8751aa7397
Having both readers and writers use a single lock word in futex system calls caused performance regression compared to SRW_LOCK_DUMMY (mutex and 2 condition variables). A contributing factor is that we did not accurately keep track of the number of waiting threads and thus had to invoke system calls to wake up any waiting threads. SUX_LOCK_GENERIC: Renamed from SRW_LOCK_DUMMY. This is the original implementation, with rw_lock (std::atomic<uint32_t>), a mutex and two condition variables. Using a separate writer mutex (as described below) is not possible, because the mutex ownership in a buf_block_t::lock must be able to transfer from a write submitter thread to an I/O completion thread, and pthread_mutex_lock() may assume that the submitter thread is recursively acquiring the mutex that it already holds, while in reality the I/O completion thread is the real owner. POSIX does not define an interface for requesting a mutex to be non-recursive. On Microsoft Windows, srw_lock_low will remain a simple wrapper of SRWLOCK. On 32-bit Microsoft Windows, sizeof(SRWLOCK)=4 while sizeof(srw_lock_low)=8. On other platforms, srw_lock_low is an alias of ssux_lock_low, the Simple (non-recursive) Shared/Update/eXclusive lock. In the futex-based implementation of ssux_lock_low (Linux, OpenBSD, Microsoft Windows), we shall use a dedicated mutex for exclusive requests (writer), and have a WRITER flag in the 'readers' lock word to inform that a writer is holding the lock or waiting for the lock to be granted. When the WRITER flag is set, all lock requests must acquire the writer mutex. Normally, shared (S) lock requests simply perform a compare-and-swap on the 'readers' word. Update locks are implemented as a combination of writer mutex and a normal counter in the 'readers' lock word. The conflict between U and X locks is guaranteed by the writer mutex. Unlike SUX_LOCK_GENERIC, wr_u_downgrade() will not wake up any pending rd_lock() waits. They will wait until u_unlock() releases the writer mutex. The ssux_lock_low is always wrapped by sux_lock (with a recursion count of U and X locks), used for dict_index_t::lock and buf_block_t::lock. Their memory footprint for the futex-based implementation will increase by sizeof(srw_mutex), or 4 bytes. This change addresses a performance regression in read-only benchmarks, such as sysbench oltp_read_only. Also write performance was improved. On 32-bit Linux and OpenBSD, lock_sys_t::hash_table will allocate two hash table elements for each srw_lock (14 instead of 15 hash table cells per 64-byte cache line on IA-32). On Microsoft Windows, sizeof(SRWLOCK)==sizeof(void*) and there is no change. Reviewed by: Vladislav Vaintroub Tested by: Axel Schwenke and Vladislav Vaintroub |
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| BUILD | ||
| client | ||
| cmake | ||
| dbug | ||
| debian | ||
| Docs | ||
| extra | ||
| include | ||
| libmariadb@b6f8883d96 | ||
| libmysqld | ||
| libservices | ||
| man | ||
| mysql-test | ||
| mysys | ||
| mysys_ssl | ||
| plugin | ||
| randgen/conf | ||
| scripts | ||
| sql | ||
| sql-bench | ||
| sql-common | ||
| storage | ||
| strings | ||
| support-files | ||
| tests | ||
| tpool | ||
| unittest | ||
| vio | ||
| win | ||
| wsrep-lib@a93955ddee | ||
| zlib | ||
| .clang-format | ||
| .gitattributes | ||
| .gitignore | ||
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| .travis.compiler.sh | ||
| .travis.yml | ||
| appveyor.yml | ||
| BUILD-CMAKE | ||
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| config.h.cmake | ||
| configure.cmake | ||
| CONTRIBUTING.md | ||
| COPYING | ||
| CREDITS | ||
| dml_handler.diff | ||
| EXCEPTIONS-CLIENT | ||
| information_schema.diff | ||
| INSTALL-SOURCE | ||
| INSTALL-WIN-SOURCE | ||
| KNOWN_BUGS.txt | ||
| README.md | ||
| THIRDPARTY | ||
| VERSION | ||
Code status:
travis-ci.org (10.5 branch)ci.appveyor.com
MariaDB: The open source relational database
MariaDB was designed as a drop-in replacement of MySQL(R) with more features, new storage engines, fewer bugs, and better performance.
MariaDB is brought to you by the MariaDB Foundation and the MariaDB Corporation. Please read the CREDITS file for details about the MariaDB Foundation, and who is developing MariaDB.
MariaDB is developed by many of the original developers of MySQL who now work for the MariaDB Corporation, the MariaDB Foundation and by many people in the community.
MySQL, which is the base of MariaDB, is a product and trademark of Oracle Corporation, Inc. For a list of developers and other contributors, see the Credits appendix. You can also run 'SHOW authors' to get a list of active contributors.
A description of the MariaDB project and a manual can be found at:
https://mariadb.com/kb/en/mariadb-vs-mysql-features/
https://mariadb.com/kb/en/mariadb-versus-mysql-compatibility/
https://mariadb.com/kb/en/new-and-old-releases/
Help
More help is available from the Maria Discuss mailing list https://launchpad.net/~maria-discuss, MariaDB's Zulip instance, https://mariadb.zulipchat.com/ and the #maria IRC channel on Freenode.
Live QA for beginner contributors
MariaDB has a dedicated time each week when we answer new contributor questions live on Zulip and IRC. From 8:00 to 10:00 UTC on Mondays, and 10:00 to 12:00 UTC on Thursdays, anyone can ask any questions they’d like, and a live developer will be available to assist.
New contributors can ask questions any time, but we will provide immediate feedback during that interval.
Licensing
NOTE:
MariaDB is specifically available only under version 2 of the GNU General Public License (GPLv2). (I.e. Without the "any later version" clause.) This is inherited from MySQL. Please see the README file in the MySQL distribution for more information.
License information can be found in the COPYING file. Third party license information can be found in the THIRDPARTY file.
Bug Reports
Bug and/or error reports regarding MariaDB should be submitted at: https://jira.mariadb.org
For reporting security vulnerabilities see: https://mariadb.org/about/security-policy/
The code for MariaDB, including all revision history, can be found at: https://github.com/MariaDB/server