55. Profile Your Application

The following sections describe methods of profiling DPDK applications on different architectures.

55.1. Profiling on x86

Intel processors provide performance counters to monitor events. Some tools provided by Intel, such as Intel® VTune™ Amplifier, can be used to profile and benchmark an application. See the VTune Performance Analyzer Essentials publication from Intel Press for more information.

For a DPDK application, this can be done in a Linux* application environment only.

The main situations that should be monitored through event counters are:

  • Cache misses
  • Branch mis-predicts
  • DTLB misses
  • Long latency instructions and exceptions

Refer to the Intel Performance Analysis Guide for details about application profiling.

55.1.1. 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.

55.2. Profiling on ARM64

55.2.1. 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.

55.2.2. 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

Warning

The PMU based scheme is useful for high accuracy performance profiling with rte_rdtsc(). However, this method can not be used in conjunction with Linux userspace profiling tools like perf as this scheme alters the PMU registers state.