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zscores

Smoothed, rolling Z-Scores for selected metrics or charts.

This collector uses the Netdata rest api to get the mean and stddev for each dimension on specified charts over a time range (defined by train_secs and offset_secs). For each dimension it will calculate a Z-Score as z = (x - mean) / stddev (clipped at z_clip). Scores are then smoothed over time (z_smooth_n) and, if mode: 'per_chart', aggregated across dimensions to a smoothed, rolling chart level Z-Score at each time step.

Charts#

Two charts are produced:

  • Z-Score (zscores.z): This chart shows the calculated Z-Score per chart (or dimension if mode='per_dim').
  • Z-Score >3 (zscores.3stddev): This chart shows a 1 if the absolute value of the Z-Score is greater than 3 or a 0 otherwise.

Below is an example of the charts produced by this collector and a typical example of how they would look when things are 'normal' on the system. Most of the zscores tend to bounce randomly around a range typically between 0 to +3 (or -3 to +3 if z_abs: 'false'), a few charts might stay steady at a more constant higher value depending on your configuration and the typical workload on your system (typically those charts that do not change that much have a smaller range of values on which to calculate a zscore and so tend to have a higher typical zscore).

So really its a combination of the zscores values themselves plus, perhaps more importantly, how they change when something strange occurs on your system which can be most useful.

zscores-collector-normal

For example, if we go onto the system and run a command like stress-ng --all 2 to create some stress, we see many charts begin to have zscores that jump outside the typical range. When the absolute zscore for a chart is greater than 3 you will see a corresponding line appear on the zscores.3stddev chart to make it a bit clearer what charts might be worth looking at first (for more background information on why 3 stddev see here) .

In the example below we basically took a sledge hammer to our system so its not suprising that lots of charts light up after we run the stress command. In a more realistic setting you might just see a handful of charts with strange zscores and that could be a good indication of where to look first.

zscores-collector-abnormal

Then as the issue passes the zscores should settle back down into their normal range again as they are calculated in a rolling and smoothed way (as defined by your zscores.conf file).

zscores-collector-normal-again

Requirements#

This collector will only work with Python 3 and requires the below packages be installed.

# become netdata user
sudo su -s /bin/bash netdata
# install required packages
pip3 install numpy pandas requests netdata-pandas==0.0.38

Configuration#

Install the underlying Python requirements, Enable the collector and restart Netdata.

cd /etc/netdata/
sudo ./edit-config python.d.conf
# Set `zscores: no` to `zscores: yes`
sudo systemctl restart netdata

The configuration for the zscores collector defines how it will behave on your system and might take some experimentation with over time to set it optimally. Out of the box, the config comes with some sane defaults to get you started.

If you are unsure about any of the below configuration options then it's best to just ignore all this and leave the zscores.conf files alone to begin with. Then you can return to it later if you would like to tune things a bit more once the collector is running for a while.

Edit the python.d/zscores.conf configuration file using edit-config from the your agent's config directory, which is usually at /etc/netdata.

cd /etc/netdata # Replace this path with your Netdata config directory, if different
sudo ./edit-config python.d/zscores.conf

The default configuration should look something like this. Here you can see each parameter (with sane defaults) and some information about each one and what it does.

# what host to pull data from
host: '127.0.0.1:19999'
# What charts to pull data for - A regex like 'system\..*|' or 'system\..*|apps.cpu|apps.mem' etc.
charts_regex: 'system\..*'
# length of time to base calulcations off for mean and stddev
train_secs: 14400 # use last 4 hours to work out the mean and stddev for the zscore
# offset preceeding latest data to ignore when calculating mean and stddev
offset_secs: 300 # ignore last 5 minutes of data when calculating the mean and stddev
# recalculate the mean and stddev every n steps of the collector
train_every_n: 900 # recalculate mean and stddev every 15 minutes
# smooth the z score by averaging it over last n values
z_smooth_n: 15 # take a rolling average of the last 15 zscore values to reduce sensitivity to temporary 'spikes'
# cap absolute value of zscore (before smoothing) for better stability
z_clip: 10 # cap each zscore at 10 so as to avoid really large individual zscores swamping any rolling average
# set z_abs: 'true' to make all zscores be absolute values only.
z_abs: 'true'
# burn in period in which to initially calculate mean and stddev on every step
burn_in: 2 # on startup of the collector continually update the mean and stddev in case any gaps or inital calculations fail to return
# mode can be to get a zscore 'per_dim' or 'per_chart'
mode: 'per_chart' # 'per_chart' means individual dimension level smoothed zscores will be aggregated to one zscore per chart per time step
# per_chart_agg is how you aggregate from dimension to chart when mode='per_chart'
per_chart_agg: 'mean' # 'absmax' will take the max absolute value accross all dimensions but will maintain the sign. 'mean' will just average.

Notes#

  • Python 3 is required as the netdata-pandas package uses python async libraries (asks and trio) to make asynchronous calls to the netdata rest api to get the required data for each chart when calculating the mean and stddev.
  • It may take a few hours or so for the collector to 'settle' into it's typical behaviour in terms of the scores you will see in the normal running of your system.
  • The zscore you see for each chart when using mode: 'per_chart' as actually an aggregated zscore accross all the dimensions on the underlying chart.
  • If you set mode: 'per_dim' then you will see a zscore for each dimension on each chart as opposed to one per chart.
  • As this collector does some calculations itself in python you may want to try it out first on a test or development system to get a sense of its performance characteristics. Most of the work in calculating the mean and stddev will be pushed down to the underlying Netdata C libraries via the rest api. But some data wrangling and calculations are then done using Pandas and Numpy within the collector itself.
  • On a development n1-standard-2 (2 vCPUs, 7.5 GB memory) vm running Ubuntu 18.04 LTS and not doing any work some of the typical performance characteristics we saw from running this collector were:
    • A runtime (netdata.runtime_zscores) of ~50ms when doing scoring and ~500ms when recalculating the mean and stddev.
    • Typically 3%-3.5% cpu usage from scoring, jumping to ~35% for one second when recalculating the mean and stddev.
    • About ~50mb of ram (apps.mem) being continually used by the python.d.plugin.
  • If you activate this collector on a fresh node, it might take a little while to build up enough data to calculate a proper zscore. So until you actually have train_secs of available data the mean and stddev calculated will be subject to more noise.

Reach out

If you need help after reading this doc, feel free to create an issue in the netdata/netdata repo or join our community forum to search for an answer. There's a good chance someone else has already found a solution to the same issue.

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