Low latency serial communication on Linux

Question

I'm implementing a protocol over serial ports on Linux. The protocol is based on a request answer scheme so the throughput is limited by the time it takes to send a packet to a device and get an answer. The devices are mostly arm based and run Linux >= 3.0. I'm having troubles reducing the round trip time below 10ms (115200 baud, 8 data bit, no parity, 7 byte per message).

What IO interfaces will give me the lowest latency: select, poll, epoll or polling by hand with ioctl? Does blocking or non blocking IO impact latency?

I tried setting the low_latency flag with setserial. But it seemed like it had no effect.

Are there any other things I can try to reduce latency? Since I control all devices it would even be possible to patch the kernel, but its preferred not to.

---- Edit ----

The serial controller uses is an 16550A.

Answer 1

Request / answer schemes tends to be inefficient, and it shows up quickly on serial port. If you are interested in throughtput, look at windowed protocol, like kermit file sending protocol.

Now if you want to stick with your protocol and reduce latency, select, poll, read will all give you roughly the same latency, because as Andy Ross indicated, the real latency is in the hardware FIFO handling.

If you are lucky, you can tweak the driver behaviour without patching, but you still need to look at the driver code. However, having the ARM handle a 10 kHz interrupt rate will certainly not be good for the overall system performance...

Another options is to pad your packet so that you hit the FIFO threshold every time. It will also confirm that if it is or not a FIFO threshold problem.

10 msec @ 115200 is enough to transmit 100 bytes (assuming 8N1), so what you are seeing is probably because the low_latency flag is not set. Try

setserial /dev/<tty_name> low_latency

It will set the low_latency flag, which is used by the kernel when moving data up in the tty layer:

void tty_flip_buffer_push(struct tty_struct *tty)
{
         unsigned long flags;
         spin_lock_irqsave(&tty->buf.lock, flags);
         if (tty->buf.tail != NULL)
                 tty->buf.tail->commit = tty->buf.tail->used;
         spin_unlock_irqrestore(&tty->buf.lock, flags);
 
         if (tty->low_latency)
                 flush_to_ldisc(&tty->buf.work);
         else
                 schedule_work(&tty->buf.work);
}

The schedule_work call might be responsible for the 10 msec latency you observe.

Answer 2

Having talked to to some more engineers about the topic I came to the conclusion that this problem is not solvable in user space. Since we need to cross the bridge into kernel land, we plan to implement an kernel module which talks our protocol and gives us latencies < 1ms.

--- edit ---

Turns out I was completely wrong. All that was necessary was to increase the kernel tick rate. The default 100 ticks added the 10ms delay. 1000Hz and a negative nice value for the serial process gives me the time behavior I wanted to reach.