Rodrigo Arias
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64 lines
2.7 KiB
Markdown
64 lines
2.7 KiB
Markdown
# Ovni model
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The ovni model tracks the state of threads and CPUs and provides the
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basis for other models.
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## Thread model
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The thread state is modelled as a finite state machine, depicted below:
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{: style="width:80%"}
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All threads begin in the `unknown` state and perform a transition to
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running then the `execute` event (with MCV `OHx`) is received.
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Notice that the runtime thread started the execution **before** emitting
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the execute event, as it is not posible to emit the event at the exact
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moment the thread begins the execution.
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The asterisk `*` in the transitions states when the event is emitted. If
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appears *before* the transition name, it happens *before* the real
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event. If appears *after* then it is emitted *after* the real event.
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In the case of the thread beginning the execution, the event is emitted
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*after* the thread begins the execution, so the asterisk appears after.
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### Cooling and warming states
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In the Linux kernel, it is generally not possible to switch from a
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thread A to another thread B in a single atomic operation, from
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user-space. The thread A must first wake B and then put itself to sleep.
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But the kernel is free to continue the execution of B instead of just
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executing the last instructions of A so it can stop.
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This limitation causes a time region at which more than one thread is
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scheduled to the same CPU, which is a property that is generally
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considered an error in parallel runtimes. To avoid this situation, the
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additional states *cooling* and *warming* are introduced.
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The cooling state indicates that the thread will be put to sleep and may
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temporarily wake another thread in the same CPU. Similarly, the warming
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state indicates that a thread is waking up, but it may be another thread
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in its own CPU for a while.
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These states allow the emulator to ignore the threads in the cooling and
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warming states when only interested in those in the running state, but
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at the same time allow the developers to identify the real transition
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sequence to explain potential delays.
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## CPU model
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The CPU can only execute one thread at a time, and it may have multiple
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threads assigned. When two or more threads are in the execution state,
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the emulator cannot determine which one is really being executed, so it
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will set all the channels to an error state.
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!!! Caution
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Multiple threads running in the same CPU typically indicates an
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error of instrumentation or a problem in the runtime.
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The emulator automatically switches the channels from one thread to
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another when a thread is switched from the CPU. So the different models
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don't need to worry about thread transitions. See the
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[channels](../../dev/channels.md) section for more information.
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