XML CO631 Anonymous Questions and Answers Keyword Index

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Keyword reference for par

Question 60 (2004):

Hi, could you just give an explanation of how the following code works (taken from "sort_pump.occ"):

    PROC sort (CHAN OF BYTE in, out)
      [total - 2]CHAN OF BYTE c:
      PAR
        cell (in, c[0])
        PAR p = 1 FOR total - 3
          cell (c[p - 1], c[p])
        cell (c[total - 3], out)
    :

Thanks.

Answer 60:

This code creates a pipeline of processes (of `total - 1' cells) -- see slides 5-17 and 5-18. The two end cells are catered for specially (since these plug into the external environment -- via `in' and `out'). The cells between these are created using a replicated PAR. If you substitute a constant value for `total' (say 10), then expand and flatten the PAR replicator, it becomes a bit clearer:

    [8]CHAN OF BYTE c:
    PAR
      cell (in, c[0])

      cell (c[0], c[1])        -- PAR p = 1 FOR 7
      cell (c[1], c[2])        --   cell (c[p - 1], c[p])
      cell (c[2], c[3])
      cell (c[3], c[4])
      cell (c[4], c[5])
      cell (c[5], c[6])
      cell (c[6], c[7])

      cell (c[7], out)

Keywords: sort-pump , par , replicator

Question 78 (2003):

I'm quite confused on how I can assign each philosopher an identity. I've modified the header of the PROC philosopher in the following way:

    PROC philosopher(CHAN OF INT id, CHAN OF BOOL left, right,down, up, CHAN OF MESSAGE report)

This means that the display PROC has 16 input channels. I'm not sure this is the right way of doing it.

Answer 78:

Assigning IDs to the philosophers is better done by simply using a ``VAL INT'', passed to each philosopher from the PAR replicator that sets the network up.

Communicating the ID to each philosopher (using the `id' channel you added) is certainly possible, but definitely not the simplest way.

Keywords: par , q7

Question 2 (2000):

Is PAR used *only* when connecting all PROCs to build the final one big PROC?

Answer 2:

No. See the above answer for an example use of PAR to build some very fine-grained parallel code. It all depends on the code you write into the components of the PAR. In the above, they are short-lived primitives (a channel input and an assigment). If the statements controlled by the PAR and instances of PROC calls and those PROCs are long-lived (e.g. they contain loops - possibly infinite), then we get a long-lived process network. See, for example, slide 4-26 from the course notes. [BTW - has everyone found the deliberate mistake on slide 4-26?]

occam lets us easilly build parallelism at any level in the system - this is much more flexible than Java, for example, whose Thread mechanism is considerably more complex to set up.

Keywords: par , process-networks