[hackers] [st][PATCH] force redraw on pselect after key is pressed

From: Kenneth Maples <kenneth.maples_AT_gmail.com>
Date: Sun, 19 Apr 2020 17:09:40 +0800

While profiling terminal latency in st, I observed that data read from
the tty was not immediately written to the screen, but instead queued in
a rate limiting step. This results in a delay of ~17ms on my computer
between the XEvent capture by the terminal and the new character written
to the buffer when there is no system load.

This patch creates a global flag which is set when a keypress is sent
from X which forces the terminal to check for new data on the tty fd on
every return from pselect(). When new data read from the tty results in
a line being redrawn, the flag is reset.

On my computer, this results in a speedup from 17ms to 400us, which
includes the round-trip time to the shell.

---
 x.c | 7 +++++++
 1 file changed, 7 insertions(+)
diff --git a/x.c b/x.c
index e5f1737..7b3feef 100644
--- a/x.c
+++ b/x.c
_AT_@ -220,6 +220,7 @@ static DC dc;
 static XWindow xw;
 static XSelection xsel;
 static TermWindow win;
+static int pendingkpress = 0; 
 
 /* Font Ring Cache */
 enum {
_AT_@ -1604,6 +1605,8 @@ xdrawline(Line line, int x1, int y1, int x2)
 	Glyph base, new;
 	XftGlyphFontSpec *specs = xw.specbuf;
 
+	pendingkpress = 0;
+
 	numspecs = xmakeglyphfontspecs(specs, &line[x1], x2 - x1, x1, y1);
 	i = ox = 0;
 	for (x = x1; x < x2 && i < numspecs; x++) {
_AT_@ -1793,6 +1796,8 @@ kpress(XEvent *ev)
 	Status status;
 	Shortcut *bp;
 
+	pendingkpress = 1;
+
 	if (IS_SET(MODE_KBDLOCK))
 		return;
 
_AT_@ -1922,6 +1927,8 @@ run(void)
 		tv = &drawtimeout;
 
 		dodraw = 0;
+		if (pendingkpress)
+			dodraw = 1;
 		if (blinktimeout && TIMEDIFF(now, lastblink) > blinktimeout) {
 			tsetdirtattr(ATTR_BLINK);
 			win.mode ^= MODE_BLINK;
-- 
2.26.1

Received on Sun Apr 19 2020 - 11:09:40 CEST