ReadFile的串行设备延迟不一致

Question

I need to read data from a serial port device(which sends data per second) on Windows in REALTIME (<= 5 ms). But the time cost by ReadFile is unpredictable, which drives me to crazy. Some piece of the code can be found at: https://gist.github.com/morris-stock/62b1674b4cda0e9df84d4738e54773f8

the delay is dumped at https://gist.github.com/morris-stock/62b1674b4cda0e9df84d4738e54773f8#file-serialport_win-cc-L283

    Poco::Timestamp now;
    if (!ReadFile(_handle, buffer, size, &bytesRead, NULL))
        throw Poco::IOException("failed to read from serial port");

    Poco::Timestamp::TimeDiff elapsed = now.elapsed();
    std::cout << Poco::DateTimeFormatter::format(now, "%Y-%m-%d %H:%M:%S.%i")
              << ", elapsed: " << elapsed << ", data len: " << bytesRead << std::endl << std::flush;

Sometimes ReadFile costs about 3000 us(which is OK, affected by COMMTIMEOUTS ) to return, but sometimes, it costs 15600 us(NOT affected by COMMTIMEOUTS ).

Please let me know if there is anything I can do to make the problem clear.

PS

COMMTIMEOUTS :

COMMTIMEOUTS cto;
cto.ReadIntervalTimeout         = 1;
cto.ReadTotalTimeoutConstant    = 1;
cto.ReadTotalTimeoutMultiplier  = 0;
cto.WriteTotalTimeoutConstant   = MAXDWORD;
cto.WriteTotalTimeoutMultiplier = 0;

the main reading thread part:

https://gist.github.com/morris-stock/62b1674b4cda0e9df84d4738e54773f8#file-serialdevice-cc-L31

device data type

baudrate : 9600, it sends about 400 bytes per second(continuously, then no data in the rest of the second).

consle output

      wPacketLength: 64
     wPacketVersion: 2
      dwServiceMask: 1
        dwReserved1: 0
       dwMaxTxQueue: 0
       dwMaxRxQueue: 0
          dwMaxBaud: 268435456
      dwProvSubType: 1
 dwProvCapabilities: 255
   dwSettableParams: 127
     dwSettableBaud: 268959743
      wSettableData: 15
wSettableStopParity: 7943
   dwCurrentTxQueue: 0
   dwCurrentRxQueue: 68824
        dwProvSpec1: 0
        dwProvSpec2: 1128813859
         wcProvChar: 0039F16C
2018-01-22 03:35:52.658, elapsed: 15600, data len: 0
2018-01-22 03:35:52.673, elapsed: 15600, data len: 0
2018-01-22 03:35:52.689, elapsed: 15600, data len: 0
2018-01-22 03:35:52.704, elapsed: 15600, data len: 0
2018-01-22 03:35:52.720, elapsed: 15600, data len: 0
2018-01-22 03:35:52.736, elapsed: 15600, data len: 0
2018-01-22 03:35:52.751, elapsed: 15600, data len: 0

Answer 1

In my case, it's the Windows system clock resolution that matters.

ClockRes gives me:

C:\work\utils\ClockRes>Clockres.exe

Clockres v2.1 - Clock resolution display utility
Copyright (C) 2016 Mark Russinovich
Sysinternals

Maximum timer interval: 15.600 ms
Minimum timer interval: 0.500 ms
Current timer interval: 1.000 ms

and

C:\work\utils\ClockRes>Clockres.exe

Clockres v2.1 - Clock resolution display utility
Copyright (C) 2016 Mark Russinovich
Sysinternals

Maximum timer interval: 15.600 ms
Minimum timer interval: 0.500 ms
Current timer interval: 15.600 ms

by calling timeBeginPeriod (1) when my app starts, I can get a more consistent result.

Thanks everyone for your kindly help.