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base repository: postgresql-cfbot/postgresql
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head repository: postgresql-cfbot/postgresql
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  • 3 commits
  • 3 files changed
  • 2 contributors

Commits on Aug 4, 2025

  1. Optimize LISTEN/NOTIFY signaling with a lock-free atomic state machine 

    This commit introduces a powerful pattern for modernizing inter-process
communication by refactoring the LISTEN/NOTIFY subsystem to use a
lock-free, atomic finite state machine (FSM). This directly addresses
the historical lack of safe, efficient state synchronization primitives.

Previously, if multiple transactions sent notifications concurrently,
each would unconditionally attempt to signal all listening backends.
This resulted in a storm of superfluous signals to listeners that were
already pending a wakeup, causing unnecessary system call overhead.

By introducing an atomic per-backend state (IDLE, SIGNALLED, PROCESSING)
in shared memory and manipulated via compare-and-swap (CAS), this
inefficiency is eliminated. A notifier can now atomically transition a
listener's state from IDLE to SIGNALLED, ensuring that only the first
notifier for a given idle listener dispatches a wakeup. The FSM also
robustly handles race conditions where new notifications arrive while a
listener is PROCESSING, guaranteeing no work is ever missed.

This FSM pattern is a generalizable solution for managing concurrency in
PostgreSQL. By modeling inter-process interactions as explicit state
transitions, we can build more robust and performant subsystems. This
commit demonstrates the pattern's effectiveness within async.c, and by
cleanly solving the state management problem first, it enables a
subsequent, trivial optimization of the wakeup mechanism itself.
    @joelonsql
    joelonsql authored and Commitfest Bot committed Aug 4, 2025
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  2. Optimize LISTEN/NOTIFY wakeup by replacing signal with direct SetLatch 

    Building upon the robust atomic state machine introduced in the previous
commit, this change completes the modernization of NOTIFY IPC by
replacing its wakeup mechanism. With inter-process state now managed
reliably, the heavyweight SIGUSR1 signal is no longer necessary and is
replaced with a much more efficient, direct "poke."

The async.c notifier now replaces its call to SendProcSignal with a
direct call to SetLatch on the target backend's procLatch. This is a
significant optimization because WaitLatch, which listeners already use
for blocking, is underpinned by the modern WaitEventSet abstraction
(kqueue, epoll, etc.). We now leverage this existing, highly efficient
infrastructure for the wakeup, completely bypassing the kill() syscall
and the SIGUSR1 signal handler for all NOTIFY events.

This demonstrates a powerful, two-step migration pattern:

1. First, solve a subsystem's state synchronization problem with a
   lock-free, atomic FSM to eliminate redundant signaling.

2. Then, with state management handled, make the wakeup itself cheaper
   by replacing the expensive signal with a direct SetLatch.

This staged approach allows us to modernize subsystems incrementally and
safely. By applying this pattern to async.c, we prove its viability and
simplicity, creating a clear template for other parts of the system to
follow in moving towards a more performant, signal-free IPC model.
    @joelonsql
    joelonsql authored and Commitfest Bot committed Aug 4, 2025
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  3. [CF 5912] Optimize LISTEN/NOTIFY 

    This branch was automatically generated by a robot using patches from an
email thread registered at:

https://commitfest.postgresql.org/patch/5912

The branch will be overwritten each time a new patch version is posted to
the thread, and also periodically to check for bitrot caused by changes
on the master branch.

Patch(es): https://www.postgresql.org/message-id/[email protected]
Author(s): Joel Jacobson
    Commitfest Bot committed Aug 4, 2025
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