T3048: add documentation for "system options performance" command

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+.. _system_options_performance:
+
+###########
+Performance
+###########
+
+As more and more routers run on Hypervisors, expecially with a :abbr:`NOS
+(Network Operating System)` as VyOS, it makes fewer and fewer sense to use
+static resource bindings like ``smp-affinity`` as present in VyOS 1.2 and
+earlier to pin certain interrupt handlers to specific CPUs.
+
+We now utilize `tuned` for dynamic resource balancing based on profiles.
+
+  .. seealso:: https://access.redhat.com/sites/default/files/attachments/201501-perf-brief-low-latency-tuning-rhel7-v2.1.pdf
+
+Configureation
+==============
+
+.. cfgcmd:: set system options performance < throughput | latency >
+
+  Configure one of the predefined system performance profiles.
+
+  * ``throughput``: A server profile focused on improving network throughput.
+    This profile favors performance over power savings by setting ``intel_pstate``
+    and ``max_perf_pct=100`` and increasing kernel network buffer sizes.
+
+    It enables transparent huge pages, and uses cpupower to set the performance
+    cpufreq governor. It also sets ``kernel.sched_min_granularity_ns`` to 10 us,
+    ``kernel.sched_wakeup_granularity_ns`` to 15 uss, and ``vm.dirty_ratio`` to
+    40%.
+
+  * ``latency``: A server profile focused on lowering network latency.
+    This profile favors performance over power savings by setting ``intel_pstate``
+    and ``min_perf_pct=100``.
+
+    It disables transparent huge pages, and automatic NUMA balancing. It also
+    uses cpupower to set the performance cpufreq governor, and requests a
+    cpu_dma_latency value of 1. It also sets busy_read and busy_poll times to
+    50 us, and tcp_fastopen to 3.