71e4cc6b8e
The WAL insertion lock variable insertingAt is currently being read and written with the help of the LWLock wait list lock to avoid any read of torn values. This wait list lock can become a point of contention on a highly concurrent write workloads. This commit switches insertingAt to a 64b atomic variable that provides torn-free reads/writes. On platforms without 64b atomic support, the fallback implementation uses spinlocks to provide the same guarantees for the values read. LWLockWaitForVar(), through LWLockConflictsWithVar(), reads the new value to check if it still needs to wait with a u64 atomic operation. LWLockUpdateVar() updates the variable before waking up the waiters with an exchange_u64 (full memory barrier). LWLockReleaseClearVar() now uses also an exchange_u64 to reset the variable. Before this commit, all these steps relied on LWLockWaitListLock() and LWLockWaitListUnlock(). This reduces contention on LWLock wait list lock and improves performance of highly-concurrent write workloads. Here are some numbers using pg_logical_emit_message() (HEAD at d6677b93) with various arbitrary record lengths and clients up to 1k on a rather-large machine (64 vCPUs, 512GB of RAM, 16 cores per sockets, 2 sockets), in terms of TPS numbers coming from pgbench: message_size_b | 16 | 64 | 256 | 1024 --------------------+--------+--------+--------+------- patch_4_clients | 83830 | 82929 | 80478 | 73131 patch_16_clients | 267655 | 264973 | 250566 | 213985 patch_64_clients | 380423 | 378318 | 356907 | 294248 patch_256_clients | 360915 | 354436 | 326209 | 263664 patch_512_clients | 332654 | 321199 | 287521 | 240128 patch_1024_clients | 288263 | 276614 | 258220 | 217063 patch_2048_clients | 252280 | 243558 | 230062 | 192429 patch_4096_clients | 212566 | 213654 | 205951 | 166955 head_4_clients | 83686 | 83766 | 81233 | 73749 head_16_clients | 266503 | 265546 | 249261 | 213645 head_64_clients | 366122 | 363462 | 341078 | 261707 head_256_clients | 132600 | 132573 | 134392 | 165799 head_512_clients | 118937 | 114332 | 116860 | 150672 head_1024_clients | 133546 | 115256 | 125236 | 151390 head_2048_clients | 137877 | 117802 | 120909 | 138165 head_4096_clients | 113440 | 115611 | 120635 | 114361 Bharath has been measuring similar improvements, where the limit of the WAL insertion lock begins to be felt when more than 256 concurrent clients are involved in this specific workload. An extra patch has been discussed to introduce a fast-exit path in LWLockUpdateVar() when there are no waiters, still this does not influence the write-heavy workload cases discussed as there are always waiters. This will be considered separately. Author: Bharath Rupireddy Reviewed-by: Nathan Bossart, Andres Freund, Michael Paquier Discussion: https://postgr.es/m/CALj2ACVF+6jLvqKe6xhDzCCkr=rfd6upaGc3477Pji1Ke9G7Bg@mail.gmail.com
PostgreSQL Database Management System ===================================== This directory contains the source code distribution of the PostgreSQL database management system. PostgreSQL is an advanced object-relational database management system that supports an extended subset of the SQL standard, including transactions, foreign keys, subqueries, triggers, user-defined types and functions. This distribution also contains C language bindings. PostgreSQL has many language interfaces, many of which are listed here: https://www.postgresql.org/download/ See the file INSTALL for instructions on how to build and install PostgreSQL. That file also lists supported operating systems and hardware platforms and contains information regarding any other software packages that are required to build or run the PostgreSQL system. Copyright and license information can be found in the file COPYRIGHT. A comprehensive documentation set is included in this distribution; it can be read as described in the installation instructions. The latest version of this software may be obtained at https://www.postgresql.org/download/. For more information look at our web site located at https://www.postgresql.org/.