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WalSndWakeup() currently loops through all the walsenders slots, with a spinlock acquisition and release for every iteration, to wake up waiting walsenders. This commonly was not a problem before e101dfac3a53c. But, to allow logical decoding on standbys, we need to wake up logical walsenders after every WAL record is applied on the standby, rather just when flushing WAL or switching timelines. This causes a performance regression for workloads replaying a lot of WAL records. To solve this, we use condition variable (CV) to efficiently wake up walsenders in WalSndWakeup(). Every walsender prepares to sleep on a shared memory CV. Note that it just prepares to sleep on the CV (i.e., adds itself to the CV's waitlist), but does not actually wait on the CV (IOW, it never calls ConditionVariableSleep()). It still uses WaitEventSetWait() for waiting, because CV infrastructure doesn't handle FeBe socket events currently. The processes (startup process, walreceiver etc.) wanting to wake up walsenders use ConditionVariableBroadcast(), which in turn calls SetLatch(), helping walsenders come out of WaitEventSetWait(). We use separate shared memory CVs for physical and logical walsenders for selective wake ups, see WalSndWakeup() for more details. This approach is simple and reasonably efficient. But not very elegant. But for 16 it seems to be a better path than a larger redesign of the CV mechanism. A desirable future improvement would be to add support for CVs into WaitEventSetWait(). This still leaves us with a small regression in very extreme workloads (due to the spinlock acquisition in ConditionVariableBroadcast() when there are no waiters) - but that seems acceptable. Reported-by: Andres Freund <andres@anarazel.de> Suggested-by: Andres Freund <andres@anarazel.de> Author: Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com> Reviewed-by: "Drouvot, Bertrand" <bertranddrouvot.pg@gmail.com> Reviewed-by: Zhijie Hou <houzj.fnst@fujitsu.com> Discussion: https://www.postgresql.org/message-id/20230509190247.3rrplhdgem6su6cg%40awork3.anarazel.de
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/.