bug#57727: 29.0.50; Optimize tty display updates

[Gerd Möllmann]

This is a wishlist item for the implementation of what was discussed
recently on emacs-devel, namely using a larger output buffer, and
avoiding fflush as a means of improving tty display update speed.

The patch below is my first take on that.  It adds a low-level interface
consisting of two functions tty--set-output-buffer-size and
tty-output-buffer-size to change and retrieve the output buffer size of
a device.  A buffer size of 0 is taken to mean "do what we did since
1991, and a non-zero value is used as the buffer size set with setvbuf.

Default is currently to do everything as it always was.  One has to do
something like

  (tty--set-output-buffer-size (* 64 1024))

to change to the new behavior.

Feedback welcome, especially for the default, i.e. leaving everything
as-is, and for the Lisp interface.

>From 597d17f6ecc866328e3c5c3438b1d7fec3b2bbea Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Gerd=20M=C3=B6llmann?= <gerd@gnu.org>
Date: Sun, 11 Sep 2022 11:42:18 +0200
Subject: [PATCH] Optimize tty display updates

* src/dispnew.c (update_frame_1): Don'f flush if tty's
output_buffer_size is non-zero.
* src/sysdep.c (init_sys_modes): Setvbuf depending on the tty's
output_buffer_size.
* src/term.c (Ftty__set_output_buffer_size, Ftty__output_buffer_size):
Low-level interface for setting and retrieving a tty's output buffer
size.
(syms_of_term): Defsubr the new functions.
* src/termchar.h (struct tty_display_info): New member
output_buffer_size.
---
 src/dispnew.c  |  4 +++-
 src/sysdep.c   |  5 ++++-
 src/term.c     | 39 +++++++++++++++++++++++++++++++++++++++
 src/termchar.h |  5 +++++
 4 files changed, 51 insertions(+), 2 deletions(-)

diff --git a/src/dispnew.c b/src/dispnew.c
index 8932f103f1..b786f0f1ba 100644
--- a/src/dispnew.c
+++ b/src/dispnew.c
@@ -4929,7 +4929,9 @@ update_frame_1 (struct frame *f, bool force_p, bool 
inhibit_id_p,
     {
       if (MATRIX_ROW_ENABLED_P (desired_matrix, i))
        {
-         if (FRAME_TERMCAP_P (f))
+         /* Note that output_buffer_size being 0 means that we want the
+            old default behavior of flushing output every now and then.  */
+         if (FRAME_TERMCAP_P (f) && FRAME_TTY (f)->output_buffer_size == 0)
            {
              /* Flush out every so many lines.
                 Also flush out if likely to have more than 1k buffered
diff --git a/src/sysdep.c b/src/sysdep.c
index efd9638b07..abb385d138 100644
--- a/src/sysdep.c
+++ b/src/sysdep.c
@@ -1304,7 +1304,10 @@ init_sys_modes (struct tty_display_info *tty_out)
     }
 #endif /* F_GETOWN */
 
-  setvbuf (tty_out->output, NULL, _IOFBF, BUFSIZ);
+  const size_t buffer_size = (tty_out->output_buffer_size
+                             ? tty_out->output_buffer_size
+                             : BUFSIZ);
+  setvbuf (tty_out->output, NULL, _IOFBF, buffer_size);
 
   if (tty_out->terminal->set_terminal_modes_hook)
     tty_out->terminal->set_terminal_modes_hook (tty_out->terminal);
diff --git a/src/term.c b/src/term.c
index 2e43d89232..8e19c96672 100644
--- a/src/term.c
+++ b/src/term.c
@@ -2400,6 +2400,43 @@ DEFUN ("resume-tty", Fresume_tty, Sresume_tty, 0, 1, 0,
   return Qnil;
 }
 
+DEFUN ("tty--set-output-buffer-size", Ftty__set_output_buffer_size,
+       Stty__set_output_buffer_size, 1, 2, 0, doc:
+       /* Set the output buffer size for a TTY.
+
+SIZE zero means use the system's default value.  If SIZE is
+non-zero,this also avoids flushing the output stream.
+
+TTY may be a terminal object, a frame, or nil (meaning the selected
+frame's terminal).
+
+This function temporarily suspends and resumes the terminal
+device.  */)
+  (Lisp_Object size, Lisp_Object tty)
+{
+  if (!TYPE_RANGED_FIXNUMP (size_t, size))
+    error ("Invalid output buffer size");
+  Fsuspend_tty(tty);
+  struct terminal *terminal = decode_tty_terminal (tty);
+  terminal->display_info.tty->output_buffer_size
+    = XFIXNUM (size) <= 0 ? 0 : XFIXNUM (size);
+  return Fresume_tty(tty);
+}
+
+DEFUN ("tty--output-buffer-size", Ftty__output_buffer_size,
+       Stty__output_buffer_size, 0, 1, 0, doc:
+       /* Return the output buffer size of TTY.
+
+TTY may be a terminal object, a frame, or nil (meaning the selected
+frame's terminal).
+
+A value of zero means TTY uses the system's default value.  */)
+  (Lisp_Object tty)
+{
+  struct terminal *terminal = decode_tty_terminal (tty);
+  return make_fixnum (terminal->display_info.tty->output_buffer_size);
+}
+
 
 /***********************************************************************
                               Mouse
@@ -4556,6 +4593,8 @@ syms_of_term (void)
   defsubr (&Stty_top_frame);
   defsubr (&Ssuspend_tty);
   defsubr (&Sresume_tty);
+  defsubr (&Stty__set_output_buffer_size);
+  defsubr (&Stty__output_buffer_size);
 #ifdef HAVE_GPM
   defsubr (&Sgpm_mouse_start);
   defsubr (&Sgpm_mouse_stop);
diff --git a/src/termchar.h b/src/termchar.h
index 49560dbc2a..0f17246411 100644
--- a/src/termchar.h
+++ b/src/termchar.h
@@ -53,6 +53,11 @@ #define EMACS_TERMCHAR_H
   FILE *output;                 /* The stream to be used for terminal output.
                                    NULL if the terminal is suspended. */
 
+  /* Size of output buffer.  A value of zero means use the default of
+     BUFIZE.  If non-zero, also minimize writes to the tty by avoiding
+     calls to flush.  */
+  size_t output_buffer_size;
+
   FILE *termscript;             /* If nonzero, send all terminal output
                                    characters to this stream also.  */
 
-- 
2.37.3