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Diffstat (limited to 'Documentation')
| -rw-r--r-- | Documentation/topics/dpdk/pmd.rst | 54 |
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diff --git a/Documentation/topics/dpdk/pmd.rst b/Documentation/topics/dpdk/pmd.rst index 9006fd40f..604ac3f6b 100644 --- a/Documentation/topics/dpdk/pmd.rst +++ b/Documentation/topics/dpdk/pmd.rst @@ -324,5 +324,59 @@ A user can use this option to set a minimum frequency of Rx queue to PMD reassignment due to PMD Auto Load Balance. For example, this could be set (in min) such that a reassignment is triggered at most every few hours. +PMD load based sleeping (Experimental) +-------------------------------------- + +PMD threads constantly poll Rx queues which are assigned to them. In order to +reduce the CPU cycles they use, they can sleep for small periods of time +when there is no load or very-low load on all the Rx queues they poll. + +This can be enabled by setting the max requested sleep time (in microseconds) +for a PMD thread:: + + $ ovs-vsctl set open_vswitch . other_config:pmd-maxsleep=500 + +Non-zero values will be rounded up to the nearest 10 microseconds to avoid +requesting very small sleep times. + +With a non-zero max value a PMD may request to sleep by an incrementing amount +of time up to the maximum time. If at any point the threshold of at least half +a batch of packets (i.e. 16) is received from an Rx queue that the PMD is +polling is met, the requested sleep time will be reset to 0. At that point no +sleeps will occur until the no/low load conditions return. + +Sleeping in a PMD thread will mean there is a period of time when the PMD +thread will not process packets. Sleep times requested are not guaranteed +and can differ significantly depending on system configuration. The actual +time not processing packets will be determined by the sleep and processor +wake-up times and should be tested with each system configuration. + +Sleep time statistics for 10 secs can be seen with:: + + $ ovs-appctl dpif-netdev/pmd-stats-clear \ + && sleep 10 && ovs-appctl dpif-netdev/pmd-perf-show + +Example output, showing that during the last 10 seconds, 76.8% of iterations +had a sleep of some length. The total amount of sleep time was 9.15 seconds and +the average sleep time per iteration was 46 microseconds:: + + - sleep iterations: 153994 ( 76.8 % of iterations) + Sleep time (us): 9159399 ( 59 us/iteration avg.) + +Any potential power saving from PMD load based sleeping is dependent on the +system configuration (e.g. enabling processor C-states) and workloads. + +.. note:: + + If there is a sudden spike of packets while the PMD thread is sleeping and + the processor is in a low-power state it may result in some lost packets or + extra latency before the PMD thread returns to processing packets at full + rate. + +.. note:: + + By default Linux kernel groups timer expirations and this can add an + overhead of up to 50 microseconds to a requested timer expiration. + .. _ovs-vswitchd(8): http://openvswitch.org/support/dist-docs/ovs-vswitchd.8.html |