Date: Thu, 22 Jan 2026 15:24:13 -0600
From: Nathan Bossart <nathandbossart@gmail.com>
To: Anthonin Bonnefoy <anthonin.bonnefoy@datadoghq.com>
Cc: PostgreSQL Hackers <pgsql-hackers@postgresql.org>
Subject: Re: Fix rounding method used to compute huge pages
Message-ID: <aXKVfaTntXRn1V0m@nathan>
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On Thu, Jan 22, 2026 at 05:42:44PM +0100, Anthonin Bonnefoy wrote:
> When computing the dynamic value of shared_memory_size_in_huge_pages,
> (1+size_b/hp_size) is currently used. This works when size_b is not
> divisible by hp_size. However, it will yield an additional huge page
> when size_b is divisible by hp_size.

Oops, it looks like this is my fault.  I doubt this causes any practical
problems, but we might as well fix it.

+		if (size_b % hp_size != 0)
+			size_b = add_size(size_b, hp_size - (size_b % hp_size));
+		hp_required = size_b / hp_size;

I think we could simplify this a tad:

	hp_required = size_b / hp_size;
	if (size_b % hp_size != 0)
		hp_required = add_size(hp_required, 1);

> 0002: This patch uses add_size in CreateAnonymousSegment when the
> allocation size is rounded up, to check for possible overflow.

Seems reasonable.

-- 
nathan