20. L2 Forwarding Sample Application with Cache Allocation Technology (CAT)

Basic Forwarding sample application is a simple skeleton example of a forwarding application. It has been extended to make use of CAT via extended command line options and linking against the libpqos library.

It is intended as a demonstration of the basic components of a DPDK forwarding application and use of the libpqos library to program CAT. For more detailed implementations see the L2 and L3 forwarding sample applications.

CAT and Code Data Prioritization (CDP) features allow management of the CPU’s last level cache. CAT introduces classes of service (COS) that are essentially bitmasks. In current CAT implementations, a bit in a COS bitmask corresponds to one cache way in last level cache. A CPU core is always assigned to one of the CAT classes. By programming CPU core assignment and COS bitmasks, applications can be given exclusive, shared, or mixed access to the CPU’s last level cache. CDP extends CAT so that there are two bitmasks per COS, one for data and one for code. The number of classes and number of valid bits in a COS bitmask is CPU model specific and COS bitmasks need to be contiguous. Sample code calls this bitmask cbm or capacity bitmask. By default, after reset, all CPU cores are assigned to COS 0 and all classes are programmed to allow fill into all cache ways. CDP is off by default.

For more information about CAT please see:

White paper demonstrating example use case:

20.1. Compiling the Application

Note

Requires libpqos from Intel’s intel-cmt-cat software package hosted on GitHub repository. For installation notes, please see README file.

GIT:

  1. To compile the application export the path to PQoS lib and the DPDK source tree and go to the example directory:

    export PQOS_INSTALL_PATH=/path/to/libpqos

To compile the sample application see Compiling the Sample Applications.

The application is located in the l2fwd-cat sub-directory.

20.2. Running the Application

To run the example in a linux environment and enable CAT on cpus 0-2:

./build/l2fwd-cat -l 1 -n 4 -- --l3ca="0x3@(0-2)"

or to enable CAT and CDP on cpus 1,3:

./build/l2fwd-cat -l 1 -n 4 -- --l3ca="(0x00C00,0x00300)@(1,3)"

If CDP is not supported it will fail with following error message:

PQOS: CDP requested but not supported.
PQOS: Requested CAT configuration is not valid!
PQOS: Shutting down PQoS library...
EAL: Error - exiting with code: 1
  Cause: PQOS: L3CA init failed!

The option to enable CAT is:

  • --l3ca='<common_cbm@cpus>[,<(code_cbm,data_cbm)@cpus>...]':

    where cbm stands for capacity bitmask and must be expressed in hexadecimal form.

    common_cbm is a single mask, for a CDP enabled system, a group of two masks (code_cbm and data_cbm) is used.

    ( and ) are necessary if it’s a group.

    cpus could be a single digit/range or a group and must be expressed in decimal form.

    ( and ) are necessary if it’s a group.

    e.g. --l3ca='0x00F00@(1,3),0x0FF00@(4-6),0xF0000@7'

    • cpus 1 and 3 share its 4 ways with cpus 4, 5 and 6;
    • cpus 4, 5 and 6 share half (4 out of 8 ways) of its L3 with cpus 1 and 3;
    • cpus 4, 5 and 6 have exclusive access to 4 out of 8 ways;
    • cpu 7 has exclusive access to all of its 4 ways;

    e.g. --l3ca='(0x00C00,0x00300)@(1,3)' for CDP enabled system

    • cpus 1 and 3 have access to 2 ways for code and 2 ways for data, code and data ways are not overlapping.

Refer to DPDK Getting Started Guide for general information on running applications and the Environment Abstraction Layer (EAL) options.

To reset or list CAT configuration and control CDP please use pqos tool from Intel’s intel-cmt-cat software package.

To enabled or disable CDP:

sudo ./pqos -S cdp-on

sudo ./pqos -S cdp-off

to reset CAT configuration:

sudo ./pqos -R

to list CAT config:

sudo ./pqos -s

For more info about pqos tool please see its man page or intel-cmt-cat wiki.

20.3. Explanation

The following sections provide an explanation of the main components of the code.

All DPDK library functions used in the sample code are prefixed with rte_ and are explained in detail in the DPDK API Documentation.

20.3.1. The Main Function

The main() function performs the initialization and calls the execution threads for each lcore.

The first task is to initialize the Environment Abstraction Layer (EAL). The argc and argv arguments are provided to the rte_eal_init() function. The value returned is the number of parsed arguments:

int ret = rte_eal_init(argc, argv);
if (ret < 0)
    rte_exit(EXIT_FAILURE, "Error with EAL initialization\n");

The next task is to initialize the PQoS library and configure CAT. The argc and argv arguments are provided to the cat_init() function. The value returned is the number of parsed arguments:

int ret = cat_init(argc, argv);
if (ret < 0)
    rte_exit(EXIT_FAILURE, "PQOS: L3CA init failed!\n");

cat_init() is a wrapper function which parses the command, validates the requested parameters and configures CAT accordingly.

Parsing of command line arguments is done in parse_args(...). libpqos is then initialized with the pqos_init(...) call. Next, libpqos is queried for system CPU information and L3CA capabilities via pqos_cap_get(...) and pqos_cap_get_type(..., PQOS_CAP_TYPE_L3CA, ...) calls. When all capability and topology information is collected, the requested CAT configuration is validated. A check is then performed (on per socket basis) for a sufficient number of unassociated COS. COS are selected and configured via the pqos_l3ca_set(...) call. Finally, COS are associated to relevant CPUs via pqos_l3ca_assoc_set(...) calls.

atexit(...) is used to register cat_exit(...) to be called on a clean exit. cat_exit(...) performs a simple CAT clean-up, by associating COS 0 to all involved CPUs via pqos_l3ca_assoc_set(...) calls.