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* Return DSA area for hash table from GetNamedDSHash()
@ 2026-04-06 22:56 Sami Imseih <[email protected]>
0 siblings, 2 replies; 4+ messages in thread
From: Sami Imseih @ 2026-04-06 22:56 UTC (permalink / raw)
To: pgsql-hackers
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)
Attachments:
[application/octet-stream] v1-0001-Add-function-to-return-DSA-area-for-a-dshash-tabl.patch (1.6K, 2-v1-0001-Add-function-to-return-DSA-area-for-a-dshash-tabl.patch)
download | inline diff:
From b2f69377435fe170b1321e55d0c3141ebe8a2efc Mon Sep 17 00:00:00 2001
From: Sami Imseih <[email protected]>
Date: Mon, 6 Apr 2026 21:50:24 +0000
Subject: [PATCH v1 1/1] Add function to return DSA area for a dshash table
A caller using GetNamedDSHash() to retrieve a dshash table may also
want to limit its size. Add dshash_get_dsa_area(), which returns the
dsa_area for a given dshash_table, allowing callers to pass it to
dsa_set_size_limit().
---
src/backend/lib/dshash.c | 11 +++++++++++
src/include/lib/dshash.h | 1 +
2 files changed, 12 insertions(+)
diff --git a/src/backend/lib/dshash.c b/src/backend/lib/dshash.c
index 1999989c14f..62ac68d4add 100644
--- a/src/backend/lib/dshash.c
+++ b/src/backend/lib/dshash.c
@@ -363,6 +363,17 @@ dshash_destroy(dshash_table *hash_table)
pfree(hash_table);
}
+/*
+ * Get the DSA area used by this hash table.
+ */
+dsa_area *
+dshash_get_dsa_area(dshash_table *hash_table)
+{
+ Assert(hash_table->control->magic == DSHASH_MAGIC);
+
+ return hash_table->area;
+}
+
/*
* Get a handle that can be used by other processes to attach to this hash
* table.
diff --git a/src/include/lib/dshash.h b/src/include/lib/dshash.h
index 64b758b381b..b868084df04 100644
--- a/src/include/lib/dshash.h
+++ b/src/include/lib/dshash.h
@@ -89,6 +89,7 @@ extern dshash_table *dshash_attach(dsa_area *area,
dshash_table_handle handle,
void *arg);
extern void dshash_detach(dshash_table *hash_table);
+extern dsa_area *dshash_get_dsa_area(dshash_table *hash_table);
extern dshash_table_handle dshash_get_hash_table_handle(dshash_table *hash_table);
extern void dshash_destroy(dshash_table *hash_table);
--
2.50.1
^ permalink raw reply [nested|flat] 4+ messages in thread
* Re: Return DSA area for hash table from GetNamedDSHash()
@ 2026-04-07 03:59 jie wang <[email protected]>
parent: Sami Imseih <[email protected]>
1 sibling, 0 replies; 4+ messages in thread
From: jie wang @ 2026-04-07 03:59 UTC (permalink / raw)
To: Sami Imseih <[email protected]>; +Cc: pgsql-hackers
Sami Imseih <[email protected]> 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
^ permalink raw reply [nested|flat] 4+ messages in thread
* Re: Return DSA area for hash table from GetNamedDSHash()
@ 2026-04-08 01:08 Michael Paquier <[email protected]>
parent: Sami Imseih <[email protected]>
1 sibling, 1 reply; 4+ messages in thread
From: Michael Paquier @ 2026-04-08 01:08 UTC (permalink / raw)
To: Sami Imseih <[email protected]>; +Cc: pgsql-hackers
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
Attachments:
[application/pgp-signature] signature.asc (833B, 2-signature.asc)
download
^ permalink raw reply [nested|flat] 4+ messages in thread
* Re: Return DSA area for hash table from GetNamedDSHash()
@ 2026-04-09 21:11 Sami Imseih <[email protected]>
parent: Michael Paquier <[email protected]>
0 siblings, 0 replies; 4+ messages in thread
From: Sami Imseih @ 2026-04-09 21:11 UTC (permalink / raw)
To: Michael Paquier <[email protected]>; [email protected]; +Cc: pgsql-hackers
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
^ permalink raw reply [nested|flat] 4+ messages in thread
end of thread, other threads:[~2026-04-09 21:11 UTC | newest]
Thread overview: 4+ messages (download: mbox mbox.gz follow: Atom feed)
-- links below jump to the message on this page --
2026-04-06 22:56 Return DSA area for hash table from GetNamedDSHash() Sami Imseih <[email protected]>
2026-04-07 03:59 ` jie wang <[email protected]>
2026-04-08 01:08 ` Michael Paquier <[email protected]>
2026-04-09 21:11 ` Sami Imseih <[email protected]>
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