Re: Improve monitoring of shared memory allocations - Mailing list pgsql-hackers

On Fri, Apr 4, 2025 at 9:15 PM Rahila Syed <rahilasyed90@gmail.com> wrote:
>
> Please find attached the patch which removes the changes for non-shared hash tables
> and keeps them for shared hash tables.
>

  /* Allocate initial segments */
+ i = 0;
  for (segp = hashp->dir; hctl->nsegs < nsegs; hctl->nsegs++, segp++)
  {
- *segp = seg_alloc(hashp);
- if (*segp == NULL)
- return false;
+ /* Assign initial segments, which are also pre-allocated */
+ if (hashp->isshared)
+ {
+ *segp = (HASHSEGMENT) HASH_SEGMENT_PTR(hashp, i++);
+ MemSet(*segp, 0, HASH_SEGMENT_SIZE(hashp));
+ }
+ else
+ {
+ *segp = seg_alloc(hashp);
+ i++;
+ }

In the non-shared hash table case, previously, we used to return false
from here when seg_alloc() fails, but now it seems to be proceeding
without returning false. Is there a reason for the same?

> I tested this by running make-check, make-check world and the reproducer script shared
> by David. I also ran pgbench to test creation and expansion of some of the
> shared hash tables.
>

This covers the basic tests for this patch. I think we should do some
low-level testing of both shared and non-shared hash tables by having
a contrib module or such (we don't need to commit such a contrib
module, but it will give us confidence that the low-level data
structure allocation change is thoroughly tested). We also need to
focus on negative tests where there is insufficient memory in the
system.

--
With Regards,
Amit Kapila.