Very slow performance

[pallix]

Versions

• Python: 2.7.6
• OS: Ubuntu 14.04
• Pymodbus: 1.3.2
• Modbus Hardware (if used):

Description

I have timed the sending of one mega byte of data with modbus RTU and the transfer spead is approximatively 7 KB/s.
When using libmodbus with RTU, I got a 10x performance improvement.
I have set the same baudrate in both cases.
This difference seems very strange to me and I wonder if a bug is hidding somewhere.

[dhoomakethu]

Could you provide some more details on the client being used (sync/async), baudrate used and any script you used to measure the performance ?

[pallix]

Thank you for your quick answer.

I use the sync client and server, a baudrate of 921600 and the Linux time command to measure performance of pymodbus.

To measure the performance of libmodbus, programs are provided by the library in the tests folder.

[dhoomakethu]

@pallix Thanks, Are you doing a write coil/holding registers or a read operation ? Also when you say one mega byte of data ,are you reading it from the file ? would be helpful, if you could share your code snippet.

[pallix]

Sorry that I did not provide the code sample directly. Server side:

slave = ModbusSlaveContext()
context = ModbusServerContext(slaves=slave, single=True)
framer = ModbusRtuFramer
StartSerialServer(port=PORT,
                          baudrate=BAUDRATE,
                          parity=PARITY,
                          context=context,
                          framer=framer,
                          timeout=TIMEOUT)

Client side:

        ONE_MEGA_BYTE = 1024**2

        client = ModbusSerialClient(method='rtu',
                                  port=PORT,
                                  baudrate=BAUDRATE,
                                  parity=PARITY,
                                  timeout=TIMEOUT,
                                  retries=1000)
        client.connect()
        nb_bytes_sent = 0
        builder = BinaryPayloadBuilder(endian=Endian.Little)
        while nb_bytes_sent < ONE_MEGA_BYTE:
            builder.reset()
            builder.add_32bit_float(1.0)
            builder.add_32bit_float(2.0)
            builder.add_32bit_float(3.0)
            builder.add_32bit_float(4.0)
            builder.add_32bit_float(5.0)
            payload = builder.build()
            client.write_registers(0, payload, skip_encode=True, unit=1)
            nb_bytes_sent += 4 * 5

[dhoomakethu]

Sorry for the late reply, There are multiple problems resulting in the slow timings. To begin with , For some reason the WriteRegisters Request doesn't come with response pdu size, and hence pymodbus waits at reading the bus till timeout (it actually is looking to read 1024 bytes which is default when the response pdu size is not calculated), There is another check when the number of bytes received is less than the expected size (in case of partial response) it again waits till the complete read timeout expecting the rest of the packets. So there are two timeouts that is taking in here + compulsory 3.5char timeout per read/write operation) which is resulting in increased time + the retry count of 1000 in your code consumes some more time when no response is received.

[pallix]

Thanks for the reply!

We checked the C benchmark and it is using much more registers (121) whereas the Python version throws an error when trying to pack more data than in this example.

Do you know why I cannot send more than 20 bytes at once?

[dhoomakethu]

The max number of registers supported by pymodbus in one transaction is 125, not sure what you mean by you can not send more than 20 bytes at once ?? Could you please show us how are you trying to pack the data ?

[pallix]

I pack the example as in the code above. When adding a line such as:

builder.add_32bit_float(6.0)

then an error is thrown when sending the data.

[dhoomakethu]

Not sure how you are building here payload, I can read up to 122 registers in one shot with float and read 125 registers in one shot

def make_payload(size):
    builder = BinaryPayloadBuilder(endian=Endian.Little)
    builder.reset()
    size = size//4
    val = 1.0
    for x in range(size):
        builder.add_32bit_float(val)
        val += 1.0
    payload = builder.build()
    return payload

def write_r(payload):
    return client.write_registers(0, payload, skip_encode=True, unit=1)

def read_r():
    return client.read_holding_registers(0, 125, unit=1)
start = time.time()
while nb_bytes_sent < ONE_MEGA_BYTE:
    r = write_r(make_payload(247))
    print(r)
    r = read_r()
    print(r)
    #
    nb_bytes_sent += 20
    break

And this is my response

DEBUG:pymodbus.transaction:Running transaction 1
DEBUG:pymodbus.transaction:send: 0x1 0x10 0x0 0x0 0x0 0x7a 0xf4 0x0 0x0 0x80 0x3f 0x0 0x0 0x0 0x40 0x0 0x0 0x40 0x40 0x0 0x0 0x80 0x40 0x0 0x0 0xa0 0x40 0x0 0x0 0xc0 0x40 0x0 0x0 0xe0 0x40 0x0 0x0 0x0 0x41 0x0 0x0 0x10 0x41 0x0 0x0 0x20 0x41 0x0 0x0 0x30 0x41 0x0 0x0 0x40 0x41 0x0 0x0 0x50 0x41 0x0 0x0 0x60 0x41 0x0 0x0 0x70 0x41 0x0 0x0 0x80 0x41 0x0 0x0 0x88 0x41 0x0 0x0 0x90 0x41 0x0 0x0 0x98 0x41 0x0 0x0 0xa0 0x41 0x0 0x0 0xa8 0x41 0x0 0x0 0xb0 0x41 0x0 0x0 0xb8 0x41 0x0 0x0 0xc0 0x41 0x0 0x0 0xc8 0x41 0x0 0x0 0xd0 0x41 0x0 0x0 0xd8 0x41 0x0 0x0 0xe0 0x41 0x0 0x0 0xe8 0x41 0x0 0x0 0xf0 0x41 0x0 0x0 0xf8 0x41 0x0 0x0 0x0 0x42 0x0 0x0 0x4 0x42 0x0 0x0 0x8 0x42 0x0 0x0 0xc 0x42 0x0 0x0 0x10 0x42 0x0 0x0 0x14 0x42 0x0 0x0 0x18 0x42 0x0 0x0 0x1c 0x42 0x0 0x0 0x20 0x42 0x0 0x0 0x24 0x42 0x0 0x0 0x28 0x42 0x0 0x0 0x2c 0x42 0x0 0x0 0x30 0x42 0x0 0x0 0x34 0x42 0x0 0x0 0x38 0x42 0x0 0x0 0x3c 0x42 0x0 0x0 0x40 0x42 0x0 0x0 0x44 0x42 0x0 0x0 0x48 0x42 0x0 0x0 0x4c 0x42 0x0 0x0 0x50 0x42 0x0 0x0 0x54 0x42 0x0 0x0 0x58 0x42 0x0 0x0 0x5c 0x42 0x0 0x0 0x60 0x42 0x0 0x0 0x64 0x42 0x0 0x0 0x68 0x42 0x0 0x0 0x6c 0x42 0x0 0x0 0x70 0x42 0x0 0x0 0x74 0x42 0x13 0xe7
DEBUG:pymodbus.client.sync:will sleep to wait for 3.5 char
DEBUG:pymodbus.transaction:recv: 0x1 0x10 0x0 0x0 0x0 0x7a 0x41 0xea
DEBUG:pymodbus.factory:Factory Response[16]
DEBUG:pymodbus.transaction:adding transaction 0
DEBUG:pymodbus.transaction:getting transaction 1
DEBUG:pymodbus.transaction:Running transaction 2
DEBUG:pymodbus.transaction:send: 0x1 0x3 0x0 0x0 0x0 0x7d 0x85 0xeb
WriteMultipleRegisterResponse (0,122)
DEBUG:pymodbus.client.sync:will sleep to wait for 3.5 char
ReadRegisterResponse (125)
DEBUG:pymodbus.transaction:recv: 0x1 0x3 0xfa 0x0 0x0 0x80 0x3f 0x0 0x0 0x0 0x40 0x0 0x0 0x40 0x40 0x0 0x0 0x80 0x40 0x0 0x0 0xa0 0x40 0x0 0x0 0xc0 0x40 0x0 0x0 0xe0 0x40 0x0 0x0 0x0 0x41 0x0 0x0 0x10 0x41 0x0 0x0 0x20 0x41 0x0 0x0 0x30 0x41 0x0 0x0 0x40 0x41 0x0 0x0 0x50 0x41 0x0 0x0 0x60 0x41 0x0 0x0 0x70 0x41 0x0 0x0 0x80 0x41 0x0 0x0 0x88 0x41 0x0 0x0 0x90 0x41 0x0 0x0 0x98 0x41 0x0 0x0 0xa0 0x41 0x0 0x0 0xa8 0x41 0x0 0x0 0xb0 0x41 0x0 0x0 0xb8 0x41 0x0 0x0 0xc0 0x41 0x0 0x0 0xc8 0x41 0x0 0x0 0xd0 0x41 0x0 0x0 0xd8 0x41 0x0 0x0 0xe0 0x41 0x0 0x0 0xe8 0x41 0x0 0x0 0xf0 0x41 0x0 0x0 0xf8 0x41 0x0 0x0 0x0 0x42 0x0 0x0 0x4 0x42 0x0 0x0 0x8 0x42 0x0 0x0 0xc 0x42 0x0 0x0 0x10 0x42 0x0 0x0 0x14 0x42 0x0 0x0 0x18 0x42 0x0 0x0 0x1c 0x42 0x0 0x0 0x20 0x42 0x0 0x0 0x24 0x42 0x0 0x0 0x28 0x42 0x0 0x0 0x2c 0x42 0x0 0x0 0x30 0x42 0x0 0x0 0x34 0x42 0x0 0x0 0x38 0x42 0x0 0x0 0x3c 0x42 0x0 0x0 0x40 0x42 0x0 0x0 0x44 0x42 0x0 0x0 0x48 0x42 0x0 0x0 0x4c 0x42 0x0 0x0 0x50 0x42 0x0 0x0 0x54 0x42 0x0 0x0 0x58 0x42 0x0 0x0 0x5c 0x42 0x0 0x0 0x60 0x42 0x0 0x0 0x64 0x42 0x0 0x0 0x68 0x42 0x0 0x0 0x6c 0x42 0x0 0x0 0x70 0x42 0x0 0x0 0x74 0x42 0x0 0x1 0x0 0x1 0x0 0x1 0xe0 0x1e
DEBUG:pymodbus.factory:Factory Response[3]
DEBUG:pymodbus.transaction:adding transaction 0
DEBUG:pymodbus.transaction:getting transaction 2