Horton

Re: Shared hash table allocations

Heikki Linnakangas <hlinnaka@iki.fi>

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From: Heikki Linnakangas <hlinnaka@iki.fi>
To: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>
Cc: Tomas Vondra <tomas@vondra.me>, "pgsql-hackers@postgresql.org" <pgsql-hackers@postgresql.org>, Robert Haas <robertmhaas@gmail.com>, Rahila Syed <rahilasyed90@gmail.com>, Matthias van de Meent <boekewurm+postgres@gmail.com>
Date: 2026-04-03T17:32:07Z
Lists: pgsql-hackers 
On 03/04/2026 16:03, Ashutosh Bapat wrote:
> On Thu, Apr 2, 2026 at 10:15 PM Heikki Linnakangas <hlinnaka@iki.fi> wrote:
>>> I think the release notes should "nudge" all the
>>> users who use non-default max_locks_per_transaction to increase it if
>>> they see "out of memory" errors. I don't think it should provide a
>>> blanket advise to double their locks
>>
>> How about:
>>
>> "If you had previously set max_locks_per_transaction, you might need to
>> set it to a higher value in v19 to avoid "out of shared memory" errors.
>> If you are unsure what to set it to and don't mind the increased memory
>> usage, you can double the value to ensure that you can acquire at least
>> as many locks as before"
> 
> The wiggle room is 100KB fixed + 10% of other two structures, so value
> by which it should be increased is partly fixed and partly a multiple
> of current value. "double the value" is simplest advice we can give.
> +1.

Ok, committed these patches to remove the safety margins, make LOCK and 
PROCLOCK fixed-size, and change the default to 
max_locks_per_transaction=128. I will do one final self-review of the 
remaining earlier patches from this thread next; I believe they're ready 
to be committed too.

Thanks for the review!

- Heikki

Commits

  1. Remove HASH_DIRSIZE, always use the default algorithm to select it

  2. Allocate all parts of shmem hash table from a single contiguous area

  3. Prevent shared memory hash tables from growing beyond initial size

  4. Merge init and max size options on shmem hash tables

  5. Change default of max_locks_per_transactions to 128

  6. Make the lock hash tables fixed-sized

  7. Remove 10% safety margin from lock manager hash table estimates

  8. Remove bogus "safety margin" from predicate.c shmem estimates

  9. Make ShmemIndex visible in the pg_shmem_allocations view

  10. Change the signature of dynahash's alloc function

  11. Remove HASH_SEGMENT option

  12. Use ShmemInitStruct to allocate shmem for semaphores