X-Git-Url: http://git.droids-corp.org/?a=blobdiff_plain;f=doc%2Fguides%2Fprog_guide%2Fmetrics_lib.rst;h=702c29d94b857f4ef40746f147dcec4496cb1b7f;hb=5cd3cac9ed;hp=71c3a1bfe883a3b0a726d6fe61edb813e3bb4fd8;hpb=7e4441c8efb95ba2e14cafe63c86c335b658c18d;p=dpdk.git

diff --git a/doc/guides/prog_guide/metrics_lib.rst b/doc/guides/prog_guide/metrics_lib.rst
index 71c3a1bfe8..702c29d94b 100644
--- a/doc/guides/prog_guide/metrics_lib.rst
+++ b/doc/guides/prog_guide/metrics_lib.rst
@@ -243,3 +243,57 @@ desired, this function should be called once a second.
 	    }
         /* ... */
     }
+
+
+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.
+
+Initialization
+~~~~~~~~~~~~~~
+
+Before the library can be used, it has to be initialised by calling
+``rte_latencystats_init()``.
+
+.. code-block:: c
+
+    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");
+
+
+Triggering statistic updates
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The ``rte_latencystats_update()`` function needs to be called
+periodically so that latency statistics can be updated.
+
+.. code-block:: c
+
+    if (latencystats_lcore_id == rte_lcore_id())
+        rte_latencystats_update();
+
+Library shutdown
+~~~~~~~~~~~~~~~~
+
+When finished, ``rte_latencystats_uninit()`` needs to be called to
+de-initialise the latency library.
+
+.. code-block:: c
+
+    rte_latencystats_uninit();