Re: Performance regression in 5.x: fork-heavy $() loops slow down ~2.5x on second run when built with --with-bash-malloc=no

Chet Ramey <[email protected]> Fri, 15 May 2026 10:11:58 -0400

Newsgroups	gmane.comp.shells.bash.bugs
Organization	ITS, Case Western Reserve University
Message-ID	<[email protected]>

On 5/13/26 11:53 PM, debianyu wrote:

> In an interactive shell, executing a fork-heavy loop with command substitution twice consecutively exhibits a ~2.5x slowdown on the second and subsequent runs:
> 
> time for i in {1..10000}; do echo $(date) &gt;/dev/null; done   # ~16s
> time for i in {1..10000}; do echo $(date) &gt;/dev/null; done   # ~40s
> time for i in {1..10000}; do echo $(date) &gt;/dev/null; done   # ~40s
> 
> The slowdown persists for the shell's lifetime, including subshells spawned from it.
> 
> Bisecting build configurations and bash versions reveals the slowdown requires both:
> 
> Bash 5.x series (5.0 onward; 4.4 unaffected)
> ./configure --with-bash-malloc=no (bash uses glibc malloc)
> 
> Either condition alone does not trigger the issue:

That's interesting. I haven't seen the glibc malloc trigger such a dramatic
slowdown before.

> Trigger Condition: Loop body must involve both fork and execve. Builtins or fork-only operations (e.g., x=$(:)) do not trigger the issue.

I agree that the patch in

https://lists.gnu.org/archive/html/bug-bash/2026-05/msg00038.html

is worth looking at.

Chet

-- 
``The lyf so short, the craft so long to lerne.'' - Chaucer
		 ``Ars longa, vita brevis'' - Hippocrates
Chet Ramey, UTech, CWRU    [email protected]    http://tiswww.cwru.edu/~chet/

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