Re: busyloop in sigchld_handler

[storm@cua.dk (Kim F. Storm)]

Sam Steingold <sds@gnu.org> writes:

> GNU Emacs 22.0.95.2 (i686-pc-linux-gnu, X toolkit, Xaw3d scroll bars) of
> 2007-03-11 on loiso
>
> I have been observing the following behavior:
> emacs hangs in sigchld_handler waiting for the child process to
> terminate:
>
>       do
> 	{
> 	  errno = 0;
> 	  pid = wait3 (&w, WNOHANG | WUNTRACED, 0);
> 	}
>       while (pid < 0 && errno == EINTR);
>
> the system stops responding, loadavg goes to 5-8(!), CPU is 100% busy.
> this lasts for ~10 seconds.
> this happens on M-c compile and when I click on a URL (it is passed to
> an existing Firefox).

I've been annoyed by this behaviour for years - but have seen it occasionally
outside Emacs too, so I didn't expect it to be a problem in Emacs.

> I fixed the problem with the following patch:

Absolutely brilliant!  I can confirm that it works.

But can you explain why it works?  
And why the problem primarily hit M-x compile.

Does the fix cause a 1 second delay for other sub-processes ?

>
> Index: process.c
> ===================================================================
> RCS file: /sources/emacs/emacs/src/process.c,v
> retrieving revision 1.500
> retrieving revision 1.501
> diff -u -w -u -b -w -i -B -r1.500 -r1.501
> --- process.c	1 Mar 2007 10:17:41 -0000	1.500
> +++ process.c	11 Mar 2007 18:16:50 -0000	1.501
> @@ -6497,6 +6497,7 @@
>        /* Keep trying to get a status until we get a definitive result.  */
>        do
>  	{
> +          sleep (1);
>  	  errno = 0;
>  	  pid = wait3 (&w, WNOHANG | WUNTRACED, 0);
>  	}


-- 
Kim F. Storm <storm@cua.dk> http://www.cua.dk