"
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
...
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