39. Metrics Library

The Metrics library implements a mechanism by which producers can publish numeric information for later querying by consumers. In practice producers will typically be other libraries or primary processes, whereas consumers will typically be applications.

Metrics themselves are statistics that are not generated by PMDs. Metric information is populated using a push model, where producers update the values contained within the metric library by calling an update function on the relevant metrics. Consumers receive metric information by querying the central metric data, which is held in shared memory.

For each metric, a separate value is maintained for each port id, and when publishing metric values the producers need to specify which port is being updated. In addition there is a special id RTE_METRICS_GLOBAL that is intended for global statistics that are not associated with any individual device. Since the metrics library is self-contained, the only restriction on port numbers is that they are less than RTE_MAX_ETHPORTS - there is no requirement for the ports to actually exist.

39.1. Initialising the library

Before the library can be used, it has to be initialized by calling rte_metrics_init() which sets up the metric store in shared memory. This is where producers will publish metric information to, and where consumers will query it from.

rte_metrics_init(rte_socket_id());

This function must be called from a primary process, but otherwise producers and consumers can be in either primary or secondary processes.

39.2. Registering metrics

Metrics must first be registered, which is the way producers declare the names of the metrics they will be publishing. Registration can either be done individually, or a set of metrics can be registered as a group. Individual registration is done using rte_metrics_reg_name():

id_1 = rte_metrics_reg_name("mean_bits_in");
id_2 = rte_metrics_reg_name("mean_bits_out");
id_3 = rte_metrics_reg_name("peak_bits_in");
id_4 = rte_metrics_reg_name("peak_bits_out");

or alternatively, a set of metrics can be registered together using rte_metrics_reg_names():

const char * const names[] = {
    "mean_bits_in", "mean_bits_out",
    "peak_bits_in", "peak_bits_out",
};
id_set = rte_metrics_reg_names(&names[0], 4);

If the return value is negative, it means registration failed. Otherwise the return value is the key for the metric, which is used when updating values. A table mapping together these key values and the metrics’ names can be obtained using rte_metrics_get_names().

39.3. Updating metric values

Once registered, producers can update the metric for a given port using the rte_metrics_update_value() function. This uses the metric key that is returned when registering the metric, and can also be looked up using rte_metrics_get_names().

rte_metrics_update_value(port_id, id_1, values[0]);
rte_metrics_update_value(port_id, id_2, values[1]);
rte_metrics_update_value(port_id, id_3, values[2]);
rte_metrics_update_value(port_id, id_4, values[3]);

if metrics were registered as a single set, they can either be updated individually using rte_metrics_update_value(), or updated together using the rte_metrics_update_values() function:

rte_metrics_update_value(port_id, id_set, values[0]);
rte_metrics_update_value(port_id, id_set + 1, values[1]);
rte_metrics_update_value(port_id, id_set + 2, values[2]);
rte_metrics_update_value(port_id, id_set + 3, values[3]);

rte_metrics_update_values(port_id, id_set, values, 4);

Note that rte_metrics_update_values() cannot be used to update metric values from multiple sets, as there is no guarantee two sets registered one after the other have contiguous id values.

39.4. Querying metrics

Consumers can obtain metric values by querying the metrics library using the rte_metrics_get_values() function that return an array of struct rte_metric_value. Each entry within this array contains a metric value and its associated key. A key-name mapping can be obtained using the rte_metrics_get_names() function that returns an array of struct rte_metric_name that is indexed by the key. The following will print out all metrics for a given port:

void print_metrics() {
    struct rte_metric_value *metrics;
    struct rte_metric_name *names;
    int len;

    len = rte_metrics_get_names(NULL, 0);
    if (len < 0) {
        printf("Cannot get metrics count\n");
        return;
    }
    if (len == 0) {
        printf("No metrics to display (none have been registered)\n");
        return;
    }
    metrics = malloc(sizeof(struct rte_metric_value) * len);
    names =  malloc(sizeof(struct rte_metric_name) * len);
    if (metrics == NULL || names == NULL) {
        printf("Cannot allocate memory\n");
        free(metrics);
        free(names);
        return;
    }
    ret = rte_metrics_get_values(port_id, metrics, len);
    if (ret < 0 || ret > len) {
        printf("Cannot get metrics values\n");
        free(metrics);
        free(names);
        return;
    }
    printf("Metrics for port %i:\n", port_id);
    for (i = 0; i < len; i++)
        printf("  %s: %"PRIu64"\n",
            names[metrics[i].key].name, metrics[i].value);
    free(metrics);
    free(names);
}

39.5. Bit-rate statistics library

The bit-rate library calculates the exponentially-weighted moving average and peak bit-rates for each active port (i.e. network device). These statistics are reported via the metrics library using the following names:

  • mean_bits_in: Average inbound bit-rate
  • mean_bits_out: Average outbound bit-rate
  • ewma_bits_in: Average inbound bit-rate (EWMA smoothed)
  • ewma_bits_out: Average outbound bit-rate (EWMA smoothed)
  • peak_bits_in: Peak inbound bit-rate
  • peak_bits_out: Peak outbound bit-rate

Once initialised and clocked at the appropriate frequency, these statistics can be obtained by querying the metrics library.

39.5.1. Initialization

Before the library can be used, it has to be initialised by calling rte_stats_bitrate_create(), which will return a bit-rate calculation object. Since the bit-rate library uses the metrics library to report the calculated statistics, the bit-rate library then needs to register the calculated statistics with the metrics library. This is done using the helper function rte_stats_bitrate_reg().

struct rte_stats_bitrates *bitrate_data;

bitrate_data = rte_stats_bitrate_create();
if (bitrate_data == NULL)
    rte_exit(EXIT_FAILURE, "Could not allocate bit-rate data.\n");
rte_stats_bitrate_reg(bitrate_data);

39.5.2. Controlling the sampling rate

Since the library works by periodic sampling but does not use an internal thread, the application has to periodically call rte_stats_bitrate_calc(). The frequency at which this function is called should be the intended sampling rate required for the calculated statistics. For instance if per-second statistics are desired, this function should be called once a second.

tics_datum = rte_rdtsc();
tics_per_1sec = rte_get_timer_hz();

while( 1 ) {
    /* ... */
    tics_current = rte_rdtsc();
    if (tics_current - tics_datum >= tics_per_1sec) {
        /* Periodic bitrate calculation */
        for (idx_port = 0; idx_port < cnt_ports; idx_port++)
                rte_stats_bitrate_calc(bitrate_data, idx_port);
            tics_datum = tics_current;
        }
    /* ... */
}

39.6. Latency statistics library

The latency statistics library calculates the latency of packet processing by a DPDK application, reporting the minimum, average, and maximum nano-seconds that packet processing takes, as well as the jitter in processing delay. These statistics are then reported via the metrics library using the following names:

  • min_latency_ns: Minimum processing latency (nano-seconds)
  • avg_latency_ns: Average processing latency (nano-seconds)
  • mac_latency_ns: Maximum processing latency (nano-seconds)
  • jitter_ns: Variance in processing latency (nano-seconds)

Once initialised and clocked at the appropriate frequency, these statistics can be obtained by querying the metrics library.

39.6.1. Initialization

Before the library can be used, it has to be initialised by calling rte_latencystats_init().

lcoreid_t latencystats_lcore_id = -1;

int ret = rte_latencystats_init(1, NULL);
if (ret)
    rte_exit(EXIT_FAILURE, "Could not allocate latency data.\n");

39.6.2. Triggering statistic updates

The rte_latencystats_update() function needs to be called periodically so that latency statistics can be updated.

if (latencystats_lcore_id == rte_lcore_id())
    rte_latencystats_update();

39.6.3. Library shutdown

When finished, rte_latencystats_uninit() needs to be called to de-initialise the latency library.

rte_latencystats_uninit();