| From: | David Rowley <dgrowleyml(at)gmail(dot)com> |
|---|---|
| To: | cca5507 <cca5507(at)qq(dot)com> |
| Cc: | "David G(dot) Johnston" <david(dot)g(dot)johnston(at)gmail(dot)com>, Chao Li <li(dot)evan(dot)chao(at)gmail(dot)com>, pgsql-hackers <pgsql-hackers(at)lists(dot)postgresql(dot)org> |
| Subject: | Re: Fix incorrect comments in tuplesort.c |
| Date: | 2025-12-07 22:09:37 |
| Message-ID: | CAApHDvrN3PNF0G4uxzT87i5034wYLG1MPZtzaCzjDaX8iJ3tOQ@mail.gmail.com |
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| Lists: | pgsql-hackers |
On Sun, 7 Dec 2025 at 21:34, cca5507 <cca5507(at)qq(dot)com> wrote:
> I find that the initial size of memtuples array must be more than ALLOCSET_SEPARATE_THRESHOLD
> is not for lower overhead of allocation, but for a bug fixed in 8ea3e7a75c0d22c41c57f59c8b367059b97d0b66.
>
> Attach a new patch.
I find the current comment perfectly understandable and the text
you're proposing to be incorrect. What you might be missing is that
with aset.c, a palloc() request above ALLOCSET_SEPARATE_THRESHOLD is
certain to require a malloc() and a dedicated block. Sizes under that
may be able to use an existing AllocBlock. The comment is effectively
explaining that we don't want to make the array big enough so that a
malloc will always be required. You may need to read
AllocSetContextCreateInternal() around where allocChunkLimit is being
set to understand what this is all about.
David
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