Thread

1. Return DSA area for hash table from GetNamedDSHash()

   Sami Imseih <samimseih@gmail.com> — 2026-04-06T22:56:21Z

   Hi,
   
   While working on extending tests for dshash.c [1], I realized that a
   user that creates a hash table with GetNamedDSHash() has no way
   to cap the size of the dsa area underpinning the table by using
   dsa_set_size_limit(). This is because the dsa_area created using
   this API is not exposed to the user.
   
   This is a gap for users of the GetNamedDSHash() API,
   because it's very likely that the callers don't want runaway growth of
   these hash tables.
   
   Attached is a new API, dshash_get_dsa_area() that takes in a dshash_table
   and returns the area. The caller can then use dsa_set_size_limit() to limit
   the size.
   
   We could change the GetNamedDSHash() API to take in a size, but that
   will not be ideal since a caller may want to change the size dynamically after
   the hash table is created.
   
   I don't have a patch for this yet, but I also think it will make sense for
   pg_dsm_registry_allocations to also show the max_size
   
   postgres=# select * from pg_dsm_registry_allocations;
             name          |  type   |  size
   ------------------------+---------+---------
    test_dsm_registry_dsa  | area    | 1048576
    test_dsm_registry_hash | hash    | 1048576
    test_dsm_registry_dsm  | segment |      20
   (3 rows)
   
   Thoughts?
   
   
   [1] [https://www.postgresql.org/message-id/acXCJODjsCytdpwT%40paquier.xyz]
   
   --
   Sami Imseih
   Amazon Web Services (AWS)
   

2. Re: Return DSA area for hash table from GetNamedDSHash()

   jie wang <jugierwang@gmail.com> — 2026-04-07T03:59:14Z

   Sami Imseih <samimseih@gmail.com> 于2026年4月7日周二 06:56写道：
   
   > Hi,
   >
   > While working on extending tests for dshash.c [1], I realized that a
   > user that creates a hash table with GetNamedDSHash() has no way
   > to cap the size of the dsa area underpinning the table by using
   > dsa_set_size_limit(). This is because the dsa_area created using
   > this API is not exposed to the user.
   >
   > This is a gap for users of the GetNamedDSHash() API,
   > because it's very likely that the callers don't want runaway growth of
   > these hash tables.
   >
   > Attached is a new API, dshash_get_dsa_area() that takes in a dshash_table
   > and returns the area. The caller can then use dsa_set_size_limit() to limit
   > the size.
   >
   > We could change the GetNamedDSHash() API to take in a size, but that
   > will not be ideal since a caller may want to change the size dynamically
   > after
   > the hash table is created.
   >
   > I don't have a patch for this yet, but I also think it will make sense for
   > pg_dsm_registry_allocations to also show the max_size
   >
   > postgres=# select * from pg_dsm_registry_allocations;
   >           name          |  type   |  size
   > ------------------------+---------+---------
   >  test_dsm_registry_dsa  | area    | 1048576
   >  test_dsm_registry_hash | hash    | 1048576
   >  test_dsm_registry_dsm  | segment |      20
   > (3 rows)
   >
   > Thoughts?
   >
   >
   > [1] [https://www.postgresql.org/message-id/acXCJODjsCytdpwT%40paquier.xyz]
   >
   > --
   > Sami Imseih
   > Amazon Web Services (AWS)
   >
   
   
   Hi,
   
   I think an assert check could be added in this patch for better safety.
    Assert(hash_table != NULL);
   
   Best regards,
   --
   wang jie
   

3. Re: Return DSA area for hash table from GetNamedDSHash()

   Michael Paquier <michael@paquier.xyz> — 2026-04-08T01:08:46Z

   On Mon, Apr 06, 2026 at 05:56:21PM -0500, Sami Imseih wrote:
   > Attached is a new API, dshash_get_dsa_area() that takes in a dshash_table
   > and returns the area. The caller can then use dsa_set_size_limit() to limit
   > the size.
   
   +dsa_area *
   +dshash_get_dsa_area(dshash_table *hash_table)
   +{
   +	Assert(hash_table->control->magic == DSHASH_MAGIC);
   +
   +	return hash_table->area;
   
   Rather than an API that returns the DSA area, perhaps it would be more
   natural to have a wrapper that calls dsa_set_size_limit(), using an
   existing dshash_table in input?
   --
   Michael
   

4. Re: Return DSA area for hash table from GetNamedDSHash()

   Sami Imseih <samimseih@gmail.com> — 2026-04-09T21:11:48Z

   Thanks for the replies!
   
   > I think an assert check could be added in this patch for better safety.
   >  Assert(hash_table != NULL);
   >
   
   I followed the same approach we take for dshash_destroy() and
   dshash_get_hash_table_handle(). The caller is responsible for
   not passing in a NULL hash table, else that assert will segfault.
   
   
   > +dsa_area *
   > +dshash_get_dsa_area(dshash_table *hash_table)
   > +{
   > +       Assert(hash_table->control->magic == DSHASH_MAGIC);
   > +
   > +       return hash_table->area;
   >
   > Rather than an API that returns the DSA area, perhaps it would be more
   > natural to have a wrapper that calls dsa_set_size_limit(), using an
   > existing dshash_table in input?
   
   hm, having GetNamedDSA return dsa_area for direct use while requiring
   a special wrapper for the dshash case creates an inconsistent API in
   dsm_registry.h. dshash_get_dsa_area() means either way the dsa_area is
   obtained, dsa_set_size_limit() can be used to set the size.
   
   --
   Sami