From 81496460b4d79005f8bfeb884e39b9d44b2638ff Mon Sep 17 00:00:00 2001 From: Harry van Haaren Date: Tue, 8 Mar 2016 17:16:35 +0000 Subject: [PATCH] doc: add statistics read frequency to fm10k guide This patch documents that the statistics of fm10k based NICs must be read regularly in order to avoid an undetected 32 bit integer-overflow. Signed-off-by: Harry van Haaren Acked-by: John McNamara --- doc/guides/nics/fm10k.rst | 16 ++++++++++++++++ 1 file changed, 16 insertions(+) diff --git a/doc/guides/nics/fm10k.rst b/doc/guides/nics/fm10k.rst index 4206b7f91c..63b495d254 100644 --- a/doc/guides/nics/fm10k.rst +++ b/doc/guides/nics/fm10k.rst @@ -65,3 +65,19 @@ The FM10000 family of NICS support a maximum of a 15K jumbo frame. The value is fixed and cannot be changed. So, even when the ``rxmode.max_rx_pkt_len`` member of ``struct rte_eth_conf`` is set to a value lower than 15364, frames up to 15364 bytes can still reach the host interface. + +Statistic Polling Frequency +~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +The FM10000 NICs expose a set of statistics via the PCI BARs. These statistics +are read from the hardware registers when ``rte_eth_stats_get()`` or +``rte_eth_xstats_get()`` is called. The packet counting registers are 32 bits +while the byte counting registers are 48 bits. As a result, the statistics must +be polled regularly in order to ensure the consistency of the returned reads. + +Given the PCIe Gen3 x8, about 50Gbps of traffic can occur. With 64 byte packets +this gives almost 100 million packets/second, causing 32 bit integer overflow +after approx 40 seconds. To ensure these overflows are detected and accounted +for in the statistics, it is necessary to read statistic regularly. It is +suggested to read stats every 20 seconds, which will ensure the statistics +are accurate. -- 2.20.1