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Object: “Task Configuration”

More and more of today's controllers have processors with multiple CPU cores. In order to take advantage of this for the IEC program and improve performance, CODESYS provides options for distributing the IEC tasks over these CPU cores. Moreover, the IEC program should be divided into multiple tasks.

There are two basic different strategies in CODESYS for distributing IEC tasks over CPU cores:

  • Fixed linking of an IEC task to a CPU core:

    The task is always executed on this specific CPU core. In this case, multiple tasks can also be bundled to one and the same CPU core if the IEC program is not yet able to be executed on multiple CPU cores at the same time.

  • The IEC task is executed on all CPU cores:

    The operating system takes control of distributing the task on the CPU cores.

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When the IEC tasks are distributed over CPU cores, some changes result in the behavior in the IEC program, which have to be considered.

  • The processing of IEC tasks by priority is no longer a given. They are processed by priority only if the tasks are bundled together to one CPU core.

  • The cycle consistency of the data in the IEC task with the highest priority is no longer a given. Therefore, the data has to be copied locally at the beginning of the IEC task cycle if the values should not change during the cycle.

  • For consistent counters (incrementer, decrementer), the atomic external library function ‎SysCpuAtomicAdd()‎ should always be used (for more details, see ‎SysCpuHandling.library‎).

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Data consistency

  • Bit access (data type ‎BIT‎) is not processed consistently (atomically) on multicore CPUs in the IEC program. For this we recommend that you use the external library function ‎SysCpuTestAndSetBit()‎. (For details, see: ‎SysCpuHandling.library‎)

  • Simple data types up to a width of 32 bits (‎BYTE, WORD/INT, DWORD/DINT‎, etc.) are processed consistently (atomically) in the IEC program on multicore CPUs as well.

  • Data types with 64 bits (‎LINT, LWORD, LREAL)‎ are processed consistently (atomically) in the IEC program on 64-bit systems and multicore systems only. To do this, you do not need to take any precautions.

  • To access complex data types (‎STRINGs, FBs, STRUCTs, ARRAYs‎), you have to make arrangements yourself for the synchronization/consistency.

  • In the task configuration on the “Variable Usage” tab, you can define whether a variable in an IEC task has read or write access.

  • On multicore systems, a "memory reordering effect" can occur. For more information, see the IEC operator ‎__MemoryBarrier()‎.

  • Distributing tasks over multiple processor cores
  • Displaying the processor load per processor core in the trace