5. Service Cores

DPDK has a concept known as service cores, which enables a dynamic way of performing work on DPDK lcores. Service core support is built into the EAL, and an API is provided to optionally allow applications to control how the service cores are used at runtime.

The service cores concept is built up out of services (components of DPDK that require CPU cycles to operate) and service cores (DPDK lcores, tasked with running services). The power of the service core concept is that the mapping between service cores and services can be configured to abstract away the difference between platforms and environments.

For example, the Eventdev has hardware and software PMDs. Of these the software PMD requires an lcore to perform the scheduling operations, while the hardware PMD does not. With service cores, the application would not directly notice that the scheduling is done in software.

For detailed information about the service core API, please refer to the docs.

5.1. Service Core Initialization

There are two methods to having service cores in a DPDK application, either by using the service coremask, or by dynamically adding cores using the API. The simpler of the two is to pass the -s coremask argument to EAL, which will take any cores available in the main DPDK coremask, and if the bits are also set in the service coremask the cores become service-cores instead of DPDK application lcores.

5.2. Enabling Services on Cores

Each registered service can be individually mapped to a service core, or set of service cores. Enabling a service on a particular core means that the lcore in question will run the service. Disabling that core on the service stops the lcore in question from running the service.

Using this method, it is possible to assign specific workloads to each service core, and map N workloads to M number of service cores. Each service lcore loops over the services that are enabled for that core, and invokes the function to run the service.

5.3. Service Core Statistics

The service core library is capable of collecting runtime statistics like number of calls to a specific service, and number of cycles used by the service. The cycle count collection is dynamically configurable, allowing any application to profile the services running on the system at any time.

5.4. Service Core Tracing

The service core library is instrumented with tracepoints using the DPDK Trace Library. These tracepoints allow you to track the service and logical cores state. To activate tracing when launching a DPDK program it is necessary to use the --trace option to specify a regular expression to select which tracepoints to enable. Here is an example if you want to only specify service core tracing:

./dpdk/examples/service_cores/build/service_cores --trace="lib.eal.thread*" --trace="lib.eal.service*"

See the Trace Library documentation for details.