ui/ncurses: Reduce unnecessary calls to redrawwin

All current *_post() methods in ui/ncurses call redrawwin() and
wrefresh() together. wrefresh() updates any lines on the screen that
have been marked as changed or invalid. However redrawwin() marks the
entire screen as invalid unconditionally. We can reduce the amount of
data written to the screen by avoiding calls to redrawwin().

Screen transitions are the primary use case of redrawwin(), where the
whole screen must be invalidated to avoid stale data remaining on
screen. All other 'in-screen' updates such as changes to widgets or
changing focus do not require a call to redrawwin(). The most noticeable
performance improvement is in nc-menu, which makes an unnecssary call to
redrawwin() after every addition to the boot option menu.

eg. The number of bytes written to STDOUT in the main menu:
	# Boot options	| Before | After
	--------------------------------
		8	| 5488   | 1149
		133	| 422454 | 4652

Signed-off-by: Samuel Mendoza-Jonas <sam.mj@au1.ibm.com>

1 files changed, 7 insertions, 1 deletions

diff --git a/ui/ncurses/nc-config.c b/ui/ncurses/nc-config.c
index 17cc380..c45df34 100644
--- a/ui/ncurses/nc-config.c
+++ b/ui/ncurses/nc-config.c
@@ -57,6 +57,7 @@ struct config_screen {
 
 	bool			exit;
 	bool			show_help;
+	bool			need_redraw;
 	void			(*on_exit)(struct cui *);
 
 	int			scroll_y;
@@ -146,6 +147,7 @@ static void config_screen_process_key(struct nc_scr *scr, int key)
 
 	} else if (screen->show_help) {
 		screen->show_help = false;
+		screen->need_redraw = true;
 		cui_show_help(screen->cui, _("System Configuration"),
 				&config_help_text);
 
@@ -165,7 +167,10 @@ static int config_screen_post(struct nc_scr *scr)
 	struct config_screen *screen = config_screen_from_scr(scr);
 	widgetset_post(screen->widgetset);
 	nc_scr_frame_draw(scr);
-	redrawwin(scr->main_ncw);
+	if (screen->need_redraw) {
+		redrawwin(scr->main_ncw);
+		screen->need_redraw = false;
+	}
 	wrefresh(screen->scr.main_ncw);
 	pad_refresh(screen);
 	return 0;
@@ -859,6 +864,7 @@ struct config_screen *config_screen_init(struct cui *cui,
 
 	screen->cui = cui;
 	screen->on_exit = on_exit;
+	screen->need_redraw = false;
 	screen->label_x = 2;
 	screen->field_x = 17;