Eye tracker: ft-eyes sleeps until the next frame is due

ft-eyes looked for new frames about 1,000 times a second: each pass of its loop asked the
control socket with a non-blocking recvfrom (a BlockingIOError nearly every time), read
both cameras' counters, and slept 1 ms. The frames come every 11.1 ms per camera, and only
as counters in ft-eyegrab's shared memory, so there's no fd to wait on.

Now each pass ends in select() on the control socket, with a timeout until 2 ms before the
next frame of either camera is due (from when its last one was seen), then every 1 ms until
it comes. A command wakes it at once. A camera with no frame for 0.1 s (headset off,
grabber idle) isn't waited for, and with both stopped it looks every 20 ms. Waiting for the
frame grabber's file uses the same select, 0.2 s at a time, instead of sleeping through
commands.

A new frame is still seen within about 1 ms of when it lands. On a synthetic share at 90 Hz
per camera, on a heavily loaded headset (load average 23, so ft-eyes rarely sat idle), its
waits went from 178 to 81 a second; unloaded, the old loop's 1 ms sleeps add up to about
1,000.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

gaze/tracker/ft-eyes

@@ -50,6 +50,7 @@ import json
 import math
 import mmap
 import os
+import select
 import socket
 import struct
 import sys
@@ -87,6 +88,15 @@ GAP = 3.0             # s without frames: the headset was off, and may sit diffe
 CLICK_BEFORE = 0.3    # a click's frames: the 300 ms before it (like the probe's fixation)
 CALIB_MIN = 15        # frames an eye needs in a calibration dot's window
 CALIB_SPREAD = 4.0    # px: more than this and the eye moved during the dot
+# Waiting for frames. They come only as a counter in shared memory, so there's nothing to block
+# on: sleep until a camera's next frame is due, then look every POLL. ft-eyegrab passes each one
+# on within a ms or two of when its camera starts the one after, so they arrive 11.1 ms apart,
+# give or take that.
+PERIOD = 1 / 90       # s between a camera's frames
+EARLY = 0.002         # start looking this long before a frame is due
+POLL = 0.001          # then this often until it comes
+STALLED = 0.1         # s without a frame: that camera stopped (headset off), and isn't waited for
+IDLE_POLL = 0.02      # how often to look while both are stopped
 
 
 class Cams:
@@ -357,7 +367,7 @@ def wait_for_cams(sock, tracker, want):
             return Cams()
         except (OSError, ValueError, RuntimeError):
             serve(sock, tracker)
-            time.sleep(0.2)
+            select.select([sock], [], [], 0.2)
 
 
 def serve(sock, tracker):
@@ -420,6 +430,7 @@ def main():
     print("ft-eyes: frames found, tracking", file=sys.stderr, flush=True)
     seen = [cams.count(0), cams.count(1)]
     report = time.monotonic()
+    arrived = [0.0, 0.0]  # when each camera's newest frame was seen (monotonic)
     while True:
         serve(sock, tracker)
         want()
@@ -429,6 +440,7 @@ def main():
             if n == seen[c]:
                 continue
             seen[c] = n
+            arrived[c] = time.monotonic()
             got = cams.frame(c, n - 1)  # only the newest: never fall behind
             if got:
                 eyes[c].feed(*got)
@@ -447,8 +459,6 @@ def main():
                     shifts += [float(v) for v in e.shift.value]
                     pupils += [e.gaze[3], e.gaze[4]] if fresh else [math.nan, math.nan]
                 out.write(latest, (yaw, pitch), flags, per, shifts, pupils)
-        else:
-            time.sleep(0.001)
         now = time.monotonic()
         if now - report >= 5:
             if verbose:
@@ -466,6 +476,12 @@ def main():
                 print("ft-eyes: frames went away; waiting", file=sys.stderr, flush=True)
                 cams = wait_for_cams(sock, tracker, want)
                 seen = [cams.count(0), cams.count(1)]
+        # Until the next frame is due (a command on the socket wakes us sooner).
+        wait = IDLE_POLL
+        for c in (0, 1):
+            if now - arrived[c] < STALLED:
+                wait = min(wait, max(arrived[c] + PERIOD - EARLY - now, POLL))
+        select.select([sock], [], [], wait)
 
 
 if __name__ == "__main__":