fd1dd0eac7
" 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 ...
Scylla
What is Scylla?
Scylla is the real-time big data database that is API-compatible with Apache Cassandra and Amazon DynamoDB. Scylla embraces a shared-nothing approach that increases throughput and storage capacity to realize order-of-magnitude performance improvements and reduce hardware costs.
For more information, please see the ScyllaDB web site.
Build Prerequisites
Scylla is fairly fussy about its build environment, requiring very recent versions of the C++20 compiler and of many libraries to build. The document HACKING.md includes detailed information on building and developing Scylla, but to get Scylla building quickly on (almost) any build machine, Scylla offers a frozen toolchain, This is a pre-configured Docker image which includes recent versions of all the required compilers, libraries and build tools. Using the frozen toolchain allows you to avoid changing anything in your build machine to meet Scylla's requirements - you just need to meet the frozen toolchain's prerequisites (mostly, Docker or Podman being available).
Building Scylla
Building Scylla with the frozen toolchain dbuild is as easy as:
$ git submodule update --init --force --recursive
$ ./tools/toolchain/dbuild ./configure.py
$ ./tools/toolchain/dbuild ninja build/release/scylla
For further information, please see:
- Developer documentation for more information on building Scylla.
- Build documentation on how to build Scylla binaries, tests, and packages.
- Docker image build documentation for information on how to build Docker images.
Running Scylla
To start Scylla server, run:
$ ./tools/toolchain/dbuild ./build/release/scylla --workdir tmp --smp 1 --developer-mode 1
This will start a Scylla node with one CPU core allocated to it and data files stored in the tmp directory.
The --developer-mode is needed to disable the various checks Scylla performs at startup to ensure the machine is configured for maximum performance (not relevant on development workstations).
Please note that you need to run Scylla with dbuild if you built it with the frozen toolchain.
For more run options, run:
$ ./tools/toolchain/dbuild ./build/release/scylla --help
Testing
See test.py manual.
Scylla APIs and compatibility
By default, Scylla is compatible with Apache Cassandra and its APIs - CQL and Thrift. There is also support for the API of Amazon DynamoDB™, which needs to be enabled and configured in order to be used. For more information on how to enable the DynamoDB™ API in Scylla, and the current compatibility of this feature as well as Scylla-specific extensions, see Alternator and Getting started with Alternator.
Documentation
Documentation can be found in ./docs and on the wiki. There is currently no clear definition of what goes where, so when looking for something be sure to check both. Seastar documentation can be found here. User documentation can be found here.
Training
Training material and online courses can be found at Scylla University. The courses are free, self-paced and include hands-on examples. They cover a variety of topics including Scylla data modeling, administration, architecture, basic NoSQL concepts, using drivers for application development, Scylla setup, failover, compactions, multi-datacenters and how Scylla integrates with third-party applications.
Contributing to Scylla
If you want to report a bug or submit a pull request or a patch, please read the contribution guidelines.
If you are a developer working on Scylla, please read the developer guidelines.
Contact
- The users mailing list and Slack channel are for users to discuss configuration, management, and operations of the ScyllaDB open source.
- The developers mailing list is for developers and people interested in following the development of ScyllaDB to discuss technical topics.