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diff --git a/docs/developer/design/ndrpdr.rst b/docs/developer/design/ndrpdr.rst new file mode 100644 index 0000000..dd769c0 --- /dev/null +++ b/docs/developer/design/ndrpdr.rst @@ -0,0 +1,83 @@ +.. This work is licensed under a Creative Commons Attribution 4.0 International +.. License. +.. http://creativecommons.org/licenses/by/4.0 +.. (c) Cisco Systems, Inc + +NDR/PDR Binary Search +===================== + +The NDR/PDR binary search algorithm used by NFVbench is based on the algorithm used by the +FD.io CSIT project, with some additional optimizations. + +Algorithm Outline +----------------- + +The ServiceChain class (nfvbench/service_chain.py) is responsible for calculating the NDR/PDR +or all frame sizes requested in the configuration. +Calculation for 1 frame size is delegated to the TrafficClient class (nfvbench/traffic_client.py) + +Call chain for calculating the NDR-PDR for a list of frame sizes: + +- ServiceChain.run() + - ServiceChain._get_chain_results() + - for every frame size: + - ServiceChain.__get_result_per_frame_size() + - TrafficClient.get_ndr_pdr() + - TrafficClient.__range_search() recursive binary search + +The search range is delimited by a left and right rate (expressed as a % of line rate per direction). +The search always start at line rate per port, e.g. in the case of 2x10Gbps, the first iteration +will send 10Gbps of traffic on each port. + +The load_epsilon configuration parameter defines the accuracy of the result as a % of line rate. +The default value of 0.1 indicates for example that the measured NDR and PDR are within 0.1% of line rate of the +actual NDR/PDR (e.g. 0.1% of 10Gbps is 10Mbps). It also determines how small the search range must be in the binary search. +Smaller values of load_epsilon will result in more iterations and will take more time but may not +always be beneficial if the absolute value falls below the precision level of the measurement. +For example a value of 0.01% would translate to an absolute value of 1Mbps (for a 10Gbps port) or +around 10kpps (at 64 byte size) which might be too fine grain. + +The recursion narrows down the range by half and stops when: + +- the range is smaller than the configured load_epsilon value +- or when the search hits 100% or 0% of line rate + +Optimization +------------ + +Binary search algorithms assume that the drop rate curve is monotonically increasing with the Tx rate. +To save time, the algorithm used by NFVbench is capable of calculating the optimal Tx rate for an +arbitrary list of target maximum drop rates in one pass instead of the usual 1 pass per target maximum drop rate. +This saves time linearly to the number target drop rates. +For example, a typical NDR/PDR search will have 2 target maximum drop rates: + +- NDR = 0.001% +- PDR = 0.1% + +The binary search will then start with a sorted list of 2 target drop rates: [0.1, 0.001]. +The first part of the binary search will then focus on finding the optimal rate for the first target +drop rate (0.1%). When found, the current target drop rate is removed from the list and +iteration continues with the next target drop rate in the list but this time +starting from the upper/lower range of the previous target drop rate, which saves significant time. +The binary search continues until the target maximum drop rate list is empty. + +Results Granularity +------------------- +The binary search results contain per direction stats (forward and reverse). +In the case of multi-chaining, results contain per chain stats. +The current code only reports aggregated stats (forward + reverse for all chains) but could be enhanced +to report per chain stats. + + +CPU Limitations +--------------- +One particularity of using a software traffic generator is that the requested Tx rate may not always be met due to +resource limitations (e.g. CPU is not fast enough to generate a very high load). The algorithm should take this into +consideration: + +- always monitor the actual Tx rate achieved as reported back by the traffic generator +- actual Tx rate is always <= requested Tx rate +- the measured drop rate should always be relative to the actual Tx rate +- if the actual Tx rate is < requested Tx rate and the measured drop rate is already within threshold + (<NDR/PDR threshold) then the binary search must stop with proper warning because the actual NDR/PDR + might probably be higher than the reported values |