CODEWITHSUNDEEP
linuxubuntuembeddedposixembedded-linux
amo
amo

Reputation: 4340

What are reasons that read() on a POSIX serial port might be slow?

I'm writing a console application under Ubuntu that uses the serial port. It needs to read and write from the serial port at 60 Hz.

I find that the call to read() is often, but not always, slow. I have set O_NDELAY, so often it returns immediately (great). Sometimes it takes up to 50 ms to finish, and that is too slow for my application. Before calling read(), I check the number of chars available, so it should not be waiting for data.

What is read() doing that takes so long? How can I speed it up?

Options on the port are:

options.c_cflag |= (CLOCAL | CREAD);
options.c_cflag &= ~PARENB;
options.c_cflag &= ~CSTOPB;
options.c_cflag &= ~CSIZE;
options.c_cflag |= CS8;

options.c_lflag &= ~(ICANON | ECHO | ECHOE | ISIG);
options.c_iflag &= ~IXON;

options.c_oflag = 0;

edit: I'd been using select() earlier but it turned out to be orthogonal to the question. Updated with my latest information.

Upvotes: 2

Views: 3352

Answers (2)

amo
amo

Reputation: 4340

The solution is to set the low_latency flag on the serial port.

See High delay in RS232 communication on a PXA270 and http://osdir.com/ml/serial/2003-11/msg00020.html

Upvotes: 2

David Schwartz
David Schwartz

Reputation: 182827

It's not what select is doing, it's what the system is doing. Your thread eventually uses up its timeslice and the system allows other code to run. If you use a sensible timeout, rather than trying to return immediately, the system should treat your process as interactive and the delays should go away.

If there's a point to selecting on a single descriptor with a 0 timeout, I can't figure out what it is. Why not just try the operation and see if you get a EWOULDBLOCK error?

Why not use a sensible timeout so the system lets other processes run when you have nothing to do?

Upvotes: 0

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