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From: Alicja Kucharczyk <zaledwie10minut@gmail.com>
Date: Thu, 29 Jan 2026 16:09:08 +0100
Message-ID: 
 <CAM=sWa7wFBK26fNFsmNdOgH27TgeYfqk=MmTGHCbyfN0VWzQBQ@mail.gmail.com>
Subject: Re: Setting huge_pages=off when HugePages are allocated in Linux?
To: Laurenz Albe <laurenz.albe@cybertec.at>
Cc: Ron Johnson <ronljohnsonjr@gmail.com>,
 Pgsql-admin <pgsql-admin@lists.postgresql.org>
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=C5=9Br., 28 sty 2026 o 18:02 Laurenz Albe <laurenz.albe@cybertec.at> napis=
a=C5=82(a):

> > In the example in this (quite helpful) Cybertec post, 10475 HugePages
> are allocated.
> > https://www.cybertec-postgresql.com/en/huge-pages-postgresql/
> >
> > If we set "huge_pages =3D off" within PG and then restart PG, will Linu=
x
> see those 10475 HugePages as off-limits to normal 4KiB allocations?
>
> Yes.  That's why it is a good idea to set "huge_pages =3D on" if you inte=
nd
> to use them with
> PostgreSQL.  Otherwise it may happen that PostgreSQL resorts to allocatin=
g
> shared buffers
> using normal memory, and your huge pages just sit around and are wasted.
>
> Yours,
> Laurenz Albe
>

I fully agree with Laurenz.

And it=E2=80=99s basically the same story the other way around: if you set
huge_pages =3D on but you reserved too many huge pages, the extra ones will
also just sit there unused, i.e. wasted.

What slightly worries me is this line in the docs:
=E2=80=9CWhile we need at least 3170 huge pages in this example, a larger s=
etting
would be appropriate if other programs on the machine also need huge pages.=
=E2=80=9D

I get what it=E2=80=99s trying to say (size for all consumers), but it can =
read a
bit like =E2=80=9Cmore is fine=E2=80=9D. On a dedicated PostgreSQL server, =
=E2=80=9Cmore=E2=80=9D is
usually just wasted memory unless you actually have another hugepage user.

It might be worth adding a short note that unused huge pages don=E2=80=99t =
get used
for regular memory allocations, so it=E2=80=99s best to size them as close =
to what
you need as possible (especially on dedicated servers).

regards,
A.Kucharczyk

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<div dir=3D"ltr"><div dir=3D"ltr">=C5=9Br., 28 sty 2026 o 18:02=C2=A0Lauren=
z Albe &lt;<a href=3D"mailto:laurenz.albe@cybertec.at">laurenz.albe@cyberte=
c.at</a>&gt; napisa=C5=82(a):</div><div class=3D"gmail_quote gmail_quote_co=
ntainer"><blockquote class=3D"gmail_quote" style=3D"margin:0px 0px 0px 0.8e=
x;border-left:1px solid rgb(204,204,204);padding-left:1ex">
&gt; In the example in this (quite helpful) Cybertec post,=C2=A010475 HugeP=
ages are allocated.=C2=A0<br>
&gt; <a href=3D"https://www.cybertec-postgresql.com/en/huge-pages-postgresq=
l/" rel=3D"noreferrer" target=3D"_blank">https://www.cybertec-postgresql.co=
m/en/huge-pages-postgresql/</a><br>
&gt; <br>
&gt; If we set &quot;huge_pages =3D off&quot; within PG and then restart PG=
, will Linux see those 10475 HugePages as off-limits to normal 4KiB allocat=
ions?<br>
<br>
Yes.=C2=A0 That&#39;s why it is a good idea to set &quot;huge_pages =3D on&=
quot; if you intend to use them with<br>
PostgreSQL.=C2=A0 Otherwise it may happen that PostgreSQL resorts to alloca=
ting shared buffers<br>
using normal memory, and your huge pages just sit around and are wasted.<br=
>
<br>
Yours,<br>
Laurenz Albe<br></blockquote><div><br></div>I fully agree with Laurenz.<br>=
<br>And it=E2=80=99s basically the same story the other way around: if you =
set huge_pages =3D on but you reserved too many huge pages, the extra ones =
will also just sit there unused, i.e. wasted.<br><br>What slightly worries =
me is this line in the docs:<br>=E2=80=9CWhile we need at least 3170 huge p=
ages in this example, a larger setting would be appropriate if other progra=
ms on the machine also need huge pages.=E2=80=9D<br><br>I get what it=E2=80=
=99s trying to say (size for all consumers), but it can read a bit like =E2=
=80=9Cmore is fine=E2=80=9D. On a dedicated PostgreSQL server, =E2=80=9Cmor=
e=E2=80=9D is usually just wasted memory unless you actually have another h=
ugepage user.<br><br><div>It might be worth adding a short note that unused=
 huge pages don=E2=80=99t get used for regular memory allocations, so it=E2=
=80=99s best to size them as close to what you need as possible (especially=
 on dedicated servers).=C2=A0</div><div><br></div><div>regards,</div><div>A=
.Kucharczyk</div></div></div>

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