| PSCI Performance Measurements on Arm Juno Development Platform |
| ============================================================== |
| |
| This document summarises the findings of 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. |
| |
| Method |
| ------ |
| |
| We used the `Juno R1 platform`_ for these tests, which has 4 x Cortex-A53 and 2 |
| x Cortex-A57 clusters running at the following frequencies: |
| |
| +-----------------+--------------------+ |
| | Domain | Frequency (MHz) | |
| +=================+====================+ |
| | Cortex-A57 | 900 (nominal) | |
| +-----------------+--------------------+ |
| | Cortex-A53 | 650 (underdrive) | |
| +-----------------+--------------------+ |
| | AXI subsystem | 533 | |
| +-----------------+--------------------+ |
| |
| Juno supports CPU, cluster and system power down states, corresponding to power |
| levels 0, 1 and 2 respectively. It does not support any retention states. |
| |
| We used the upstream `TF master as of 31/01/2017`_, building the platform using |
| the ``ENABLE_RUNTIME_INSTRUMENTATION`` option: |
| |
| .. code:: shell |
| |
| make PLAT=juno ENABLE_RUNTIME_INSTRUMENTATION=1 \ |
| SCP_BL2=<path/to/scp-fw.bin> \ |
| BL33=<path/to/test-fw.bin> \ |
| all fip |
| |
| When using the debug build of TF, there was no noticeable difference in the |
| results. |
| |
| The tests are based on an ARM-internal test framework. 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. |
| |
| In the results below, CPUs 0-3 refer to CPUs in the little cluster (A53) and |
| CPUs 4-5 refer to CPUs in the big cluster (A57). In all cases CPU 4 is the lead |
| CPU. |
| |
| ``PSCI_ENTRY`` refers to the 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)``. |
| |
| ``PSCI_EXIT`` refers to the 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)``. |
| |
| ``CFLUSH_OVERHEAD`` refers to the part of ``PSCI_ENTRY`` taken to flush the |
| caches. This corresponds to: ``(RT_INSTR_EXIT_CFLUSH - RT_INSTR_ENTER_CFLUSH)``. |
| |
| 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. |
| |
| Results and Commentary |
| ---------------------- |
| |
| ``CPU_SUSPEND`` to deepest power level on all CPUs in parallel |
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
| |
| +-------+---------------------+--------------------+--------------------------+ |
| | CPU | ``PSCI_ENTRY`` (us) | ``PSCI_EXIT`` (us) | ``CFLUSH_OVERHEAD`` (us) | |
| +=======+=====================+====================+==========================+ |
| | 0 | 27 | 20 | 5 | |
| +-------+---------------------+--------------------+--------------------------+ |
| | 1 | 114 | 86 | 5 | |
| +-------+---------------------+--------------------+--------------------------+ |
| | 2 | 202 | 58 | 5 | |
| +-------+---------------------+--------------------+--------------------------+ |
| | 3 | 375 | 29 | 94 | |
| +-------+---------------------+--------------------+--------------------------+ |
| | 4 | 20 | 22 | 6 | |
| +-------+---------------------+--------------------+--------------------------+ |
| | 5 | 290 | 18 | 206 | |
| +-------+---------------------+--------------------+--------------------------+ |
| |
| A large variance in ``PSCI_ENTRY`` and ``PSCI_EXIT`` times across CPUs is |
| observed due to TF PSCI lock contention. In the worst case, CPU 3 has to wait |
| for the 3 other CPUs in the cluster (0-2) to complete ``PSCI_ENTRY`` and release |
| the lock before proceeding. |
| |
| The ``CFLUSH_OVERHEAD`` times for CPUs 3 and 5 are higher because they are the |
| last CPUs in their respective clusters to power down, therefore both the L1 and |
| L2 caches are flushed. |
| |
| The ``CFLUSH_OVERHEAD`` time for CPU 5 is a lot larger than that for CPU 3 |
| because the L2 cache size for the big cluster is lot larger (2MB) compared to |
| the little cluster (1MB). |
| |
| ``CPU_SUSPEND`` to power level 0 on all CPUs in parallel |
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
| |
| +-------+---------------------+--------------------+--------------------------+ |
| | CPU | ``PSCI_ENTRY`` (us) | ``PSCI_EXIT`` (us) | ``CFLUSH_OVERHEAD`` (us) | |
| +=======+=====================+====================+==========================+ |
| | 0 | 116 | 14 | 8 | |
| +-------+---------------------+--------------------+--------------------------+ |
| | 1 | 204 | 14 | 8 | |
| +-------+---------------------+--------------------+--------------------------+ |
| | 2 | 287 | 13 | 8 | |
| +-------+---------------------+--------------------+--------------------------+ |
| | 3 | 376 | 13 | 9 | |
| +-------+---------------------+--------------------+--------------------------+ |
| | 4 | 29 | 15 | 7 | |
| +-------+---------------------+--------------------+--------------------------+ |
| | 5 | 21 | 15 | 8 | |
| +-------+---------------------+--------------------+--------------------------+ |
| |
| There is no lock contention in TF generic code at power level 0 but the large |
| variance in ``PSCI_ENTRY`` times across CPUs is due to lock contention in Juno |
| platform code. The platform lock is used to mediate access to a single SCP |
| communication channel. This is compounded by the SCP firmware waiting for each |
| AP CPU to enter WFI before making the channel available to other CPUs, which |
| effectively serializes the SCP power down commands from all CPUs. |
| |
| On platforms with a more efficient CPU power down mechanism, it should be |
| possible to make the ``PSCI_ENTRY`` times smaller and consistent. |
| |
| The ``PSCI_EXIT`` times are consistent across all CPUs because TF does not |
| require locks at power level 0. |
| |
| The ``CFLUSH_OVERHEAD`` times for all CPUs are small and consistent since only |
| the cache associated with power level 0 is flushed (L1). |
| |
| ``CPU_SUSPEND`` to deepest power level on all CPUs in sequence |
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
| |
| +-------+---------------------+--------------------+--------------------------+ |
| | CPU | ``PSCI_ENTRY`` (us) | ``PSCI_EXIT`` (us) | ``CFLUSH_OVERHEAD`` (us) | |
| +=======+=====================+====================+==========================+ |
| | 0 | 114 | 20 | 94 | |
| +-------+---------------------+--------------------+--------------------------+ |
| | 1 | 114 | 20 | 94 | |
| +-------+---------------------+--------------------+--------------------------+ |
| | 2 | 114 | 20 | 94 | |
| +-------+---------------------+--------------------+--------------------------+ |
| | 3 | 114 | 20 | 94 | |
| +-------+---------------------+--------------------+--------------------------+ |
| | 4 | 195 | 22 | 180 | |
| +-------+---------------------+--------------------+--------------------------+ |
| | 5 | 21 | 17 | 6 | |
| +-------+---------------------+--------------------+--------------------------+ |
| |
| The ``CFLUSH_OVERHEAD`` times for lead CPU 4 and all CPUs in the non-lead cluster |
| are large because all other CPUs in the cluster are powered down during the |
| test. The ``CPU_SUSPEND`` call powers down to the cluster level, requiring a |
| flush of both L1 and L2 caches. |
| |
| The ``CFLUSH_OVERHEAD`` time for CPU 4 is a lot larger than those for the little |
| CPUs because the L2 cache size for the big cluster is lot larger (2MB) compared |
| to the little cluster (1MB). |
| |
| The ``PSCI_ENTRY`` and ``CFLUSH_OVERHEAD`` times for CPU 5 are low because lead |
| CPU 4 continues to run while CPU 5 is suspended. Hence CPU 5 only powers down to |
| level 0, which only requires L1 cache flush. |
| |
| ``CPU_SUSPEND`` to power level 0 on all CPUs in sequence |
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
| |
| +-------+---------------------+--------------------+--------------------------+ |
| | CPU | ``PSCI_ENTRY`` (us) | ``PSCI_EXIT`` (us) | ``CFLUSH_OVERHEAD`` (us) | |
| +=======+=====================+====================+==========================+ |
| | 0 | 22 | 14 | 5 | |
| +-------+---------------------+--------------------+--------------------------+ |
| | 1 | 22 | 14 | 5 | |
| +-------+---------------------+--------------------+--------------------------+ |
| | 2 | 21 | 14 | 5 | |
| +-------+---------------------+--------------------+--------------------------+ |
| | 3 | 22 | 14 | 5 | |
| +-------+---------------------+--------------------+--------------------------+ |
| | 4 | 17 | 14 | 6 | |
| +-------+---------------------+--------------------+--------------------------+ |
| | 5 | 18 | 15 | 6 | |
| +-------+---------------------+--------------------+--------------------------+ |
| |
| Here the times are small and consistent since there is no contention and it is |
| only necessary to flush the cache to power level 0 (L1). This is the best case |
| scenario. |
| |
| The ``PSCI_ENTRY`` times for CPUs in the big cluster are slightly smaller than |
| for the CPUs in little cluster due to greater CPU performance. |
| |
| The ``PSCI_EXIT`` times are generally lower than in the last test because the |
| cluster remains powered on throughout the test and there is less code to execute |
| on power on (for example, no need to enter CCI coherency) |
| |
| ``CPU_OFF`` on all non-lead CPUs in sequence then ``CPU_SUSPEND`` on lead CPU to deepest power level |
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
| |
| The test sequence here is as follows: |
| |
| 1. Call ``CPU_ON`` and ``CPU_OFF`` on each non-lead CPU in sequence. |
| |
| 2. Program wake up timer and suspend the lead CPU to the deepest power level. |
| |
| 3. Call ``CPU_ON`` on non-lead CPU to get the timestamps from each CPU. |
| |
| +-------+---------------------+--------------------+--------------------------+ |
| | CPU | ``PSCI_ENTRY`` (us) | ``PSCI_EXIT`` (us) | ``CFLUSH_OVERHEAD`` (us) | |
| +=======+=====================+====================+==========================+ |
| | 0 | 110 | 28 | 93 | |
| +-------+---------------------+--------------------+--------------------------+ |
| | 1 | 110 | 28 | 93 | |
| +-------+---------------------+--------------------+--------------------------+ |
| | 2 | 110 | 28 | 93 | |
| +-------+---------------------+--------------------+--------------------------+ |
| | 3 | 111 | 28 | 93 | |
| +-------+---------------------+--------------------+--------------------------+ |
| | 4 | 195 | 22 | 181 | |
| +-------+---------------------+--------------------+--------------------------+ |
| | 5 | 20 | 23 | 6 | |
| +-------+---------------------+--------------------+--------------------------+ |
| |
| The ``CFLUSH_OVERHEAD`` times for all little CPUs are large because all other |
| CPUs in that cluster are powerered down during the test. The ``CPU_OFF`` call |
| powers down to the cluster level, requiring a flush of both L1 and L2 caches. |
| |
| The ``PSCI_ENTRY`` and ``CFLUSH_OVERHEAD`` times for CPU 5 are small because |
| lead CPU 4 is running and CPU 5 only powers down to level 0, which only requires |
| an L1 cache flush. |
| |
| The ``CFLUSH_OVERHEAD`` time for CPU 4 is a lot larger than those for the little |
| CPUs because the L2 cache size for the big cluster is lot larger (2MB) compared |
| to the little cluster (1MB). |
| |
| The ``PSCI_EXIT`` times for CPUs in the big cluster are slightly smaller than |
| for CPUs in the little cluster due to greater CPU performance. These times |
| generally are greater than the ``PSCI_EXIT`` times in the ``CPU_SUSPEND`` tests |
| because there is more code to execute in the "on finisher" compared to the |
| "suspend finisher" (for example, GIC redistributor register programming). |
| |
| ``PSCI_VERSION`` on all CPUs in parallel |
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
| |
| Since very little code is associated with ``PSCI_VERSION``, this test |
| approximates the round trip latency for handling a fast SMC at EL3 in TF. |
| |
| +-------+-------------------+ |
| | CPU | TOTAL TIME (ns) | |
| +=======+===================+ |
| | 0 | 3020 | |
| +-------+-------------------+ |
| | 1 | 2940 | |
| +-------+-------------------+ |
| | 2 | 2980 | |
| +-------+-------------------+ |
| | 3 | 3060 | |
| +-------+-------------------+ |
| | 4 | 520 | |
| +-------+-------------------+ |
| | 5 | 720 | |
| +-------+-------------------+ |
| |
| The times for the big CPUs are less than the little CPUs due to greater CPU |
| performance. |
| |
| We suspect the time for lead CPU 4 is shorter than CPU 5 due to subtle cache |
| effects, given that these measurements are at the nano-second level. |
| |
| -------------- |
| |
| *Copyright (c) 2019-2023, Arm Limited and Contributors. All rights reserved.* |
| |
| .. _Juno R1 platform: https://developer.arm.com/documentation/100122/latest/ |
| .. _TF master as of 31/01/2017: https://git.trustedfirmware.org/TF-A/trusted-firmware-a.git/tree/?id=c38b36d |