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diff --git a/docs/perf/psci-performance-methodology.rst b/docs/perf/psci-performance-methodology.rst new file mode 100644 index 0000000..a9f379d --- /dev/null +++ b/docs/perf/psci-performance-methodology.rst @@ -0,0 +1,55 @@ +Runtime Instrumentation Methodology +=================================== + +This document outlines steps for undertaking performance measurements of key +operations in the Trusted Firmware-A Power State Coordination Interface (PSCI) +implementation, using the in-built Performance Measurement Framework (PMF) and +runtime instrumentation timestamps. + +Framework +~~~~~~~~~ + +The tests are based on the ``runtime-instrumentation`` test suite provided by +the Trusted Firmware Test Framework (TFTF). The release build of this framework +was used because the results in the debug build became skewed; the console +output prevented some of the tests from executing in parallel. + +The tests consist of both parallel and sequential tests, which are broadly +described as follows: + +- **Parallel Tests** This type of test powers on all the non-lead CPUs and + brings them and the lead CPU to a common synchronization point. The lead CPU + then initiates the test on all CPUs in parallel. + +- **Sequential Tests** This type of test powers on each non-lead CPU in + sequence. The lead CPU initiates the test on a non-lead CPU then waits for the + test to complete before proceeding to the next non-lead CPU. The lead CPU then + executes the test on itself. + +Note there is very little variance observed in the values given (~1us), although +the values for each CPU are sometimes interchanged, depending on the order in +which locks are acquired. Also, there is very little variance observed between +executing the tests sequentially in a single boot or rebooting between tests. + +Given that runtime instrumentation using PMF is invasive, there is a small +(unquantified) overhead on the results. PMF uses the generic counter for +timestamps, which runs at 50MHz on Juno. + +Metrics +~~~~~~~ + +.. glossary:: + + Powerdown Latency + Time taken from entering the TF PSCI implementation to the point the hardware + enters the low power state (WFI). Referring to the TF runtime instrumentation points, this + corresponds to: ``(RT_INSTR_ENTER_HW_LOW_PWR - RT_INSTR_ENTER_PSCI)``. + + Wakeup Latency + Time taken from the point the hardware exits the low power state to exiting + the TF PSCI implementation. This corresponds to: ``(RT_INSTR_EXIT_PSCI - + RT_INSTR_EXIT_HW_LOW_PWR)``. + + Cache Flush Latency + Time taken to flush the caches during powerdown. This corresponds to: + ``(RT_INSTR_EXIT_CFLUSH - RT_INSTR_ENTER_CFLUSH)``. |