Scheduling Latency of a Blocking UART Read.

I have a Python script that is running as a systemd service. It is using a UART and SPI to communicate with an ARM processor. The process performs a blocking read on the serial port waiting for 4 bytes. Using an oscilloscope I measure the difference between when the serial port packet is transmitted and when the read call returns. Most often this value is around 2 ms which is fine. Sometimes (within 20 seconds to a minute) this value is 500 ms and I’ve seen it be larger than 1 second. If attempted to use chrt to change the static priority and scheduling policy. It reports that the values are set correctly but the large latencies remain.

This is with the 3.8.13-bone62 that I rebuilt to increase the SPI buffer. The buffer size was the only change.

Any thoughts on how to improve this? I can probably tolerate up to 100 ms.

Are you measuring the time to return to Python, or return from the read()

I toggle an output pin on the Beaglebone in Python after the call to I also measure the time in Python using time.time() and it jibes with what I see on the scope. Also the call to the SPI driver seems to have little latency and also have the SPI lines on the scope. So I can see the packet on the UART RX of the BBB and I can see the output pin go high at about the same time as the SPI clock burst and chip select.

Another detail is that I have had serial port code running for more than year on several hundred units that do not exhibit this issue. At first I thought it was because in this case I am using a blocking. I rewrote the code to use read with a timeout as with the earlier code and the problem persists.

I misstated the packet size above. It is 8 bytes. But that is also difference. My packets before were around 1 kB minimum. I’m wondering if there is some latency getting those bytes out of the hardware buffer or something like that.