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Andres Freund bc971f4025 Optimize walsender wake up logic using condition variables 
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 e101dfac3a. 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
2023-05-21 09:44:55 -07:00
config Fix typos in comments 2023-05-02 12:23:08 +09:00
contrib Expand some more uses of "deleg" to "delegation" or "delegated". 2023-05-21 10:55:18 -04:00
doc doc: PG 16 relnotes, add commits 2023-05-21 11:56:16 -04:00
src Optimize walsender wake up logic using condition variables 2023-05-21 09:44:55 -07:00
.cirrus.yml ci: Remove OpenSSL 3.1 workaround for missing system CA 2023-04-24 11:40:23 +02:00
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aclocal.m4 autoconf: Move export_dynamic determination to configure 2022-12-06 18:55:28 -08:00
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COPYRIGHT Update copyright for 2023 2023-01-02 15:00:37 -05:00
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README Canonicalize some URLs 2020-02-10 20:47:50 +01:00
README.git Canonicalize some URLs 2020-02-10 20:47:50 +01:00

README 

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/.