Benedict created CASSANDRA-9870:
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Summary: Improve cassandra-stress graphing
Key: CASSANDRA-9870
URL: https://issues.apache.org/jira/browse/CASSANDRA-9870
Project: Cassandra
Issue Type: Improvement
Components: Tools
Reporter: Benedict
CASSANDRA-7918 introduces graph output from a stress run, but these graphs are
a little limited. Attached to the ticket is an example of some improved graphs
which can serve as the *basis* for some improvements, which I will briefly
describe. They should not be taken as the exact end goal, but we should aim for
at least their functionality. Preferably with some Javascript advantages thrown
in, such as the hiding of datasets/graphs for clarity. Any ideas for
improvements are *definitely* encouraged.
Firstly, one overriding design principle was to display _on *one* screen_ all
of the information necessary to get a good idea of how two or more branches
compare to each other. Ideally we will reintroduce this, painting multiple
graphs onto one screen, stretched to fit.
Secondly, axes must be truncated to only the interesting dimensions, to ensure
there is no wasted space.
Thirdly, each graph displaying multiple kinds of data should use colour _and
shape_ to help easily distinguish the different datasets.
Fourthly, each graph should be tailored to the data it is representing, and we
should have multiple views of each data.
The data can roughly be partitioned into three kinds: throughput, latency and
gc.
These can each be viewed in different ways:
* as a continuous plot of:
** raw data
** scaled/compared to a "base" branch, or other metric
** cumulatively
* as box plots
** ideally, these will plot median, outer quartiles, outer deciles and absolute
limits of the distribution, so the shape of the data can be best understood
Each compresses the information differently, losing different information, so
that collectively they help to understand the data.
Some basic rules that work well:
* Latency information should be plotted to a logarithmic scale, to avoid high
latencies drowning out low ones
* GC information should be plotted cumulatively, to avoid differing throughputs
giving the impression of worse GC. It should also have a line that is rescaled
by the amount of work (number of operations) completed
* Throughput should be plotted as the actual numbers
To walk the graphs top-left to bottom-right, we have:
* Spot throughput comparison of branches to the baseline branch, as an
improvement ratio (which can of course be negative, but is not in this example)
* Raw throughput of all branches (no baseline)
* Raw throughput as a box plot
* Latency percentiles, compared to baseline. The percentage improvement at any
point in time vs baseline is calculated, and then multiplied by the overall
median for the entire run. This simply permits the non-baseline branches to
scatter their wins/loss around a relatively clustered line for each percentile.
It's probably the most "dishonest" graph but comparing something like latency
where each data point can have very high variance is difficult, and this gives
you an idea of clustering of improvements/losses.
* Latency percentiles, raw, each with a different shape; lowest percentiles
plotted as a solid line as they vary least, with higher percentiles each
getting their own subtly different shape to scatter.
* Latency box plots
* GC time, plotted cumulatively and also scaled by work done
* GC Mb, plotted cumulatively and also scaled by work done
* GC time, raw
* GC time as a box plot
These do mostly introduce the concept of a "baseline" branch. It may be that,
ideally, this baseline be selected by a dropdown so the javascript can
transform the output dynamically. This would permit more interesting
comparisons to be made on the fly.
There are also some complexities, such as deciding which datapoints to compare
against baseline when times get out-of-whack (due to GC, etc, causing a lack of
output for a period). The version I uploaded does a merge of the times,
permitting a small degree of variance, and ignoring those datapoints we cannot
pair. One option here might be to change stress' behaviour to always print to a
strict schedule, instead of trying to get absolutely accurate apportionment of
timings. If this makes things much simpler, it can be done.
As previously stated, but may be lost in the wall-of-text, these should be
taken as a starting point / sign post, rather than a golden rule for the end
goal. But ideally they will be the lower bound of what we can deliver.
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