Horton

Re: Improve LWLock tranche name visibility across backends

Sami Imseih <samimseih@gmail.com>

View thread View on postgr.es
From: Sami Imseih <samimseih@gmail.com>
To: Nathan Bossart <nathandbossart@gmail.com>
Cc: Tom Lane <tgl@sss.pgh.pa.us>, Andres Freund <andres@anarazel.de>, Bertrand Drouvot <bertranddrouvot.pg@gmail.com>, Rahila Syed <rahilasyed90@gmail.com>, pgsql-hackers <pgsql-hackers@postgresql.org>
Date: 2025-08-26T19:56:22Z
Lists: pgsql-hackers

Attachments

> > On Mon, Aug 25, 2025 at 04:59:41PM -0500, Sami Imseih wrote:
> > > hmm, can we really avoid a shared lock when reading from shared memory?
> > > considering access for both reads and writes can be concurrent to shared
> > > memory. We are also taking an exclusive lock when writing a new tranche.
> >
> > We probably want to hold a lock while we 1) increment LWLockCounter and
> > copy a new tranche name to memory and
>
> In the last rev, I removed the spinlock acquired on ShmemLock in-lieu of
> a LWLock. This is because I wanted a single LWLock acquisition while
> both incrementing LWLockCounter and writing to shared memory, and
> doing this much work, particularly writing to shared memory,
> with a spinlock seemed inappropriate. With that said, this is not high
> concurrency of performance sensitive activity at all, so perhaps I was
> being overly paranoid.

Here is v12 that replaces the LWLock to access the shared memory with a
ShmemLock and implements a local counter.

--
Sami

Commits

  1. test_dsa: Avoid leaking LWLock tranches.

  2. Teach DSM registry to ERROR if attaching to an uninitialized entry.

  3. Add a test harness for the LWLock tranche code.

  4. Revert recent change to RequestNamedLWLockTranche().

  5. Move dynamically-allocated LWLock tranche names to shared memory.

  6. Prepare DSM registry for upcoming changes to LWLock tranche names.

  7. Add GetNamedDSA() and GetNamedDSHash().