1b7eea0f52
This patch flips two "switches": 1) It switches admission to be up-front. 2) It changes the admission algorithm. (1) by now all permits are obtained up-front, so this patch just yanks out the restricted reader from all reader stacks and simultaneously switches all `obtain_permit_nowait()` calls to `obtain_permit()`. By doing this admission is now waited on when creating the permit. (2) we switch to an admission algorithm that adds a new aspect to the existing resource availability: the number of used/blocked reads. Namely it only admits new reads if in addition to the necessary amount of resources being available, all currently used readers are blocked. In other words we only admit new reads if all currently admitted reads requires something other than CPU to progress. They are either waiting on I/O, a remote shard, or attention from their consumers (not used currently). We flip these two switches at the same time because up-front admission means cache reads now need to obtain a permit too. For cache reads the optimal concurrency is 1. Anything above that just increases latency (without increasing throughput). So we want to make sure that if a cache reader hits it doesn't get any competition for CPU and it can run to completion. We admit new reads only if the read misses and has to go to disk. Another change made to accommodate this switch is the replacement of the replica side read execution stages which the reader concurrency semaphore as an execution stage. This replacement is needed because with the introduction of up-front admission, reads are not independent of each other any-more. One read executed can influence whether later reads executed will be admitted or not, and execution stages require independent operations to work well. By moving the execution stage into the semaphore, we have an execution stage which is in control of both admission and running the operations in batches, avoiding the bad interaction between the two.
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 here. 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.