86bbf1763d
" The reader concurrency semaphore timing out or its queue being overflown are fairly common events both in production and in testing. At the same time it is a hard to diagnose problem that often has a benign cause (especially during testing), but it is equally possible that it points to something serious. So when this error starts to appear in logs, usually we want to investigate and the investigation is lengthy... either involves looking at metrics or coredumps or both. This patch intends to jumpstart this process by dumping a diagnostics on semaphore timeout or queue overflow. The diagnostics is printed to the log with debug level to avoid excessive spamming. It contains a histogram of all the permits associated with the problematic semaphore organized by table, operation and state. Example: DEBUG 2020-10-08 17:05:26,115 [shard 0] reader_concurrency_semaphore - Semaphore _read_concurrency_sem: timed out, dumping permit diagnostics: Permits with state admitted, sorted by memory memory count name 3499M 27 ks.test:data-query 3499M 27 total Permits with state waiting, sorted by count count memory name 1 0B ks.test:drain 7650 0B ks.test:data-query 7651 0B total Permits with state registered, sorted by count count memory name 0 0B total Total: permits: 7678, memory: 3499M This allows determining several things at glance: * What are the tables involved * What are the operations involved * Where is the memory This can speed up a follow-up investigation greatly, or it can even be enough on its own to determine that the issue is benign. Tests: unit(dev, debug) " * 'dump-diagnostics-on-semaphore-timeout/v2' of https://github.com/denesb/scylla: reader_concurrency_semaphore: dump permit diagnostics on timeout or queue overflow utils: add to_hr_size() reader_concurrency_semaphore: link permits into an intrusive list reader_concurrency_semaphore: move expiry_handler::operator()() out-of-line reader_concurrency_semaphore: move constructors out-of-line reader_concurrency_semaphore: add state to permits reader_concurrency_semaphore: name permits querier_cache_test: test_immediate_evict_on_insert: use two permits multishard_combining_reader: reader_lifecycle_policy: add permit param to create_reader() multishard_combining_reader: add permit parameter multishard_combining_reader: shard_reader: use multishard reader's permit
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.