Rather than a loop subtracting 1e6 to convert usecs to secs/usecs, just

use div/mod.

io.c

@@ -405,14 +405,14 @@ static void sleep_for_bwlimit(int bytes_written)
 
 	if (!bwlimit)
 		return;
+
+	assert(bytes_written > 0);
+	assert(bwlimit > 0);
 	
-	tv.tv_sec = 0;
 	tv.tv_usec = bytes_written * 1000 / bwlimit;
+	tv.tv_sec  = tv.tv_usec / 1000000;
+	tv.tv_usec = tv.tv_usec % 1000000;
 
-	while (tv.tv_usec > 1000000) {
-		tv.tv_sec++;
-		tv.tv_usec -= 1000000;
-	}
 	select(0, NULL, NULL, NULL, tv);
 }