Multiple Loss Ratio Search for Packet Throughput (MLRsearch)
draft-ietf-bmwg-mlrsearch-02
| Document | Type | Active Internet-Draft (bmwg WG) | |
|---|---|---|---|
| Authors | Maciek Konstantynowicz , Vratko Polák | ||
| Last updated | 2022-03-07 | ||
| Replaces | draft-vpolak-mkonstan-bmwg-mlrsearch | ||
| Stream | Internet Engineering Task Force (IETF) | ||
| Intended RFC status | (None) | ||
| Formats | plain text html xml htmlized pdfized bibtex | ||
| Stream | WG state | WG Document | |
| Document shepherd | (None) | ||
| IESG | IESG state | I-D Exists | |
| Consensus boilerplate | Unknown | ||
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| Responsible AD | (None) | ||
| Send notices to | (None) |
draft-ietf-bmwg-mlrsearch-02
Benchmarking Working Group M. Konstantynowicz, Ed.
Internet-Draft V. Polak
Intended status: Informational Cisco Systems
Expires: 8 September 2022 7 March 2022
Multiple Loss Ratio Search for Packet Throughput (MLRsearch)
draft-ietf-bmwg-mlrsearch-02
Abstract
TODO: Update after all sections are ready.
This document proposes changes to [RFC2544], specifically to packet
throughput search methodology, by defining a new search algorithm
referred to as Multiple Loss Ratio search (MLRsearch for short).
Instead of relying on binary search with pre-set starting offered
load, it proposes a novel approach discovering the starting point in
the initial phase, and then searching for packet throughput based on
defined packet loss ratio (PLR) input criteria and defined final
trial duration time. One of the key design principles behind
MLRsearch is minimizing the total test duration and searching for
multiple packet throughput rates (each with a corresponding PLR)
concurrently, instead of doing it sequentially.
The main motivation behind MLRsearch is the new set of challenges and
requirements posed by NFV (Network Function Virtualization),
specifically software based implementations of NFV data planes.
Using [RFC2544] in the experience of the authors yields often not
repetitive and not replicable end results due to a large number of
factors that are out of scope for this draft. MLRsearch aims to
address this challenge in a simple way of getting the same result
sooner, so more repetitions can be done to describe the
replicability.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
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Internet-Drafts are draft documents valid for a maximum of six months
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time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 8 September 2022.
Copyright Notice
Copyright (c) 2022 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Intentions of this document . . . . . . . . . . . . . . . . . 5
3. RFC2544 . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1. Throughput search . . . . . . . . . . . . . . . . . . . . 5
4. Problems . . . . . . . . . . . . . . . . . . . . . . . . . . 6
4.1. Repeatability and Comparability . . . . . . . . . . . . . 6
4.2. Non-Zero Target Loss Ratios . . . . . . . . . . . . . . . 6
5. Solution ideas . . . . . . . . . . . . . . . . . . . . . . . 7
5.1. Short duration trials . . . . . . . . . . . . . . . . . . 8
5.2. FRMOL as reasonable start . . . . . . . . . . . . . . . . 8
5.3. Non-zero loss ratios . . . . . . . . . . . . . . . . . . 8
5.4. Concurrent ratio search . . . . . . . . . . . . . . . . . 9
5.5. Load selection heuristics and shortcuts . . . . . . . . . 9
6. Non-compliance with RFC2544 . . . . . . . . . . . . . . . . . 9
7. Additional Requirements . . . . . . . . . . . . . . . . . . . 10
7.1. TODO: Search Stop Criteria . . . . . . . . . . . . . . . 10
7.2. Reliability of Test Equipment . . . . . . . . . . . . . . 10
7.2.1. Very late frames . . . . . . . . . . . . . . . . . . 10
8. MLRsearch Background . . . . . . . . . . . . . . . . . . . . 11
9. MLRsearch Overview . . . . . . . . . . . . . . . . . . . . . 13
10. Sample Implementation . . . . . . . . . . . . . . . . . . . . 16
10.1. Input Parameters . . . . . . . . . . . . . . . . . . . . 16
10.2. Initial Phase . . . . . . . . . . . . . . . . . . . . . 17
10.3. Non-Initial Phases . . . . . . . . . . . . . . . . . . . 18
11. FD.io CSIT Implementation . . . . . . . . . . . . . . . . . . 22
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11.1. Additional details . . . . . . . . . . . . . . . . . . . 22
11.1.1. FD.io CSIT Input Parameters . . . . . . . . . . . . 24
11.2. Example MLRsearch Run . . . . . . . . . . . . . . . . . 25
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 27
13. Security Considerations . . . . . . . . . . . . . . . . . . . 27
14. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 27
15. References . . . . . . . . . . . . . . . . . . . . . . . . . 27
15.1. Normative References . . . . . . . . . . . . . . . . . . 27
15.2. Informative References . . . . . . . . . . . . . . . . . 28
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 28
1. Terminology
TODO: Update after most other sections are updated.
* TODO: The current text uses Throughput for the zero loss ratio
load. Is the capital T needed/useful?
* DUT and SUT: see the definitions in https://gerrit.fd.io/r/c/
csit/+/35545
* Traffic Generator (TG) and Traffic Analyzer (TA): see
https://datatracker.ietf.org/doc/html/rfc6894#section-4 TODO:
Maybe there is an earlier RFC?
* Overall search time: the time it takes to find all required loads
within their precision goals, starting from zero trials measured
at given DUT configuration and traffic profile.
* TODO: traffic profile?
* Intended load: https://datatracker.ietf.org/doc/html/
rfc2285#section-3.5.1
* Offered load: https://datatracker.ietf.org/doc/html/
rfc2285#section-3.5.2
* Maximum offered load (MOL): see
https://datatracker.ietf.org/doc/html/rfc2285#section-3.5.3
* Forwarding rate at maximum offered load (FRMOL)
https://datatracker.ietf.org/doc/html/rfc2285#section-3.6.2
* Trial Loss Count: the number of frames transmitted minus the
number of frames received. Negative count is possible, e.g. when
SUT duplicates some frames.
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* Trial Loss Ratio: ratio of frames received relative to frames
transmitted over the trial duration. For bi-directional
throughput tests, the aggregate ratio is calculated, based on the
aggregate number of frames transmitted and received. If the trial
loss count is negative, its absolute value MUST be used to keep
compliance with RFC2544.
* Safe load: any value, such that trial measurement at this (or
lower) intended load is correcrly handled by both TG and TA,
regardless of SUT behavior. Frequently, it is not known what the
safe load is.
* Max load (TODO rename?): Maximal intended load to be used during
search. Benchmarking team decides which value is low enough to
guarantee values reported by TG and TA are reliable. It has to be
a safe load, but it can be lower than a safe load estimate for
added safety. See the subsection on unreliable test equipment
below. This value MUST NOT be higher than MOL, which itself MUST
NOT be higher than Maximum Frame Rate
https://datatracker.ietf.org/doc/html/rfc2544#section-20
* Min load: Minimal intended load to be used during search.
Benchmarking team decides which value is high enough to guarantee
the trial measurement results are valid. E.g. considerable
overall search time can be saved by declaring SUT faulty if min
load trial shows too high loss rate. Zero frames per second is a
valid min load value
* Effective loss ratio: a corrected value of trial loss ratio chosen
to avoid difficulties if SUT exhibits decreasing loss ratio with
increasing load. It is the maximum of trial loss ratios measured
at the same duration on all loads smaller than (and including) the
current one.
* Target loss ratio: a loss ratio value acting as an input for the
search. The search is finding tight enough lower and upper bounds
in intended load, so that the measurement at the lower bound has
smaller or equal trial loss ratio, and upper bound has strictly
larger trial loss ratio. For the tightest upper bound, the
effective loss ratio is the same as trial loss ratio at that upper
bound load. For the tightest lower bound, the effective loss
ratio can be higher than the trial loss ratio at that lower bound,
but still not larger than the target loss ratio.
* TODO: Search algorithm.
* TODO: Precision goal.
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* TODO: Define a "benchmarking group".
* TODO: Upper and lower bound.
* TODO: Valid and invalid bound?
* TODO: Interval and interval width?
TODO: Mention NIC/PCI bandwidth/pps limits can be lower than
bandwidth of medium.
2. Intentions of this document
The intention of this document is to provide recommendations for: *
optimizing search for multiple target loss ratios at once, * speeding
up the overall search time, * improve search results repeatability
and comparability.
No part of RFC2544 is intended to be obsoleted by this document.
3. RFC2544
3.1. Throughput search
It is useful to restate the key requirements of RFC2544 using the new
terminology (see section Terminology).
The following sections of RFC2544 are of interest for this document.
* https://datatracker.ietf.org/doc/html/rfc2544#section-20 Mentions
the max load SHOULD not be larget than the theoretical maximum
rate for the frame size on the media.
* https://datatracker.ietf.org/doc/html/rfc2544#section-23 Lists the
actions to be done for each trial measurement, it also mentions
loss rate as an example of trial measurement results. This
document uses loss count instead, as that is the quantity that is
easier for the current test equipment to measure, e.g. it is not
affected by the real traffic duration. TODO: Time uncertainty
again.
* https://datatracker.ietf.org/doc/html/rfc2544#section-24 Mentions
"full length trials" leading to the Throughput found, as opposed
to shorter trial durations, allowed in an attempt to "minimize the
length of search procedure". This document talks about "final
trial duration" and aims to "optimize overal search time".
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* https://datatracker.ietf.org/doc/html/rfc2544#section-26.1 with
https://www.rfc-editor.org/errata/eid422 finaly states
requirements for the search procedure. It boils down to "increase
intended load upon zero trial loss and decrease intended load upon
non-zero trial loss".
No additional constraints are placed on the load selection, and there
is no mention of an exit condition, e.g. when there is enough trial
measurements to proclaim the largest load with zero trial loss (and
final trial duration) to be the Throughput found.
4. Problems
4.1. Repeatability and Comparability
RFC2544 does not suggest to repeat Throughput search, and from just
one Throughput value, it cannot be determined how repeatable that
value is (how likely it is for a repeated Throughput search to end up
with a value less then the precision goal away from the first value).
Depending on SUT behavior, different benchmark groups can report
significantly different Througput values, even when using identical
SUT and test equipment, just because of minor differences in their
search algorithm (e.g. different max load value).
While repeatability can be addressed by repeating the search several
times, the differences in the comparability scenario may be
systematic, e.g. seeming like a bias in one or both benchmark groups.
MLRsearch algorithm does not really help with the repeatability
problem. This document RECOMMENDS to repeat a selection of
"important" tests ten times, so users can ascertain the repeatability
of the results.
TODO: How to report? Average and standard deviation?
Following MLRsearch algorithm leaves less freedom for the benchmark
groups to encounter the comparability problem, alghough more research
is needed to determine the effect of MLRsearch's tweakable
parameters.
4.2. Non-Zero Target Loss Ratios
https://datatracker.ietf.org/doc/html/rfc1242#section-3.17 defines
Throughput as: The maximum rate at which none of the offered frames
are dropped by the device.
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and then it says: Since even the loss of one frame in a data stream
can cause significant delays while waiting for the higher level
protocols to time out, it is useful to know the actual maximum data
rate that the device can support.
New "software DUTs" (traffic forwarding programs running on
commercial-off-the-shelf compute server hardware) frequently exhibit
quite low repeatability of Throughput results per above definition.
This is due to, in general, throughput rates of software DUTs
(programs) being sensitive to server resource allocation by OS during
runtime, as well as any interrupts or blocking of software threads
involved in packet processing.
To deal with this, this document recommends discovery of multiple
throughput rates of interest for software DUTs that run on general
purpose COTS servers (with x86, AArch64 Instruction Set
Architectures): * throughput rate with target of zero packet loss
ratio. * at least one throughput rate with target of non-zero packet
loss ratio.
In our experience, the higher the target loss ratio is, the better is
the repeatability of the corresponding load found.
TODO: Define a good name for a load corresponding to a specific non-
zero target loss ration, while keeping Throughput for the load
corresponding to zero target loss ratio.
This document RECOMMENDS the benchmark groups to search for
corresponding loads to at least one non-zero target loss ratio. This
document does not suggest any particular non-zero target loss ratio
value to search the corresponding load for.
5. Solution ideas
This document gives several independent ideas on how to lower the
(average) overall search time, while remaining unconditionally
compliant with RFC2544 (and adding some of extensions).
This document also specifies one particular way to combine all the
ideas into a single search algorithm class (single logic with few
tweakable parameters).
Little to no research has been done into the question of which
combination of ideas achieves the best compromise with respect to
overal search time, high repeatability and high comparability.
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TODO: How important it is to discuss particular implementation
choices, especially when motivated by non-deterministic SUT behavior?
5.1. Short duration trials
https://datatracker.ietf.org/doc/html/rfc2544#section-24 already
mentions the possibity of using shorter duration for trials that are
not part of "final determination".
Obviously, the upper and lower bound from a smaller duration trial
can be used as the initial upper and lower bound for the final
determination.
MLRsearch makes it clear a re-measurement is always needed (new trial
measurement with the same load but longer duration). It also
specifes what to do if the longer trial is no longer a valid bound
(TODO define?), e.g. start an external search. Additionaly one
halving can be saved during the shorter duration search.
5.2. FRMOL as reasonable start
TODO expand: Overal search ends with "final determination" search,
preceded by "shorter duration search" preceded by "bound
initialization", where the bounds can be considerably different from
min and max load.
For SUTs with high repeatability, the FRMOL is usually a good
approximation of Throughput. But for less repeatable SUTs,
forwarding rate (TODO define) is frequently a bad approximation to
Throughput, therefore halving and other robust-to-worst-case
approaches have to be used. Still, forwarding rate at FRMOL load can
be a good initial bound.
5.3. Non-zero loss ratios
See the "Popularity of non-zero target loss ratios" section above.
TODO: Define "trial measurement result classification criteria", or
keep reusing long phrases without definitions?
A search for a load corresponding to a non-zero target loss rate is
very similar to a search for Throughput, just the criterion when to
increase or decrease the intended load for the next trial measurement
uses the comparison of trial loss ratio to the target loss ratio
(instead of comparing loss count to zero) Any search algorithm that
works for Throughput can be easily used also for non-zero target loss
rates, perhaps with small modifications in places where the measured
forwarding rate is used.
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Note that it is possible to search for multiple loss ratio goals if
needed.
5.4. Concurrent ratio search
A single trial measurement result can act as an upper bound for a
lower target loss ratio, and as a lower bound for a higher target
loss ratio at the same time. This is an example of how it can be
advantageous to search for all loss ratio goals "at once", or at
least "reuse" trial measurement result done so far.
Even when a search algorithm is fully deterministic in load selection
while focusing on a single loss ratio and trial duration, the choice
of iteration order between target loss ratios and trial durations can
affect the obtained results in subtle ways. MLRsearch offers one
particular ordering.
5.5. Load selection heuristics and shortcuts
Aside of the two heuristics already mentioned (FRMOL based initial
bounds and saving one halving when increasing trial duration), there
are other tricks that can save some overall search time at the cost
of keeping the difference between final lower and upper bound
intentionally large (but still within the precision goal).
TODO: Refer implementation subsections on: * Uneven splits. *
Rounding the interval width up. * Using old invalid bounds for
interval width guessing.
The impact on overall duration is probably small, and the effect on
result distribution maybe even smaller. TODO: Is the two-liner above
useful at all?
6. Non-compliance with RFC2544
It is possible to achieve even faster search times by abandoning some
requirements and suggestions of RFC2544, mainly by reducing the wait
times at start and end of trial.
Such results are therefore no longer compliant with RFC2544 (or at
least not unconditionally), but they may still be useful for internal
usage, or for comparing results of different DUTs achieved with an
identical non-compliant algorithm.
TODO: Refer to the subsection with CSIT customizations.
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7. Additional Requirements
RFC2544 can be understood as having a number of implicit
requirements. They are made explicit in this section (as
requirements for this document, not for RFC2544).
Recommendations on how to properly address the implicit requirements
are out of scope of this document.
7.1. TODO: Search Stop Criteria
TODO: Mention the timeout parameter?
7.2. Reliability of Test Equipment
Both TG and TA MUST be able to handle correctly every intended load
used during the search.
On TG side, the difference between Intended Load and Offered Load
MUST be small.
TODO: How small? Difference of one packet may not be measurable due
to time uncertainties.
TODO expand: time uncertainty.
To ensure that, max load (see Terminology) has to be set to low
enough value. Benchmark groups MAY list the max load value used,
especially if the Throughput value is equal (or close) to the max
load.
Solutions (even problem formulations) for the following open problems
are outside of the scope of this document: * Detecting when the test
equipment operates above its safe load. * Finding a large but safe
load value. * Correcting any result affected by max load value not
being a safe load.
7.2.1. Very late frames
RFC2544 requires quite conservative time delays see
https://datatracker.ietf.org/doc/html/rfc2544#section-23 to prevent
frames buffered in one trial measurement to be counted as received in
a subsequent trial measurement.
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However, for some SUTs it may still be possible to buffer enough
frames, so they are still sending them (perhaps in bursts) when the
next trial measurement starts. Sometimes, this can be detected as a
negative trial loss count, e.g. TA receiving more frames than TG has
sent during this trial measurement. Frame duplication is another way
of causing the negative trial loss count.
https://datatracker.ietf.org/doc/html/rfc2544#section-10 recommends
to use sequence numbers in frame payloads, but generating and
verifying them requires test equipment resources, which may be not
plenty enough to suport at high loads. (Using low enough max load
would work, but frequently that would be smaller than SUT's sctual
Throughput.)
RFC2544 does not offer any solution to the negative loss problem,
except implicitly treating negative trial loss counts the same way as
positive trial loss counts.
This document also does not offer any practical solution.
Instead, this document SUGGESTS the search algorithm to take any
precaution necessary to avoid very late frames.
This document also REQUIRES any detected duplicate frames to be
counted as additional lost frames. This document also REQUIRES, any
negative trial loss ratio to be treated as positive trial loss ratio
of the same absolute value.
!!! Nothing below is up-to-date with draft v02. !!!
8. MLRsearch Background
TODO: Old section, probably obsoleted by preceding section(s).
Multiple Loss Ratio search (MLRsearch) is a packet throughput search
algorithm suitable for deterministic systems (as opposed to
probabilistic systems). MLRsearch discovers multiple packet
throughput rates in a single search, each rate is associated with a
distinct Packet Loss Ratio (PLR) criterion.
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For cases when multiple rates need to be found, this property makes
MLRsearch more efficient in terms of time execution, compared to
traditional throughput search algorithms that discover a single
packet rate per defined search criteria (e.g. a binary search
specified by [RFC2544]). MLRsearch reduces execution time even
further by relying on shorter trial durations of intermediate steps,
with only the final measurements conducted at the specified final
trial duration. This results in the shorter overall search execution
time when compared to a traditional binary search, while guaranteeing
the same results for deterministic systems.
In practice, two rates with distinct PLRs are commonly used for
packet throughput measurements of NFV systems: Non Drop Rate (NDR)
with PLR=0 and Partial Drop Rate (PDR) with PLR>0. The rest of this
document describes MLRsearch with NDR and PDR pair as an example.
Similarly to other throughput search approaches like binary search,
MLRsearch is effective for SUTs/DUTs with PLR curve that is non-
decreasing with growing offered load. It may not be as effective for
SUTs/DUTs with abnormal PLR curves, although it will always converge
to some value.
MLRsearch relies on traffic generator to qualify the received packet
stream as error-free, and invalidate the results if any disqualifying
errors are present e.g. out-of-sequence frames.
MLRsearch can be applied to both uni-directional and bi-directional
throughput tests.
For bi-directional tests, MLRsearch rates and ratios are aggregates
of both directions, based on the following assumptions:
* Traffic transmitted by traffic generator and received by SUT/DUT
has the same packet rate in each direction, in other words the
offered load is symmetric.
* SUT/DUT packet processing capacity is the same in both directions,
resulting in the same packet loss under load.
MLRsearch can be applied even without those assumptions, but in that
case the aggregate loss ratio is less useful as a metric.
MLRsearch can be used for network transactions consisting of more
than just one packet, or anything else that has intended load as
input and loss ratio as output (duration as input is optional). This
text uses mostly packet-centric language.
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9. MLRsearch Overview
The main properties of MLRsearch:
* MLRsearch is a duration aware multi-phase multi-rate search
algorithm:
- Initial Phase determines promising starting interval for the
search.
- Intermediate Phases progress towards defined final search
criteria.
- Final Phase executes measurements according to the final search
criteria.
- Final search criteria are defined by following inputs:
o Target PLRs (e.g. 0.0 and 0.005 when searching for NDR and
PDR).
o Final trial duration.
o Measurement resolution.
* Initial Phase:
- Measure MRR over initial trial duration.
- Measured MRR is used as an input to the first intermediate
phase.
* Multiple Intermediate Phases:
- Trial duration:
o Start with initial trial duration in the first intermediate
phase.
o Converge geometrically towards the final trial duration.
- Track all previous trial measurement results:
o Duration, offered load and loss ratio are tracked.
o Effective loss ratios are tracked.
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+ While in practice, real loss ratios can decrease with
increasing load, effective loss ratios never decrease.
This is achieved by sorting results by load, and using
the effective loss ratio of the previous load if the
current loss ratio is smaller than that.
o The algorithm queries the results to find best lower and
upper bounds.
+ Effective loss ratios are always used.
o The phase ends if all target loss ratios have tight enough
bounds.
- Search:
o Iterate over target loss ratios in increasing order.
o If both upper and lower bound are in measurement results for
this duration, apply bisect until the bounds are tight
enough, and continue with next loss ratio.
o If a bound is missing for this duration, but there exists a
bound from the previous duration (compatible with the other
bound at this duration), re-measure at the current duration.
o If a bound in one direction (upper or lower) is missing for
this duration, and the previous duration does not have a
compatible bound, compute the current "interval size" from
the second tightest bound in the other direction (lower or
upper respectively) for the current duration, and choose
next offered load for external search.
o The logic guarantees that a measurement is never repeated
with both duration and offered load being the same.
o The logic guarantees that measurements for higher target
loss ratio iterations (still within the same phase duration)
do not affect validity and tightness of bounds for previous
target loss ratio iterations (at the same duration).
- Use of internal and external searches:
o External search:
+ It is a variant of "exponential search".
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+ The "interval size" is multiplied by a configurable
constant (powers of two work well with the subsequent
internal search).
o Internal search:
+ A variant of binary search that measures at offered load
between the previously found bounds.
+ The interval does not need to be split into exact halves,
if other split can get to the target width goal faster.
* The idea is to avoid returning interval narrower than
the current width goal. See sample implementation
details, below.
* Final Phase:
- Executed with the final test trial duration, and the final
width goal that determines resolution of the overall search.
* Intermediate Phases together with the Final Phase are called Non-
Initial Phases.
* The returned bounds stay within prescribed min_rate and max_rate.
- When returning min_rate or max_rate, the returned bounds may be
invalid.
o E.g. upper bound at max_rate may come from a measurement
with loss ratio still not higher than the target loss ratio.
The main benefits of MLRsearch vs. binary search include:
* In general, MLRsearch is likely to execute more trials overall,
but likely less trials at a set final trial duration.
* In well behaving cases, e.g. when results do not depend on trial
duration, it greatly reduces (>50%) the overall duration compared
to a single PDR (or NDR) binary search over duration, while
finding multiple drop rates.
* In all cases MLRsearch yields the same or similar results to
binary search.
* Note: both binary search and MLRsearch are susceptible to
reporting non-repeatable results across multiple runs for very bad
behaving cases.
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Caveats:
* Worst case MLRsearch can take longer than a binary search, e.g. in
case of drastic changes in behaviour for trials at varying
durations.
- Re-measurement at higher duration can trigger a long external
search. That never happens in binary search, which uses the
final duration from the start.
10. Sample Implementation
Following is a brief description of a sample MLRsearch
implementation, which is a simplified version of the existing
implementation.
10.1. Input Parameters
1. *max_rate* - Maximum Transmit Rate (MTR) of packets to be used by
external traffic generator implementing MLRsearch, limited by the
actual Ethernet link(s) rate, NIC model or traffic generator
capabilities.
2. *min_rate* - minimum packet transmit rate to be used for
measurements. MLRsearch fails if lower transmit rate needs to be
used to meet search criteria.
3. *final_trial_duration* - required trial duration for final rate
measurements.
4. *initial_trial_duration* - trial duration for initial MLRsearch
phase.
5. *final_relative_width* - required measurement resolution
expressed as (lower_bound, upper_bound) interval width relative
to upper_bound.
6. *packet_loss_ratios* - list of maximum acceptable PLR search
criteria.
7. *number_of_intermediate_phases* - number of phases between the
initial phase and the final phase. Impacts the overall MLRsearch
duration. Less phases are required for well behaving cases, more
phases may be needed to reduce the overall search duration for
worse behaving cases.
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10.2. Initial Phase
1. First trial measures at configured maximum transmit rate (MTR)
and discovers maximum receive rate (MRR).
* IN: trial_duration = initial_trial_duration.
* IN: offered_transmit_rate = maximum_transmit_rate.
* DO: single trial.
* OUT: measured loss ratio.
* OUT: MRR = measured receive rate. Received rate is computed
as intended load multiplied by pass ratio (which is one minus
loss ratio). This is useful when loss ratio is computed from
a different metric than intended load. For example, intended
load can be in transactions (multiple packets each), but loss
ratio is computed on level of packets, not transactions.
* Example: If MTR is 10 transactions per second, and each
transaction has 10 packets, and receive rate is 90 packets per
second, then loss rate is 10%, and MRR is computed to be 9
transactions per second.
If MRR is too close to MTR, MRR is set below MTR so that interval
width is equal to the width goal of the first intermediate phase.
If MRR is less than min_rate, min_rate is used.
2. Second trial measures at MRR and discovers MRR2.
* IN: trial_duration = initial_trial_duration.
* IN: offered_transmit_rate = MRR.
* DO: single trial.
* OUT: measured loss ratio.
* OUT: MRR2 = measured receive rate. If MRR2 is less than
min_rate, min_rate is used. If loss ratio is less or equal to
the smallest target loss ratio, MRR2 is set to a value above
MRR, so that interval width is equal to the width goal of the
first intermediate phase. MRR2 could end up being equal to
MTR (for example if both measurements so far had zero loss),
which was already measured, step 3 is skipped in that case.
3. Third trial measures at MRR2.
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* IN: trial_duration = initial_trial_duration.
* IN: offered_transmit_rate = MRR2.
* DO: single trial.
* OUT: measured loss ratio.
* OUT: MRR3 = measured receive rate. If MRR3 is less than
min_rate, min_rate is used. If step 3 is not skipped, the
first trial measurement is forgotten. This is done because in
practice (if MRR2 is above MRR), external search from MRR and
MRR2 is likely to lead to a faster intermediate phase than a
bisect between MRR2 and MTR.
10.3. Non-Initial Phases
1. Main phase loop:
1. IN: trial_duration for the current phase. Set to
initial_trial_duration for the first intermediate phase; to
final_trial_duration for the final phase; or to the element
of interpolating geometric sequence for other intermediate
phases. For example with two intermediate phases,
trial_duration of the second intermediate phase is the
geometric average of initial_trial_duration and
final_trial_duration.
2. IN: relative_width_goal for the current phase. Set to
final_relative_width for the final phase; doubled for each
preceding phase. For example with two intermediate phases,
the first intermediate phase uses quadruple of
final_relative_width and the second intermediate phase uses
double of final_relative_width.
3. IN: Measurement results from the previous phase (previous
duration).
4. Internal target ratio loop:
1. IN: Target loss ratio for this iteration of ratio loop.
2. IN: Measurement results from all previous ratio loop
iterations of current phase (current duration).
3. DO: According to the procedure described in point 2:
1. either exit the phase (by jumping to 1.5),
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2. or exit loop iteration (by continuing with next
target loss ratio, jumping to 1.4.1),
3. or calculate new transmit rate to measure with.
4. DO: Perform the trial measurement at the new transmit
rate and current trial duration, compute its loss ratio.
5. DO: Add the result and go to next iteration (1.4.1),
including the added trial result in 1.4.2.
5. OUT: Measurement results from this phase.
6. OUT: In the final phase, bounds for each target loss ratio
are extracted and returned.
1. If a valid bound does not exist, use min_rate or
max_rate.
2. New transmit rate (or exit) calculation (for point 1.4.3):
1. If the previous duration has the best upper and lower bound,
select the middle point as the new transmit rate.
1. See 2.5.3. below for the exact splitting logic.
2. This can be a no-op if interval is narrow enough already,
in that case continue with 2.2.
3. Discussion, assuming the middle point is selected and
measured:
1. Regardless of loss rate measured, the result becomes
either best upper or best lower bound at current
duration.
2. So this condition is satisfied at most once per
iteration.
3. This also explains why previous phase has double
width goal:
1. We avoid one more bisection at previous phase.
2. At most one bound (per iteration) is re-measured
with current duration.
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3. Each re-measurement can trigger an external
search.
4. Such surprising external searches are the main
hurdle in achieving low overall search durations.
5. Even without 1.1, there is at most one external
search per phase and target loss ratio.
6. But without 1.1 there can be two re-measurements,
each coming with a risk of triggering external
search.
2. If the previous duration has one bound best, select its
transmit rate. In deterministic case this is the last
measurement needed this iteration.
3. If only upper bound exists in current duration results:
1. This can only happen for the smallest target loss ratio.
2. If the upper bound was measured at min_rate, exit the
whole phase early (not investigating other target loss
ratios).
3. Select new transmit rate using external search:
1. For computing previous interval size, use:
1. second tightest bound at current duration,
2. or tightest bound of previous duration, if
compatible and giving a more narrow interval,
3. or target interval width if none of the above is
available.
4. In any case increase to target interval width if
smaller.
2. Quadruple the interval width.
3. Use min_rate if the new transmit rate is lower.
4. If only lower bound exists in current duration results:
1. If the lower bound was measured at max_rate, exit this
iteration (continue with next lowest target loss ratio).
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2. Select new transmit rate using external search:
1. For computing previous interval size, use:
1. second tightest bound at current duration,
2. or tightest bound of previous duration, if
compatible and giving a more narrow interval,
3. or target interval width if none of the above is
available.
4. In any case increase to target interval width if
smaller.
2. Quadruple the interval width.
3. Use max_rate if the new transmit rate is higher.
5. The only remaining option is both bounds in current duration
results.
1. This can happen in two ways, depending on how the lower
bound was chosen.
1. It could have been selected for the current loss
ratio, e.g. in re-measurement (2.2) or in initial
bisect (2.1).
2. It could have been found as an upper bound for the
previous smaller target loss ratio, in which case it
might be too low.
3. The algorithm does not track which one is the case,
as the decision logic works well regardless.
2. Compute "extending down" candidate transmit rate exactly
as in 2.3.
3. Compute "bisecting" candidate transmit rate:
1. Compute the current interval width from the two
bounds.
2. Express the width as a (float) multiple of the target
width goal for this phase.
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3. If the multiple is not higher than one, it means the
width goal is met. Exit this iteration and continue
with next higher target loss ratio.
4. If the multiple is two or less, use half of that for
new width if the lower subinterval.
5. Round the multiple up to nearest even integer.
6. Use half of that for new width if the lower
subinterval.
7. Example: If lower bound is 2.0 and upper bound is
5.0, and width goal is 1.0, the new candidate
transmit rate will be 4.0. This can save a
measurement when 4.0 has small loss. Selecting the
average (3.5) would never save a measurement, giving
more narrow bounds instead.
4. If either candidate computation want to exit the
iteration, do as bisecting candidate computation says.
5. The remaining case is both candidates wanting to measure
at some rate. Use the higher rate. This prefers
external search down narrow enough interval, competing
with perfectly sized lower bisect subinterval.
11. FD.io CSIT Implementation
The only known working implementation of MLRsearch is in the open-
source code running in Linux Foundation FD.io CSIT project
[FDio-CSIT-MLRsearch] as part of a Continuous Integration /
Continuous Development (CI/CD) framework.
MLRsearch is also available as a Python package in [PyPI-MLRsearch].
11.1. Additional details
This document so far has been describing a simplified version of
MLRsearch algorithm. The full algorithm as implemented in CSIT
contains additional logic, which makes some of the details (but not
general ideas) above incorrect. Here is a short description of the
additional logic as a list of principles, explaining their main
differences from (or additions to) the simplified description, but
without detailing their mutual interaction.
1. Logarithmic transmit rate.
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* In order to better fit the relative width goal, the interval
doubling and halving is done differently.
* For example, the middle of 2 and 8 is 4, not 5.
2. Timeout for bad cases.
* The worst case for MLRsearch is when each phase converges to
intervals way different than the results of the previous
phase.
* Rather than suffer total search time several times larger than
pure binary search, the implemented tests fail themselves when
the search takes too long (given by argument _timeout_).
3. Intended count.
* The number of packets to send during the trial should be equal
to the intended load multiplied by the duration.
- Also multiplied by a coefficient, if loss ratio is
calculated from a different metric.
o Example: If a successful transaction uses 10 packets,
load is given in transactions per second, but loss ratio
is calculated from packets, so the coefficient to get
intended count of packets is 10.
* But in practice that does not work.
- It could result in a fractional number of packets,
- so it has to be rounded in a way traffic generator chooses,
- which may depend on the number of traffic flows and traffic
generator worker threads.
4. Attempted count. As the real number of intended packets is not
known exactly, the computation uses the number of packets traffic
generator reports as sent. Unless overridden by the next point.
5. Duration stretching.
* In some cases, traffic generator may get overloaded, causing
it to take significantly longer (than duration) to send all
packets.
* The implementation uses an explicit stop,
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- causing lower attempted count in those cases.
* The implementation tolerates some small difference between
attempted count and intended count.
- 10 microseconds worth of traffic is sufficient for our
tests.
* If the difference is higher, the unsent packets are counted as
lost.
- This forces the search to avoid the regions of high
duration stretching.
- The final bounds describe the performance of not just SUT,
but of the whole system, including the traffic generator.
6. Excess packets.
* In some test (e.g. using TCP flows) Traffic generator reacts
to packet loss by retransmission. Usually, such packet loss
is already affecting loss ratio. If a test also wants to
treat retransmissions due to heavily delayed packets also as a
failure, this is once again visible as a mismatch between the
intended count and the attempted count.
* The CSIT implementation simply looks at absolute value of the
difference, so it offers the same small tolerance before it
starts marking a "loss".
7. For result processing, we use lower bounds and ignore upper
bounds.
11.1.1. FD.io CSIT Input Parameters
1. *max_rate* - Typical values: 2 * 14.88 Mpps for 64B 10GE link
rate, 2 * 18.75 Mpps for 64B 40GE NIC (specific model).
2. *min_rate* - Value: 2 * 9001 pps (we reserve 9000 pps for latency
measurements).
3. *final_trial_duration* - Value: 30.0 seconds.
4. *initial_trial_duration* - Value: 1.0 second.
5. *final_relative_width* - Value: 0.005 (0.5%).
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6. *packet_loss_ratios* - Value: 0.0, 0.005 (0.0% for NDR, 0.5% for
PDR).
7. *number_of_intermediate_phases* - Value: 2. The value has been
chosen based on limited experimentation to date. More
experimentation needed to arrive to clearer guidelines.
8. *timeout* - Limit for the overall search duration (for one
search). If MLRsearch oversteps this limit, it immediately
declares the test failed, to avoid wasting even more time on a
misbehaving SUT. Value: 600.0 (seconds).
9. *expansion_coefficient* - Width multiplier for external search.
Value: 4.0 (interval width is quadroupled). Value of 2.0 is best
for well-behaved SUTs, but value of 4.0 has been found to
decrease overall search time for worse-behaved SUT
configurations, contributing more to the overall set of different
SUT configurations tested.
11.2. Example MLRsearch Run
The following list describes a search from a real test run in CSIT
(using the default input values as above).
* Initial phase, trial duration 1.0 second.
Measurement 1, intended load 18750000.0 pps (MTR), measured loss
ratio 0.7089514628479618 (valid upper bound for both NDR and PDR).
Measurement 2, intended load 5457160.071600716 pps (MRR), measured
loss ratio 0.018650817320118702 (new tightest upper bounds).
Measurement 3, intended load 5348832.933500009 pps (slightly less
than MRR2 in preparation for first intermediate phase target interval
width), measured loss ratio 0.00964383362905351 (new tightest upper
bounds).
* First intermediate phase starts, trial duration still 1.0 seconds.
Measurement 4, intended load 4936605.579021453 pps (no lower bound,
performing external search downwards, for NDR), measured loss ratio
0.0 (valid lower bound for both NDR and PDR).
Measurement 5, intended load 5138587.208637197 pps (bisecting for
NDR), measured loss ratio 0.0 (new tightest lower bounds).
Measurement 6, intended load 5242656.244044665 pps (bisecting),
measured loss ratio 0.013523745379347257 (new tightest upper bounds).
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* Both intervals are narrow enough.
* Second intermediate phase starts, trial duration 5.477225575051661
seconds.
Measurement 7, intended load 5190360.904111567 pps (initial bisect
for NDR), measured loss ratio 0.0023533920869969953 (NDR upper bound,
PDR lower bound).
Measurement 8, intended load 5138587.208637197 pps (re-measuring NDR
lower bound), measured loss ratio 1.2080222912800403e-06 (new
tightest NDR upper bound).
* The two intervals have separate bounds from now on.
Measurement 9, intended load 4936605.381062318 pps (external NDR
search down), measured loss ratio 0.0 (new valid NDR lower bound).
Measurement 10, intended load 5036583.888432355 pps (NDR bisect),
measured loss ratio 0.0 (new tightest NDR lower bound).
Measurement 11, intended load 5087329.903232804 pps (NDR bisect),
measured loss ratio 0.0 (new tightest NDR lower bound).
* NDR interval is narrow enough, PDR interval not ready yet.
Measurement 12, intended load 5242656.244044665 pps (re-measuring PDR
upper bound), measured loss ratio 0.0101174866190136 (still valid PDR
upper bound).
* Also PDR interval is narrow enough, with valid bounds for this
duration.
* Final phase starts, trial duration 30.0 seconds.
Measurement 13, intended load 5112894.3238511775 pps (initial bisect
for NDR), measured loss ratio 0.0 (new tightest NDR lower bound).
Measurement 14, intended load 5138587.208637197 (re-measuring NDR
upper bound), measured loss ratio 2.030389804256833e-06 (still valid
PDR upper bound).
* NDR interval is narrow enough, PDR interval not yet.
Measurement 15, intended load 5216443.04126728 pps (initial bisect
for PDR), measured loss ratio 0.005620871287975237 (new tightest PDR
upper bound).
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Measurement 16, intended load 5190360.904111567 (re-measuring PDR
lower bound), measured loss ratio 0.0027629971184465604 (still valid
PDR lower bound).
* PDR interval is also narrow enough.
* Returning bounds:
* NDR_LOWER = 5112894.3238511775 pps; NDR_UPPER = 5138587.208637197
pps;
* PDR_LOWER = 5190360.904111567 pps; PDR_UPPER = 5216443.04126728
pps.
12. IANA Considerations
No requests of IANA.
13. Security Considerations
Benchmarking activities as described in this memo are limited to
technology characterization of a DUT/SUT using controlled stimuli in
a laboratory environment, with dedicated address space and the
constraints specified in the sections above.
The benchmarking network topology will be an independent test setup
and MUST NOT be connected to devices that may forward the test
traffic into a production network or misroute traffic to the test
management network.
Further, benchmarking is performed on a "black-box" basis, relying
solely on measurements observable external to the DUT/SUT.
Special capabilities SHOULD NOT exist in the DUT/SUT specifically for
benchmarking purposes. Any implications for network security arising
from the DUT/SUT SHOULD be identical in the lab and in production
networks.
14. Acknowledgements
Many thanks to Alec Hothan of OPNFV NFVbench project for thorough
review and numerous useful comments and suggestions.
15. References
15.1. Normative References
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[RFC2544] Bradner, S. and J. McQuaid, "Benchmarking Methodology for
Network Interconnect Devices", RFC 2544,
DOI 10.17487/RFC2544, March 1999,
<https://www.rfc-editor.org/info/rfc2544>.
15.2. Informative References
[FDio-CSIT-MLRsearch]
"FD.io CSIT Test Methodology - MLRsearch", November 2021,
<https://s3-docs.fd.io/csit/rls2110/report/introduction/
methodology_data_plane_throughput/
methodology_data_plane_throughput.html#mlrsearch-tests>.
[PyPI-MLRsearch]
"MLRsearch 0.4.0, Python Package Index", April 2021,
<https://pypi.org/project/MLRsearch/0.4.0/>.
Authors' Addresses
Maciek Konstantynowicz (editor)
Cisco Systems
Email: mkonstan@cisco.com
Vratko Polak
Cisco Systems
Email: vrpolak@cisco.com
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