Horton

Re: Improve LWLock tranche name visibility across backends

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: Nathan Bossart <nathandbossart@gmail.com>
Cc: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>, Sami Imseih <samimseih@gmail.com>, Rahila Syed <rahilasyed90@gmail.com>, pgsql-hackers <pgsql-hackers@postgresql.org>
Date: 2025-08-19T18:06:50Z
Lists: pgsql-hackers 
Hi,

On 2025-08-19 12:29:14 -0500, Nathan Bossart wrote:
> On Tue, Aug 19, 2025 at 08:09:53AM +0000, Bertrand Drouvot wrote:
> > On Mon, Aug 18, 2025 at 05:53:44PM -0500, Sami Imseih wrote:
> >> > (or some other shmem-based
> >> > data structure we have yet to introduce, like a dslist/dsarray).
> >> 
> >> This will be an interesting API to invest time in, if there could be more
> >> use-cases.
> > 
> > I did a quick check and I did not find current use cases: possible candidates
> > could be in ExecParallelHashTableAlloc(), PTIterationArray, PTEntryArray for
> > examples but I think they all know the final size upfront so there is no real
> > need for dsarray for those).
> > 
> >> I think it's a separate discussion at this point.
> > 
> > OTOH, that would be a valid use case to introduce this new API but I'm not sure
> > it's worth it given that the dshash looks good enough for our case (even if not
> > ideal though).
> 
> IMHO it'd be okay to proceed with dshash for now.  It would be pretty easy
> to switch to something like a "dslist" in the future.

Possibly stupid question - is it really worth having a dynamic structure here?
The number of tranches is strictly bound, it seems like it'd be simpler to
have an array of tranch nmes in shared memory.

Greetings,

Andres Freund

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().