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

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Message #46 received at 66020 <at> debbugs.gnu.org (full text, mbox):

From: Dmitry Gutov <dmitry <at> gutov.dev>
To: Eli Zaretskii <eliz <at> gnu.org>
Cc: 66020 <at> debbugs.gnu.org, monnier <at> iro.umontreal.ca, stefankangas <at> gmail.com
Subject: Re: bug#66020 (bug#64735 spin-off): regarding the default for
 read-process-output-max
Date: Thu, 21 Sep 2023 20:54:30 +0300

On 21/09/2023 16:16, Eli Zaretskii wrote:
>> 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?

No need to be surprised, I'm still growing intuition for what is fast 
and what is slow at this level of abstraction.

> 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.

There are context switches after 'read_process_output' exits (control is 
returned to Emacs's event loop, the external process runs again, we wait 
on it with 'select'), it might not be there later, especially outside of 
the lab situation where we benchmark just single external process. So I 
don't know.

I'm not majorly concerned, of course, and wouldn't be at all, if not for 
the previously recorded minor degragation with larger buffers in the 
longer-running session (last table in https://debbugs.gnu.org/66020#10).

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