: s/as_mutable_static_row/mutation_as_static_row/ flat_mutation_reader: make _buffer a tracked buffer mutation_reader: extract the two fill_buffer_result into a single one ...