GNU bug report logs - #79737
Consider increasing MAX_ALLOCA

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Message #5 received at submit <at> debbugs.gnu.org (full text, mbox):

From: Przemysław Alexander Kamiński
 <alexander <at> kaminski.se>
To: bug-gnu-emacs <at> gnu.org 
Subject: Consider increasing MAX_ALLOCA
Date: Fri, 31 Oct 2025 09:57:48 +0100

Summary:

Request to consider updating MAX_ALLOCA value in order to decrease heap
allocation (anecdotal performance boost on macOS observed).

Context:

Some critical functions (e.g. process reads, overlays, regex etc.) are
using SAFE_ALLOCA. SAFE_ALLOCA is allocating up to available
MAX_ALLOCA and defers to record_xmalloc if either:

- Requested allocation is bigger than MAX_ALLOCA
- Available pool was exhausted

This results in (usually) small, subsequent heap allocations.

MAX_ALLOCA has two functions:
- Is used as a stack allocation size
- Is used as an object allocation size

It seems rare for function to have more than single SAFE_ALLOCA and
over 21 years (when MAX_ALLOCA was introduced) default parameters for
operating systems had changed over time and could be used to decrease
allocation pressure.

I have a patch that I use for macOS that decreased temporary
heap allocation considerably (from 1:10 to 1:2, persistent to
temporary allocations).

-- 
Przemysław Alexander Kamiński (vel xlii vel exlee)
https://xlii.space  ||  https://codeberg.org/exlee

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