Re: Fix unnecessary shared memory page allocation in CalculateShmemSize()

On Mon, 14 Sept 2026 at 09:19, Chao Li <li(dot)evan(dot)chao(at)gmail(dot)com> wrote:
>
> Hi,
>
> I just noticed this item when I went through my TODO list today. I remember finding this issue a few months ago, but at that time, only bugs new to PG19 were being processed, so I put it on my TODO list.
>
> This is a small issue, but it has been there for many years. CalculateShmemSize() has logic to round size to a multiple of a typical page size:
> ```
> /* might as well round it off to a multiple of a typical page size */
> size = add_size(size, 8192 - (size % 8192));
> ```
>
> When size is already a multiple of 8192, this add_size() call is not needed; it only results in an extra 8192 bytes being allocated in shared memory. The fix is simple:
> ```
> if (size % 8192 != 0)
> /* might as well round it off to a multiple of a typical page size */
> size = add_size(size, 8192 - (size % 8192));
> ```
>
> I put the comment within the if clause because I remember Tom once mentioning that this would be the preferred style.

Shouldn't a TYPEALIGN(8192, size) do the trick here, and do it more
concise and better?

Kind regards,

Matthias van de Meent

Databricks (https://www.databricks.com)