Re: Excessive redisplay from lots of process output

[Eli Zaretskii]

> From: Spencer Baugh <sbaugh@janestreet.com>
> Date: Fri, 24 Feb 2023 15:08:17 -0500
> Cc: emacs-devel@gnu.org,
>   azeng@janestreet.com
> 
> > This is normal.
> 
> Just to make sure I understand, you said earlier:
> 
> >There are many variables and flags Emacs uses to decide when something
> >changed that needs the display to be updated.  Those should generally
> >prevent Emacs from running the expensive parts of redisplay if they
> >are unnecessary.  The question is why this doesn't work in your case.
> 
> Are you saying this does not apply in the case of reading from
> process output? That after reading process output, we always do
> an expensive redisplay?

Whether redisplay is expensive or not depends on many factors.  But we
always flush the frame in that case, yes.

> If so, it seems to me that this could be substantially improved.
> Why do we always do an expensive redisplay after reading process
> output, even if nothing has been changed to make it necessary?

We don't always do an expensive redisplay, but we always call XFlush.

> I would like to improve this so that my Emacs does not do
> expensive redisplay whenever it reads process output, which is
> substantially hurting performance for me when there's lots of
> process output to read.

AFAIU, what is problematic for you is the X traffic causes by XFlush.