GNU bug report logs - #66020
[PATCH] Reduce GC churn in read_process_output

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From: Eli Zaretskii <eliz <at> gnu.org>
To: Dmitry Gutov <dmitry <at> gutov.dev>
Cc: 66020 <at> debbugs.gnu.org, monnier <at> iro.umontreal.ca, stefankangas <at> gmail.com
Subject: bug#66020: (bug#64735 spin-off): regarding the default for read-process-output-max
Date: Thu, 21 Sep 2023 16:16:31 +0300

> Date: Thu, 21 Sep 2023 15:20:57 +0300
> Cc: Eli Zaretskii <eliz <at> gnu.org>, Stefan Kangas <stefankangas <at> gmail.com>,
>  66020 <at> debbugs.gnu.org
> From: Dmitry Gutov <dmitry <at> gutov.dev>
> 
> On 21/09/2023 05:36, Stefan Monnier wrote:
> >> make_process), although I had to use a value produced by make_uninit_string:
> >> apparently simply storing a char* field inside a managed structure creates
> >> problems for the GC and early segfaults. Anyway, the result was slightly
> > That should depend on*where*  you put that field.  Basically, it has to
> > come after:
> > 
> >      /* The thread a process is linked to, or nil for any thread.  */
> >      Lisp_Object thread;
> >      /* After this point, there are no Lisp_Objects.  */
> > 
> > since all the words up to that point will be traced by the GC (and
> > assumed to be Lisp_Object fields).
> 
> Ah, thanks. That calls for another try.
> 
> ...still no improvement, though no statistically significant slowdown 
> either this time.

Why did you expect a significant improvement?  Allocating and freeing
the same-size buffer in quick succession has got to be optimally
handled by modern malloc implementations, so I wouldn't be surprised
by what you discover.  There should be no OS calls, just reuse of a
buffer that was just recently free'd.  The overhead exists, but is
probably very small, so it is lost in the noise.

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