Date: Thu, 4 Sep 2025 10:44:00 -0500
From: Nathan Bossart <nathandbossart@gmail.com>
To: Rahila Syed <rahilasyed90@gmail.com>
Cc: Sami Imseih <samimseih@gmail.com>,
	Bertrand Drouvot <bertranddrouvot.pg@gmail.com>,
	Tom Lane <tgl@sss.pgh.pa.us>, Andres Freund <andres@anarazel.de>,
	pgsql-hackers <pgsql-hackers@postgresql.org>
Subject: Re: Improve LWLock tranche name visibility across backends
Message-ID: <aLmzwC2dRbqk14y6@nathan>
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On Wed, Sep 03, 2025 at 02:01:14PM -0500, Nathan Bossart wrote:
> Committed.

I'm having some regrets about the changes to RequestNamedLWLockTranche().
Specifically, when it is first called, it immediately allocates an array
big enough to hold 256 requests (~17 KB), whereas it used to only allocate
space for 16 requests (~1 KB) and resize as needed.  Furthermore, the
MAX_NAMED_TRANCHES check isn't actually needed because InitializeLWLocks()
will do the same check via its calls to LWLockNewTrancheId() for all the
named tranche requests.

-- 
nathan

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	filename=v1-0001-Revert-some-recent-changes-to-RequestNamedLWLockT.patch

From 6fe6590f86427c55ac831c5b48784d3d259fe1c8 Mon Sep 17 00:00:00 2001
From: Nathan Bossart <nathan@postgresql.org>
Date: Thu, 4 Sep 2025 10:36:06 -0500
Subject: [PATCH v1 1/1] Revert some recent changes to
 RequestNamedLWLockTranche().

---
 src/backend/storage/lmgr/lwlock.c | 16 +++++++++++-----
 1 file changed, 11 insertions(+), 5 deletions(-)

diff --git a/src/backend/storage/lmgr/lwlock.c b/src/backend/storage/lmgr/lwlock.c
index 258cdebd0f5..a5f6b81806d 100644
--- a/src/backend/storage/lmgr/lwlock.c
+++ b/src/backend/storage/lmgr/lwlock.c
@@ -610,6 +610,7 @@ void
 RequestNamedLWLockTranche(const char *tranche_name, int num_lwlocks)
 {
 	NamedLWLockTrancheRequest *request;
+	static int	NamedLWLockTrancheRequestsAllocated;
 
 	if (!process_shmem_requests_in_progress)
 		elog(FATAL, "cannot request additional LWLocks outside shmem_request_hook");
@@ -628,17 +629,22 @@ RequestNamedLWLockTranche(const char *tranche_name, int num_lwlocks)
 
 	if (NamedLWLockTrancheRequestArray == NULL)
 	{
+		NamedLWLockTrancheRequestsAllocated = 16;
 		NamedLWLockTrancheRequestArray = (NamedLWLockTrancheRequest *)
 			MemoryContextAlloc(TopMemoryContext,
 							   MAX_NAMED_TRANCHES
 							   * sizeof(NamedLWLockTrancheRequest));
 	}
 
-	if (NamedLWLockTrancheRequests >= MAX_NAMED_TRANCHES)
-		ereport(ERROR,
-				(errmsg("maximum number of tranches already registered"),
-				 errdetail("No more than %d tranches may be registered.",
-						   MAX_NAMED_TRANCHES)));
+	if (NamedLWLockTrancheRequests >= NamedLWLockTrancheRequestsAllocated)
+	{
+		int			i = pg_nextpower2_32(NamedLWLockTrancheRequests + 1);
+
+		NamedLWLockTrancheRequestArray = (NamedLWLockTrancheRequest *)
+			repalloc(NamedLWLockTrancheRequestArray,
+					 i * sizeof(NamedLWLockTrancheRequest));
+		NamedLWLockTrancheRequestsAllocated = i;
+	}
 
 	request = &NamedLWLockTrancheRequestArray[NamedLWLockTrancheRequests];
 	strlcpy(request->tranche_name, tranche_name, NAMEDATALEN);
-- 
2.39.5 (Apple Git-154)


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