feat(docs): Add Methodology

[csit.git] / docs / content / methodology / root_cause_analysis / perpatch_performance_tests.md

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bookToc: false
title: "Per-patch performance tests"
weight: 1
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# Per-patch performance tests

Updated for CSIT git commit id: 72b45cfe662107c8e1bb549df71ba51352a898ee.

A methodology similar to trending analysis is used for comparing performance
before a DUT code change is merged. This can act as a verify job to disallow
changes which would decrease performance without a good reason.

## Existing jobs

VPP is the only project currently using such jobs.
They are not started automatically, must be triggered on demand.
They allow full tag expressions, but some tags are enforced (such as MRR).

There are jobs available for multiple types of testbeds,
based on various processors.
Their Gerrit triggers words are of the form "perftest-{node_arch}"
where the node_arch combinations currently supported are:
2n-clx, 2n-tx2, 2n-zn2, 3n-tsh.

## Test selection

Gerrit trigger line without any additional arguments selects
a small set of test cases to run.
If additional arguments are added to the Gerrit trigger, they are treated
as Robot tag expressions to select tests to run.
While very flexible, this method of test selection also allows the user
to accidentally select too high number of tests, blocking the testbed for days.

What follows is a list of explanations and recommendations
to help users to select the minimal set of tests cases.

### Verify cycles

When Gerrit schedules multiple jobs to run for the same patch set,
it waits until all runs are complete.
While it is waiting, it is possible to trigger more jobs
(adding runs to the set Gerrit is waiting for), but it is not possible
to trigger more runs for the same job, until Gerrit is done waiting.
After Gerrit is done waiting, it becames possible to trigger
the same job again.

Example. User triggers one set of tests on 2n-icx and immediately
also triggers other set of tests on 3n-icx. Then the user notices
2n-icx run end early because of a typo in tag expression.
When the user tries to re-trigger 2n-icx (with fixed tag expression),
that comment gets ignored by Jenkins.
Only when 3n-icx job finishes, the user can trigger 2n-icx.

### One comment many jobs

In the past, the CSIT code which parses for perftest trigger comments
was buggy, which lead to bad behavior (as in selection all performance test,
because "perftest" is also a robot tag) when user included multiple
perftest trigger words in the same comment.

The worst bugs were fixed since then, but it is still recommended
to use just one trigger word per Gerrit comment, just to be safe.

### Multiple test cases in run

While Robot supports OR operator, it does not support parentheses,
so the OR operator is not very useful. It is recommended
to use space instead of OR operator.

Example template:
perftest-2n-icx {tag_expression_1} {tag_expression_2}

See below for more concrete examples.

### Suite tags

Traditionally, CSIT maintains broad Robot tags that can be used to select tests.

But it is not recommended to use them for test selection,
as it is not that easy to determine how many test cases are selected.

The recommended way is to look into CSIT repository first,
and locate a specific suite the user is interested in,
and use its suite tag. For example, "ethip4-ip4base" is a suite tag
selecting just one suite in CSIT git repository,
avoiding all scale, container, and other simialr variants.

Note that CSIT uses "autogen" code generator,
so the robot running in Jenkins has access to more suites
than visible just by looking into CSIT git repository,
so suite tag is not enough to select even the intended suite,
and user still probably wants to narrow down
to a single test case within a suite.

### Fully specified tag expressions

Here is one template to select a single test case:
{test_type}AND{nic_model}AND{nic_driver}AND{cores}AND{frame_size}AND{suite_tag}
where the variables are all lower case (so AND operator stands out).

Currently only one test type is supported by the performance comparison jobs:
"mrr".
The nic_driver options depend on nic_model. For Intel cards "drv_avf" (AVF plugin)
and "drv_vfio_pci" (DPDK plugin) are popular, for Mellanox "drv_rdma_core".
Currently, the performance using "drv_af_xdp" is not reliable enough, so do not use it
unless you are specifically testing for AF_XDP.

The most popular nic_model is "nic_intel-xxv710", but that is not available
on all testbed types.
It is safe to use "1c" for cores (unless you are suspection multi-core performance
is affected differently) and "64b" for frame size ("78b" for ip6
and more for dot1q and other encapsulated traffic;
"1518b" is popular for ipsec and other payload-bound tests).

As there are more test cases than CSIT can periodically test,
it is possible to encounter an old test case that currently fails.
To avoid that, you can look at "job spec" files we use for periodic testing,
for example
[this one](https://github.com/FDio/csit/blob/master/resources/job_specs/report_iterative/2n-icx/vpp-mrr-00.md).

### Shortening triggers

Advanced users may use the following tricks to avoid writing long trigger comments.

Robot supports glob matching, which can be used to select multiple suite tags at once.

Not specifying one of 6 parts of the recommended expression pattern
will select all available options. For example not specifying nic_driver
for nic_intel-xxv710 will select all 3 applicable drivers.
You can use NOT operator to reject some options (e.g. NOTdrv_af_xdp),
but beware, with NOT the order matters:
tag1ANDtag2NOTtag3 is not the same as tag1NOTtag3ANDtag2,
the latter is evaluated as tag1AND(NOT(tag3ANDtag2)).

Beware when not specifying nic_model. As a precaution,
CSIT code will insert the defailt NIC model for the tetsbed used.
Example: Specifying drv_rdma_core without specifying nic_model
will fail, as the default nic_model is nic_intel-xxv710
which does not support RDMA core driver.

### Complete example

A user wants to test a VPP change which may affect load balance whith bonding.
Searching tag documentation for "bonding" finds LBOND tag and its variants.
Searching CSIT git repository (directory tests/) finds 8 suite files,
all suited only for 3-node testbeds.
All suites are using vhost, but differ by the forwarding app inside VM
(DPDK or VPP), by the forwarding mode of VPP acting as host level vswitch
(MAC learning or cross connect), and by the number of DUT1-DUT2 links
available (1 or 2).

As not all NICs and testbeds offer enogh ports for 2 parallel DUT-DUT links,
the user looks at
[testbed specifications](https://github.com/FDio/csit/tree/master/topologies/available)
and finds that only xxv710 NIC on 3n-icx testbed matches the requirements.
Quick look into the suites confirm the smallest frame size is 64 bytes
(despite DOT1Q robot tag, as the encapsulation does not happen on TG-DUT links).
It is ok to use just 1 physical core, as 3n-icx has hyperthreading enabled,
so VPP vswitch will use 2 worker threads.

The user decides the vswitch forwarding mode is not important
(so choses cross connect as that has less CPU overhead),
but wants to test both NIC drivers (not AF_XDP), both apps in VM,
and both 1 and 2 parallel links.

After shortening, this is the trigger comment fianlly used:
perftest-3n-icx mrrANDnic_intel-x710AND1cAND64bAND?lbvpplacp-dot1q-l2xcbase-eth-2vhostvr1024-1vm*NOTdrv_af_xdp

## Basic operation

The job builds VPP .deb packages for both the patch under test
(called "current") and its parent patch (called "parent").

For each test (from a set defined by tag expression),
both builds are subjected to several trial measurements (BMRR).
Measured samples are grouped to "parent" sequence,
followed by "current" sequence. The same Minimal Description Length
algorithm as in trending is used to decide whether it is one big group,
or two smaller gropus. If it is one group, a "normal" result
is declared for the test. If it is two groups, and current average
is less then parent average, the test is declared a regression.
If it is two groups and current average is larger or equal,
the test is declared a progression.

The whole job fails (giving -1) if some trial measurement failed,
or if any test was declared a regression.

## Temporary specifics

The Minimal Description Length analysis is performed by
CSIT code equivalent to jumpavg-0.1.3 library available on PyPI.

In hopes of strengthening of signal (code performance) compared to noise
(all other factors influencing the measured values), several workarounds
are applied.

In contrast to trending, trial duration is set to 10 seconds,
and only 5 samples are measured for each build.
Both parameters are set in ci-management.

This decreases sensitivity to regressions, but also decreases
probability of false positives.

## Console output

The following information as visible towards the end of Jenkins console output,
repeated for each analyzed test.

The original 5 values are visible in order they were measured.
The 5 values after processing are also visible in output,
this time sorted by value (so people can see minimum and maximum).

The next output is difference of averages. It is the current average
minus the parent average, expressed as percentage of the parent average.

The next three outputs contain the jumpavg representation
of the two groups and a combined group.
Here, "bits" is the description length; for "current" sequence
it includes effect from "parent" average value
(jumpavg-0.1.3 penalizes sequences with too close averages).

Next, a sentence describing which grouping description is shorter,
and by how much bits.
Finally, the test result classification is visible.

The algorithm does not track test case names,
so test cases are indexed (from 0).