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When reads arrive, they have to wait for admission on the reader concurrency semaphore. If the node is overloaded, the reads will be queued. They can time out while in the queue, but will not time out once admitted. Once the shard is sufficiently loaded, it is possible that most queued reads will time out, because the average time it takes to for a queued read to be admitted is around that of the timeout. If a read times out, any work we already did, or are about to do on it is wasted effort. Therefore, the patch tries to prevent it by checking if an admitted read has a chance to complete in time and abort it if not. It uses the following criteria: if read's remaining time <= read's timeout when arrived to the semaphore * live updateable preemptive_abort_factor; the read is rejected and the next one from the wait list is considered. Fixes https://github.com/scylladb/scylladb/issues/14909 Fixes: SCYLLADB-353 Backport is not needed. Better to first observe its impact. Closes scylladb/scylladb#21649 * github.com:scylladb/scylladb: reader_concurrency_semaphore: Check during admission if read may timeout permit_reader::impl: Replace break with return after evicting inactive permit on timeout reader_concurrency_semaphore: Add preemptive_abort_factor to constructors config: Add parameters to control reads' preemptive_abort_factor permit_reader: Add a new state: preemptive_aborted reader_concurrency_semaphore: validate waiters counter when dequeueing a waiting permit reader_concurrency_semaphore: Remove cpu_concurrency's default value
ScyllaDB Documentation
This repository contains the source files for ScyllaDB documentation.
- The
devfolder contains developer-oriented documentation related to the ScyllaDB code base. It is not published and is only available via GitHub. - All other folders and files contain user-oriented documentation related to ScyllaDB and are sources for docs.scylladb.com/manual.
To report a documentation bug or suggest an improvement, open an issue in GitHub issues for this project.
To contribute to the documentation, open a GitHub pull request.
Key Guidelines for Contributors
- The user documentation is written in reStructuredText (RST) - a plaintext markup language similar to Markdown. If you're not familiar with RST, see ScyllaDB RST Examples.
- The developer documentation is written in Markdown. See Basic Markdown Syntax for reference.
- Follow the ScyllaDB Style Guide.
To prevent the build from failing:
-
If you add a new file, ensure it's added to an appropriate toctree, for example:
.. toctree:: :maxdepth: 2 :hidden: Page X </folder1/article1> Page Y </folder1/article2> Your New Page </folder1/your-new-article> -
Make sure the link syntax is correct. See the guidelines on creating links
-
Make sure the section headings are correct. See the guidelines on creating headings Note that the markup must be at least as long as the text in the heading. For example:
---------------------- Prerequisites ----------------------
Building User Documentation
Prerequisites
- Python
- poetry
- make
See the ScyllaDB Sphinx Theme prerequisites to check which versions of the above are currently required.
Mac OS X
You must have a working Homebrew in order to install the needed tools.
You also need the standard utility make.
Check if you have these two items with the following commands:
brew help
make -h
Linux Distributions
Building the user docs should work out of the box on most Linux distributions.
Windows
Use "Bash on Ubuntu on Windows" for the same tools and capabilities as on Linux distributions.
Building the Docs
- Run
make previewin thedocs/directory to build the documentation. - Preview the built documentation locally at http://127.0.0.1:5500/.
Cleanup
You can clean up all the build products and auto-installed Python stuff with:
make pristine
Information for Contributors
If you are interested in contributing to Scylla docs, please read the Scylla open source page at http://www.scylladb.com/opensource/ and complete a Scylla contributor agreement if needed. We can only accept documentation pull requests if we have a contributor agreement on file for you.
Third-party Documentation
-
Do any copying as a separate commit. Always commit an unmodified version first and then do any editing in a separate commit.
-
We already have a copy of the Apache license in our tree, so you do not need to commit a copy of the license.
-
Include the copyright header from the source file in the edited version. If you are copying an Apache Cassandra document with no copyright header, use:
This document includes material from Apache Cassandra.
Apache Cassandra is Copyright 2009-2014 The Apache Software Foundation.