50. Profile Your Application

50.1.1. Empty cycles tracing

Iterations that yielded no RX packets (empty cycles, wasted iterations) can be analyzed using VTune Amplifier. This profiling employs the Instrumentation and Tracing Technology (ITT) API feature of VTune Amplifier and requires only reconfiguring the DPDK library, no changes in a DPDK application are needed.

To trace wasted iterations on RX queues, first reconfigure DPDK with CONFIG_RTE_ETHDEV_RXTX_CALLBACKS and CONFIG_RTE_ETHDEV_PROFILE_ITT_WASTED_RX_ITERATIONS enabled.

Then rebuild DPDK, specifying paths to the ITT header and library, which can be found in any VTune Amplifier distribution in the include and lib directories respectively:

make EXTRA_CFLAGS=-I<path to ittnotify.h> \
     EXTRA_LDLIBS="-L<path to libittnotify.a> -littnotify"

Finally, to see wasted iterations in your performance analysis results, select the “Analyze user tasks, events, and counters” checkbox in the “Analysis Type” tab when configuring analysis via VTune Amplifier GUI. Alternatively, when running VTune Amplifier via command line, specify -knob enable-user-tasks=true option.

Collected regions of wasted iterations will be marked on VTune Amplifier’s timeline as ITT tasks. These ITT tasks have predefined names, containing Ethernet device and RX queue identifiers.

50.2.1. Using Linux perf

The ARM64 architecture provide performance counters to monitor events. The Linux perf tool can be used to profile and benchmark an application. In addition to the standard events, perf can be used to profile arm64 specific PMU (Performance Monitor Unit) events through raw events (-e -rXX).

For more derails refer to the ARM64 specific PMU events enumeration.

50.2.2. High-resolution cycle counter

The default cntvct_el0 based rte_rdtsc() provides a portable means to get a wall clock counter in user space. Typically it runs at <= 100MHz.

The alternative method to enable rte_rdtsc() for a high resolution wall clock counter is through the armv8 PMU subsystem. The PMU cycle counter runs at CPU frequency. However, access to the PMU cycle counter from user space is not enabled by default in the arm64 linux kernel. It is possible to enable cycle counter for user space access by configuring the PMU from the privileged mode (kernel space).

By default the rte_rdtsc() implementation uses a portable cntvct_el0 scheme. Application can choose the PMU based implementation with CONFIG_RTE_ARM_EAL_RDTSC_USE_PMU.

The example below shows the steps to configure the PMU based cycle counter on an armv8 machine.

git clone https://github.com/jerinjacobk/armv8_pmu_cycle_counter_el0
cd armv8_pmu_cycle_counter_el0
make
sudo insmod pmu_el0_cycle_counter.ko
cd $DPDK_DIR
make config T=arm64-armv8a-linuxapp-gcc
echo "CONFIG_RTE_ARM_EAL_RDTSC_USE_PMU=y" >> build/.config
make