2 title: Multiple Loss Ratio Search for Packet Throughput (MLRsearch)
4 docname: draft-vpolak-mkonstan-bmwg-mlrsearch-01
9 wg: Benchmarking Working Group
14 pi: # can use array (if all yes) or hash here
19 sortrefs: # defaults to yes
24 ins: M. Konstantynowicz
25 name: Maciek Konstantynowicz
28 email: mkonstan@cisco.com
34 email: vrpolak@cisco.com
45 This document proposes changes to [RFC2544], specifically to packet
46 throughput search methodology, by defining a new search algorithm
47 referred to as Multiple Loss Ratio search (MLRsearch for short). Instead
48 of relying on binary search with pre-set starting offered load, it
49 proposes a novel approach discovering the starting point in the initial
50 phase, and then searching for packet throughput based on defined packet
51 loss ratio (PLR) input criteria and defined final trial duration time.
52 One of the key design principles behind MLRsearch is minimizing the
53 total test duration and searching for multiple packet throughput rates
54 (each with a corresponding PLR) concurrently, instead of doing it
57 The main motivation behind MLRsearch is the new set of challenges and
58 requirements posed by NFV (Network Function Virtualization),
59 specifically software based implementations of NFV data planes. Using
60 [RFC2544] in the experience of the authors yields often not repetitive
61 and not replicable end results due to a large number of factors that are
62 out of scope for this draft. MLRsearch aims to address this challenge and
63 define a common (standard?) way to evaluate NFV packet throughput
64 performance that takes into account varying characteristics of NFV
71 * NDR - Non-Drop Rate, a packet throughput metric with Packet Loss Ratio
72 equal zero (a zero packet loss), expressed in packets-per-second
73 (pps). NDR packet throughput has an associated metric oftentimes
74 referred to as NDR bandwidth expressed in bits-per-second (bps), and
75 calculated as a product of:
76 * NDR packet rate for specific packet (frame) size, and
77 * Packet (L2 frame size) size in bits plus any associated L1 overhead.
78 * PLR - Packet Loss Ratio, a packet loss metric calculated as a ratio of
79 (packets_transmitted - packets_received) to packets_transmitted, over
80 the test trial duration.
81 * PDR - Partial-Drop Rate, a packet throughput metric with Packet Loss
82 Ratio greater than zero (a non-zero packet loss), expressed in
83 packets-per-second (pps). PDR packet throughput has an associated
84 metric oftentimes referred to as PDR bandwidth expressed in bits-per-
85 second (bps), and calculated as a product of:
86 * PDR packet rate for specific packet (frame) size, and
87 * Packet (L2 frame size) size in bits plus any associated L1 overhead.
89 # MLRsearch Background
91 Multiple Loss Rate search (MLRsearch) is a packet throughput search
92 algorithm suitable for deterministic (as opposed to probabilistic)
93 systems. MLRsearch discovers multiple packet throughput rates in a
94 single search, each rate associated with a distinct Packet Loss Ratio
97 Two popular names for particular PLR criteria are Non-Drop Rate (NDR,
98 with PLR=0, zero packet loss) and Partial Drop Rate (PDR, with PLR>0,
99 non-zero packet loss). MLRsearch discovers NDR and PDR in a single
100 search reducing required execution time compared to separate binary
101 searches for NDR and PDR. MLRsearch reduces execution time even further
102 by relying on shorter trial durations of intermediate steps, with only
103 the final measurements conducted at the specified final trial duration.
104 This results in the shorter overall search execution time when compared
105 to a standard NDR/PDR binary search, while guaranteeing the same or
107 (TODO: Specify "standard" in the previous sentence.)
109 If needed, MLRsearch can be easily adopted to discover more throughput
110 rates with different pre-defined PLRs.
112 Unless otherwise noted, all throughput rates are *always* bi-directional
113 aggregates of two equal (symmetric) uni-directional packet rates
114 received and reported by an external traffic generator.
118 The main properties of MLRsearch:
120 * MLRsearch is a duration aware multi-phase multi-rate search algorithm.
121 * Initial phase determines promising starting interval for the search.
122 * Intermediate phases progress towards defined final search criteria.
123 * Final phase executes measurements according to the final search
126 * Uses link rate as a starting transmit rate and discovers the Maximum
127 Receive Rate (MRR) used as an input to the first intermediate phase.
128 * Intermediate phases:
129 * Start with initial trial duration (in the first phase) and converge
130 geometrically towards the final trial duration (in the final phase).
131 * Track two values for NDR and two for PDR.
132 * The values are called (NDR or PDR) lower_bound and upper_bound.
133 * Each value comes from a specific trial measurement
134 (most recent for that transmit rate),
135 and as such the value is associated with that measurement's duration and loss.
136 * A bound can be invalid, for example if NDR lower_bound
137 has been measured with nonzero loss.
138 * Invalid bounds are not real boundaries for the searched value,
139 but are needed to track interval widths.
140 * Valid bounds are real boundaries for the searched value.
141 * Each non-initial phase ends with all bounds valid.
142 * Start with a large (lower_bound, upper_bound) interval width and
143 geometrically converge towards the width goal (measurement resolution)
144 of the phase. Each phase halves the previous width goal.
145 * Use internal and external searches:
146 * External search - measures at transmit rates outside the (lower_bound,
147 upper_bound) interval. Activated when a bound is invalid,
148 to search for a new valid bound by doubling the interval width.
149 It is a variant of "exponential search".
150 * Internal search - "binary search", measures at transmit rates within the
151 (lower_bound, upper_bound) valid interval, halving the interval width.
153 * Executed with the final test trial duration, and the final width
154 goal that determines resolution of the overall search.
155 * Intermediate phases together with the final phase are called
158 The main benefits of MLRsearch vs. binary search include:
160 * In general MLRsearch is likely to execute more search trials overall, but
161 less trials at a set final duration.
162 * In well behaving cases it greatly reduces (>50%) the overall duration
163 compared to a single PDR (or NDR) binary search duration,
164 while finding multiple drop rates.
165 * In all cases MLRsearch yields the same or similar results to binary search.
166 * Note: both binary search and MLRsearch are susceptible to reporting
167 non-repeatable results across multiple runs for very bad behaving
172 * Worst case MLRsearch can take longer than a binary search e.g. in case of
173 drastic changes in behaviour for trials at varying durations.
175 # Sample Implementation
177 Following is a brief description of a sample MLRsearch implementation
178 based on the open-source code running in FD.io CSIT project as part of a
179 Continuous Integration / Continuous Development (CI/CD) framework.
183 1. **maximum_transmit_rate** - maximum packet transmit rate to be used by
184 external traffic generator, limited by either the actual Ethernet
185 link rate or traffic generator NIC model capabilities. Sample
186 defaults: 2 * 14.88 Mpps for 64B 10GE link rate,
187 2 * 18.75 Mpps for 64B 40GE NIC maximum rate.
188 2. **minimum_transmit_rate** - minimum packet transmit rate to be used for
189 measurements. MLRsearch fails if lower transmit rate needs to be
190 used to meet search criteria. Default: 2 * 10 kpps (could be higher).
191 3. **final_trial_duration** - required trial duration for final rate
192 measurements. Default: 30 sec.
193 4. **initial_trial_duration** - trial duration for initial MLRsearch phase.
195 5. **final_relative_width** - required measurement resolution expressed as
196 (lower_bound, upper_bound) interval width relative to upper_bound.
198 6. **packet_loss_ratio** - maximum acceptable PLR search criteria for
199 PDR measurements. Default: 0.5%.
200 7. **number_of_intermediate_phases** - number of phases between the initial
201 phase and the final phase. Impacts the overall MLRsearch duration.
202 Less phases are required for well behaving cases, more phases
203 may be needed to reduce the overall search duration for worse behaving cases.
204 Default (2). (Value chosen based on limited experimentation to date.
205 More experimentation needed to arrive to clearer guidelines.)
209 1. First trial measures at maximum rate and discovers MRR.
210 * *in*: trial_duration = initial_trial_duration.
211 * *in*: offered_transmit_rate = maximum_transmit_rate.
212 * *do*: single trial.
213 * *out*: measured loss ratio.
214 * *out*: mrr = measured receive rate.
215 2. Second trial measures at MRR and discovers MRR2.
216 * *in*: trial_duration = initial_trial_duration.
217 * *in*: offered_transmit_rate = MRR.
218 * *do*: single trial.
219 * *out*: measured loss ratio.
220 * *out*: mrr2 = measured receive rate.
221 3. Third trial measures at MRR2.
222 * *in*: trial_duration = initial_trial_duration.
223 * *in*: offered_transmit_rate = MRR2.
224 * *do*: single trial.
225 * *out*: measured loss ratio.
227 ## Non-initial phases
230 * *in*: trial_duration for the current phase.
231 Set to initial_trial_duration for the first intermediate phase;
232 to final_trial_duration for the final phase;
233 or to the element of interpolating geometric sequence
234 for other intermediate phases.
235 For example with two intermediate phases, trial_duration
236 of the second intermediate phase is the geometric average
237 of initial_strial_duration and final_trial_duration.
238 * *in*: relative_width_goal for the current phase.
239 Set to final_relative_width for the final phase;
240 doubled for each preceding phase.
241 For example with two intermediate phases,
242 the first intermediate phase uses quadruple of final_relative_width
243 and the second intermediate phase uses double of final_relative_width.
244 * *in*: ndr_interval, pdr_interval from the previous main loop iteration
245 or the previous phase.
246 If the previous phase is the initial phase, both intervals have
247 lower_bound = MRR2, uper_bound = MRR.
248 Note that the initial phase is likely to create intervals with invalid bounds.
249 * *do*: According to the procedure described in point 2,
250 either exit the phase (by jumping to 1.g.),
251 or prepare new transmit rate to measure with.
252 * *do*: Perform the trial measurement at the new transmit rate
253 and trial_duration, compute its loss ratio.
254 * *do*: Update the bounds of both intervals, based on the new measurement.
255 The actual update rules are numerous, as NDR external search
256 can affect PDR interval and vice versa, but the result
257 agrees with rules of both internal and external search.
258 For example, any new measurement below an invalid lower_bound
259 becomes the new lower_bound, while the old measurement
260 (previously acting as the invalid lower_bound)
261 becomes a new and valid upper_bound.
262 Go to next iteration (1.c.), taking the updated intervals as new input.
263 * *out*: current ndr_interval and pdr_interval.
264 In the final phase this is also considered
265 to be the result of the whole search.
266 For other phases, the next phase loop is started
267 with the current results as an input.
268 2. New transmit rate (or exit) calculation (for 1.d.):
269 * If there is an invalid bound then prepare for external search:
270 * *If* the most recent measurement at NDR lower_bound transmit rate
271 had the loss higher than zero, then
272 the new transmit rate is NDR lower_bound
273 decreased by two NDR interval widths.
274 * Else, *if* the most recent measurement at PDR lower_bound
275 transmit rate had the loss higher than PLR, then
276 the new transmit rate is PDR lower_bound
277 decreased by two PDR interval widths.
278 * Else, *if* the most recent measurement at NDR upper_bound
279 transmit rate had no loss, then
280 the new transmit rate is NDR upper_bound
281 increased by two NDR interval widths.
282 * Else, *if* the most recent measurement at PDR upper_bound
283 transmit rate had the loss lower or equal to PLR, then
284 the new transmit rate is PDR upper_bound
285 increased by two PDR interval widths.
286 * If interval width is higher than the current phase goal:
287 * Else, *if* NDR interval does not meet the current phase width goal,
288 prepare for internal search. The new transmit rate is
289 (NDR lower bound + NDR upper bound) / 2.
290 * Else, *if* PDR interval does not meet the current phase width goal,
291 prepare for internal search. The new transmit rate is
292 (PDR lower bound + PDR upper bound) / 2.
293 * Else, *if* some bound has still only been measured at a lower duration,
294 prepare to re-measure at the current duration (and the same transmit rate).
295 The order of priorities is:
300 * *Else*, do not prepare any new rate, to exit the phase.
301 This ensures that at the end of each non-initial phase
302 all intervals are valid, narrow enough, and measured
303 at current phase trial duration.
305 # Known Implementations
307 The only known working implementation of MLRsearch is in Linux Foundation
308 FD.io CSIT project. https://wiki.fd.io/view/CSIT. https://git.fd.io/csit/.
310 ## FD.io CSIT Implementation Deviations
312 This document so far has been describing a simplified version of MLRsearch algorithm.
313 The full algorithm as implemented contains additional logic,
314 which makes some of the details (but not general ideas) above incorrect.
315 Here is a short description of the additional logic as a list of principles,
316 explaining their main differences from (or additions to) the simplified description,
317 but without detailing their mutual interaction.
319 1. Logarithmic transmit rate.
320 In order to better fit the relative width goal,
321 the interval doubling and halving is done differently.
322 For example, the middle of 2 and 8 is 4, not 5.
323 2. Optimistic maximum rate.
324 The increased rate is never higher than the maximum rate.
325 Upper bound at that rate is always considered valid.
326 3. Pessimistic minimum rate.
327 The decreased rate is never lower than the minimum rate.
328 If a lower bound at that rate is invalid,
329 a phase stops refining the interval further (until it gets re-measured).
330 4. Conservative interval updates.
331 Measurements above current upper bound never update a valid upper bound,
332 even if drop ratio is low.
333 Measurements below current lower bound always update any lower bound
334 if drop ratio is high.
335 5. Ensure sufficient interval width.
336 Narrow intervals make external search take more time to find a valid bound.
337 If the new transmit increased or decreased rate would result in width
338 less than the current goal, increase/decrease more.
339 This can happen if the measurement for the other interval
340 makes the current interval too narrow.
341 Similarly, take care the measurements in the initial phase
342 create wide enough interval.
343 6. Timeout for bad cases.
344 The worst case for MLRsearch is when each phase converges to intervals
345 way different than the results of the previous phase.
346 Rather than suffer total search time several times larger
347 than pure binary search, the implemented tests fail themselves
348 when the search takes too long (given by argument *timeout*).
350 # IANA Considerations
354 # Security Considerations