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doc: add statistics read frequency to ixgbe guide

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This patch adds a note to the ixgbe PMD guide, stating
the minimum time that statistics must be polled from
the hardware in order to avoid register values becoming
saturated and "sticking" to the max value.

Reported-by: Jerry Zhang <jerry.zhang@intel.com>
Tested-by: Marcin Kerlin <marcinx.kerlin@intel.com>
Signed-off-by: Harry van Haaren <harry.van.haaren@intel.com>
Acked-by: Marcin Kerlin <marcinx.kerlin@intel.com>
This commit is contained in:
Harry van Haaren 2016-03-08 14:29:39 +00:00 committed by Thomas Monjalon
parent 851b2c86bb
commit 125f6edbe8
1 changed files with 25 additions and 0 deletions
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doc/guides/nics/ixgbe.rst
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@ -199,3 +199,28 @@ When using kernel PF + DPDK VF on x550, please make sure using the kernel
driver that disables MDD or can disable MDD. (Some kernel driver can use
this CLI 'insmod ixgbe.ko MDD=0,0' to disable MDD. Some kernel driver disables
it by default.)
Statistics
----------
The statistics of ixgbe hardware must be polled regularly in order for it to
remain consistent. Running a DPDK application without polling the statistics will
cause registers on hardware to count to the maximum value, and "stick" at
that value.
In order to avoid statistic registers every reaching the maximum value,
read the statistics from the hardware using ``rte_eth_stats_get()`` or
``rte_eth_xstats_get()``.
The maximum time between statistics polls that ensures consistent results can
be calculated as follows:
.. code-block:: c
max_read_interval = UINT_MAX / max_packets_per_second
max_read_interval = 4294967295 / 14880952
max_read_interval = 288.6218096127183 (seconds)
max_read_interval = ~4 mins 48 sec.
In order to ensure valid results, it is recommended to poll every 4 minutes.