lguest: Adaptive timeout

Since the correct timeout value varies, use a heuristic which adjusts
the timeout depending on how many packets we've seen. This gives
slightly worse results, but doesn't need tweaking when GSO is
introduced.

500 usec 19.1887 xmit 561141 recv 1 timeout 559657
Dynamic (278) 20.1974 xmit 214510 recv 5 timeout 214491 usec 278

Signed-off-by: Rusty Russell <[e-mail]>

Documentation/lguest/lguest.c

···
 static unsigned long guest_limit, guest_max;
 /* The pipe for signal hander to write to. */
 static int timeoutpipe[2];
+static unsigned int timeout_usec = 500;
 
 /* a per-cpu variable indicating whose vcpu is currently running */
 static unsigned int __thread cpu_id;
···
 	itm.it_interval.tv_sec = 0;
 	itm.it_interval.tv_usec = 0;
 	itm.it_value.tv_sec = 0;
-	itm.it_value.tv_usec = 500;
+	itm.it_value.tv_usec = timeout_usec;
 
 	setitimer(ITIMER_REAL, &itm, NULL);
 }
···
 	unsigned int head, out, in, num = 0;
 	int len;
 	struct iovec iov[vq->vring.num];
+	static int last_timeout_num;
 
 	/* Keep getting output buffers from the Guest until we run out. */
 	while ((head = get_vq_desc(vq, iov, &out, &in)) != vq->vring.num) {
···
 	/* Block further kicks and set up a timer if we saw anything. */
 	if (!timeout && num)
 		block_vq(vq);
+
+	if (timeout) {
+		if (num < last_timeout_num)
+			timeout_usec += 10;
+		else if (timeout_usec > 1)
+			timeout_usec--;
+		last_timeout_num = num;
+	}
 }
 
 /* This is where we handle a packet coming in from the tun device to our