On 06/06/2013 11:58 AM, Johan Hovold wrote:
> On Thu, Jun 06, 2013 at 10:50:36AM +0200, Tomaž Šolc wrote:
>> I have noticed that the ftdi_sio serial driver in recent kernel versions
>> has very bad performance when used through the Python's serial library.
>>
>> As a test case I have a custom device that will send a continuous block
>> of 5k characters once every few seconds over a RS-232 line (115200 baud)
>> to an Olimex programmer (based on FT2232C, also tried one with FT2232H).
>>
>> Programmer is connected to a Linux system where a simple Python script
>> reads the device:
>>
>> import serial
>> comm = serial.Serial("/dev/ttyUSB0", 115200)
>> while True:
>> line = comm.readline()
>>
>> With kernels before 3.7.0 the script reads uncorrupted data while using
>> newer kernels (including 3.9.4) the Python script sees heavy byte loss.
>> "top" shows an 95% "idle" CPU. Only very slow transmissions (on the
>> order of tens of bytes per second) will come through uncorrupted.
>>
>> Using git-bisect, I have found the commit that introduced this problem:
>>
>> 6f602912c9d0c84c2edbd446dd9f72660b701605
>> usb: serial: ftdi_sio: Add missing chars_in_buffer function
>>
>> This might also be related with the unusual way Python serial library
>> reads the device. It uses select() with no timeout and single byte
>> read()s in a loop. strace output:
>>
>> select(4, [3], [], [], NULL) = 1 (in [3])
>> read(3, "D", 1) = 1
>> select(4, [3], [], [], NULL) = 1 (in [3])
>> read(3, "E", 1) = 1
>> ...
>>
>> With sufficiently large read()s the byte loss can be eliminated.
>>
>> With the commit above, each select() now causes an additional round trip
>> over USB to read the state of the hardware buffer. It's possible that
>> constant status querying triggers some bug in the hardware or the query
>> is simply too slow and causes overflows in the hardware buffer.
>
> You're absolutely right. This is a known issue (the select overhead)
> that was just recently fixed by commit a37025b5c7 ("USB: ftdi_sio: fix
> chars_in_buffer overhead") in v3.10-rc3. Care to give v3.10-rc4 a try?
Johan, thanks for the prompt reply. I have tried v3.10-rc4 and indeed
the issue is fixed.
Best regards
Tomaž
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