56 lines
2.6 KiB
Markdown
56 lines
2.6 KiB
Markdown
# Mux
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The emulator provides a mechanism to interconnect [channels](../channels) in a
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similar way as an [analog
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multiplexer](https://en.wikipedia.org/wiki/Multiplexer) by using the `mux`
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module.
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Multiplexers or muxers have one output channel, one select channel and a
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variable number of input channels (including zero). Each input channel is
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registered in the multiplexer by using a key with a given value. In the normal
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operation of a mux, the value of the select channel is used to find an input
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with a matching key. When there is no match, the output value is set to null.
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Multiplexers can be configured to execute a custom select function, which will
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take the value of the select channel and determine which input should be
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selected. This allows a multiplexer to act as a filter too.
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## Tracking
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The typical use of multiplexers is to implement the tracking modes of channels.
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As an example, the following diagram shows two multiplexers used to implement
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the subsystem view of [Nanos6](../nanos6):
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The first mux0 selects the input corresponding to the thread running in that
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CPU (thread0), and forwards the value to the output. If no thread is running the
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output is set to null. The output channel *nanos6.cpu0.subsystem.run* is then
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connected to the Paraver timeline in the row corresponding to the CPU0, which
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shows the subsystem of the currently **running** thread.
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The Nanos6 subsystem channel is also connected to the second mux1, which forwards the
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value to the output only when the thread state is **Active** (not paused or
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dead). The output is directly connected to the Paraver row assigned to that
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thread. This view shows the subsystem of the thread when is active.
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Notice that when the thread0 is no longer Running (switches to the Cooling
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state shown in yellow) the CPU subsystem timeline no longer shows the subsystem,
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as the mux0 is not selecting any input (because there are no threads in the CPU0
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in the Running state at that moment).
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However, the thread view still shows the subsystem, as the thread0 is still
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active (not paused yet). The mux1 is still selecting the thread subsystem
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channel as input.
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## Separation of concerns
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Multiplexers allow models to interact with each other in a controlled way. In
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the example, the blue channel (*nanos6.thread0.subsystem*) is directly modified by
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the Nanos6 model when a new event is received. While the red channels are
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controlled by the ovni model. The rest of the channels are automatically updated
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in the propagation phase of the [bay](../patchbay) allowing the ovni model to
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modify the Nanos6 Paraver view of the subsystems.
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