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path: root/src/sim/probe/Probe.py
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2014-01-24base: add support for probe points and common probesMatt Horsnell
The probe patch is motivated by the desire to move analytical and trace code away from functional code. This is achieved by the probe interface which is essentially a glorified observer model. What this means to users: * add a probe point and a "notify" call at the source of an "event" * add an isolated module, that is being used to carry out *your* analysis (e.g. generate a trace) * register that module as a probe listener Note: an example is given for reference in src/cpu/o3/simple_trace.[hh|cc] and src/cpu/SimpleTrace.py What is happening under the hood: * every SimObject maintains has a ProbeManager. * during initialization (src/python/m5/simulate.py) first regProbePoints and the regProbeListeners is called on each SimObject. this hooks up the probe point notify calls with the listeners. FAQs: Why did you develop probe points: * to remove trace, stats gathering, analytical code out of the functional code. * the belief that probes could be generically useful. What is a probe point: * a probe point is used to notify upon a given event (e.g. cpu commits an instruction) What is a probe listener: * a class that handles whatever the user wishes to do when they are notified about an event. What can be passed on notify: * probe points are templates, and so the user can generate probes that pass any type of argument (by const reference) to a listener. What relationships can be generated (1:1, 1:N, N:M etc): * there isn't a restriction. You can hook probe points and listeners up in a 1:1, 1:N, N:M relationship. They become useful when a number of modules listen to the same probe points. The idea being that you can add a small number of probes into the source code and develop a larger number of useful analysis modules that use information passed by the probes. Can you give examples: * adding a probe point to the cpu's commit method allows you to build a trace module (outputting assembler), you could re-use this to gather instruction distribution (arithmetic, load/store, conditional, control flow) stats. Why is the probe interface currently restricted to passing a const reference: * the desire, initially at least, is to allow an interface to observe functionality, but not to change functionality. * of course this can be subverted by const-casting. What is the performance impact of adding probes: * when nothing is actively listening to the probes they should have a relatively minor impact. Profiling has suggested even with a large number of probes (60) the impact of them (when not active) is very minimal (<1%).