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From: Heikki Linnakangas <[email protected]>
To: Matthias van de Meent <[email protected]>
Cc: Andrey Borodin <[email protected]>
Cc: PostgreSQL Hackers <[email protected]>
Subject: Re: Refactor PROCLOCK hash table into partitioned list allocator
Date: Tue, 6 Jan 2026 17:17:13 +0200
Message-ID: <[email protected]> (raw)
In-Reply-To: <CAEze2Wh5Qwit7WZhcJyYRWxVM_M-bf1VBkHqqp9nCJ6rfHTWMQ@mail.gmail.com>
References: <[email protected]>
	<[email protected]>
	<CAEze2Wh5Qwit7WZhcJyYRWxVM_M-bf1VBkHqqp9nCJ6rfHTWMQ@mail.gmail.com>

On 06/01/2026 16:58, Matthias van de Meent wrote:
> On Tue, 6 Jan 2026 at 15:24, Heikki Linnakangas <[email protected]> wrote:
>>
>> On 30/12/2025 14:37, Andrey Borodin wrote:
>>> Hi hackers,
>>>
>>> Following up on the Discord discussion about the PROCLOCK hash table being
>>> a "weird allocator" that we never actually use for lookups - I took a stab at
>>> replacing it with a simpler partitioned free list approach as was suggested.
>>> I was doing this mostly to educate myself on Lock Manager internals.
>>>
>>> The current implementation uses LockMethodProcLockHash purely as an allocator.
>>> We never do hash lookups by key; we only allocate entries, link them to the lock's
>>> procLocks list, and free them later. Using a full hash table for this adds
>>> unnecessary complexity and maybe even overhead (I did not measure this).
>>>
>>> The attached patch replaces this with:
>>> - ProcLockArray: A fixed-size array of all PROCLOCK structs (allocated at startup)
>>> - ProcLockFreeList: Partitioned free lists, one per lock partition to reduce contention
>>> - ProcLockAlloc/Free: Simple push/pop operations on the free lists
>>> - PROCLOCK lookup: Linear traversal of lock->procLocks (see LockRefindAndRelease()
>>>     and FastPathGetRelationLockEntry())
>>>
>>> The last point bothers me most. It seems like this traversals are expected to be short.
>>> But I'm not 100% sure.
>>
>> Hmm, yeah the last point contradicts the premise that the hash table is
>> used purely as an allocator. It *is* used for lookups, and you're
>> replacing them with linear scans. That doesn't seem like an improvement.
> 
> There are 2 types of PROCLOCK lookup used in LockRefindAndRelease and
> FastPathGetRelationLockEntry:
> - An active backend's PROCLOCK entries (in both LRAR and FPGRLE).
> - Prepared transaction's PROCLOCK entries (only in LRAR, called from
> lock_twophase_postcommit).

There are also lookups in SetupLockInTable and in LockRelease.

> For the backend's PROCLOCK entry lookup, we can use a backend-local
> hash table, which only keeps track of where this backend's entries
> are.

Hmm, good point. In fact we already have that: there's a pointer to the 
current process's PROCLOCK entry in LOCALLOCK already. Can we use that 
to avoid the linear scans? There's this LockAcquireExtended:

> 	/*
> 	 * Find or create lock and proclock entries with this tag
> 	 *
> 	 * Note: if the locallock object already existed, it might have a pointer
> 	 * to the lock already ... but we should not assume that that pointer is
> 	 * valid, since a lock object with zero hold and request counts can go
> 	 * away anytime.  So we have to use SetupLockInTable() to recompute the
> 	 * lock and proclock pointers, even if they're already set.
> 	 */
> 	proclock = SetupLockInTable(lockMethodTable, MyProc, locktag,
> 								hashcode, lockmode);

So that comment suggests that the 'proclock' pointer cannot be trusted 
here. I don't remember how all this works, so I'm not sure if that is a 
show-stopper or if we could somehow leverage the 'proclock' pointer in 
LOCALLOCK anyway.

- Heikki

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