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

> On Sep 14, 2026, at 16:14, Matthias van de Meent <boekewurm+postgres(at)gmail(dot)com> wrote:
>
> 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?
>

TYPEALIGN(8192, size) would handle rounding more concisely, but I see add_size() has an overflow protection, maybe that matters?

Best regards,
--
Chao Li (Evan)
HighGo Software Co., Ltd.
https://www.highgo.com/