.TL
Garlic: the postprocess pipeline
.AU
Rodrigo Arias Mallo
.AI
Barcelona Supercomputing Center
.AB
.LP
This document covers the format used to store the results of the
execution and the postprocess steps used to generate a set of
figures from the results to present the data.
.AE
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.NH 1
Introduction
.LP
After the correct execution of an experiment some measurements are
recorded in the results for further investigation. Typically the time of
the execution or other quantities are measured and presented later in a
figure (generally a plot or a table).
The
.I "postprocess pipeline"
consists of all the steps required to create a set of figures from the
results. Similarly to the execution pipeline where several stages run
sequentially,
.[
garlic execution
.]
the postprocess pipeline is also formed by multiple stages executed
in order.
.PP
The rationale behind dividing execution and postprocess is
that usually the experiments are costly to run (they take a long time to
complete) while generating a figure require less time. Refining the
figures multiple times reusing the same experimental results doesn't
require the execution of the complete experiment, so the experimenter
can try multiple ways to present the data without waiting a large delay.
.NH 1
Results
.LP
The results are generated in the same
.I "target"
machine where the experiment is executed and are stored in the garlic
.I output ,
organized into a directory structure following the experiment name, the
unit name and the run number (governed by the
.I control
stage):
.QS
.CW
 |-- 6lp88vlj7m8hvvhpfz25p5mvvg7ycflb-experiment
 |   |-- 8lpmmfix52a8v7kfzkzih655awchl9f1-unit 
 |   |   |-- 1 
 |   |   |   |-- stderr.log
 |   |   |   |-- stdout.log
 |   |   |   |-- ...
 |   |   |-- 2 
 ...
.QE
In order to provide an easier access to the results, an index is also
created by taking the
.I expName
and
.I unitName
attributes (defined in the experiment configuration) and linking them to
the appropriate experiment and unit directories. These links are
overwritten by the last experiment with the same names so they are only
valid for the last execution. The output and index directories are
placed into a per-user directory, as we cannot guarantee the complete
execution of each unit when multiple users can share units.
.PP
The messages printed to the standard output and error are
are stored in the log files with the same name inside each run
directory. Additional data is sometimes generated by the experiments,
and is found in each run directory. As the generated data can be very
large, is ignored by default when considering the results.
.NH 1
Fetching the results
.LP
Consider a program of interest for which an experiment has been designed to
measure some properties that the experimenter wants to present in a
visual plot. When the experiment is launched, the execution
pipeline (EP) is completely executed and it will generate some
results. In this escenario, the execution pipeline depends on the
program\[em]any changes in the program will cause nix to build it again
using the updated program. The results will also depend on the
execution pipeline, and the graph on the results. This chain of
dependencies can be shown in the following dependency graph:
.PS
circlerad=0.22;
linewid=0.35;
right
circle "Prog"
arrow
circle "EP"
arrow
circle "Result"
arrow
circle "PP"
arrow
circle "Plot"
.PE
Ideally, the dependencies should be handled by nix, so it can detect any
change and rebuild the necessary parts automatically. Unfortunately, nix
is not able to build the result as a derivation directly as it requires access
to the
.I "target"
machine with several user accounts. In order to let several users reuse
the same results from a cache, we
use the
.I "nix store"
to make them available. To generate the results from the
experiment, we add some extra steps that must be executed manually.
.PS
circle "Prog"
arrow
diag=linewid + circlerad;
far=circlerad*3 + linewid*4
E: circle "EP"
R: circle "Result" at E + (far,0)
RUN: circle "Run" at E + (diag,-diag) dashed
FETCH: circle "Fetch" at R + (-diag,-diag) dashed
move to R.e
arrow
P: circle "PP"
arrow
circle "Plot"
arrow dashed from E to RUN chop
arrow dashed from RUN to FETCH chop
arrow dashed from FETCH to R chop
arrow from E to R chop
.PE
The run and fetch steps are provided by the helper tool
.I "garlic(1)" ,
which launches the experiment using the user credentials at the
.I "target"
machine and then fetches the results, placing them in a directory known
by nix.  When the result derivation needs to be built, nix will look in
this directory for the results of the execution. If the directory is not
found, a message is printed to suggest the user to launch the experiment
and the build process is stopped. When the result is successfully built
by any user, is stored in the
.I "nix store"
and it won't need to be rebuilt again until the experiment changes, as
the hash only depends on the experiment and not on the contents of the
results.
.PP
Notice that this mechanism violates the deterministic nature of the nix
store, as from a given input (the experiment) we can generate different
outputs (each result from different executions). We knowingly relaxed
this restriction by providing a guarantee that the results are
equivalent and there is no need to execute an experiment more than once.
.PP
To force the execution of an experiment you can use the
.I rev
attribute which is a number assigned to each experiment
and can be incremented to create copies that only differs on that
number. The experiment hash will change but the experiment will be the
same, as long as the revision number is ignored along the execution
stages.
.NH 1
Postprocess stages
.LP
Once the results are completely generated in the
.I "target"
machine there are several stages required to build a set of figures:
.PP
.I fetch \[em]
waits until all the experiment units are completed and then executes the
next stage. This stage is performed by the
.I garlic(1)
tool using the
.I -F
option and also reports the current state of the execution.
.PP
.I store \[em]
copies from the
.I target
machine into the nix store all log files generated by the experiment, 
keeping the same directory structure. It tracks the execution state of
each unit and only copies the results once the experiment is complete.
Other files are ignored as they are often very large and not required
for the subsequent stages.
.PP
.I timetable \[em]
converts the results of the experiment into a NDJSON file with one
line per run for each unit. Each line is a valid JSON object, containing
the
.I exp ,
.I unit
and
.I run
keys and the unit configuration (as a JSON object) in the
.I config
key. The execution time is captured from the standard output and is
added in the
.I time
key.
.PP
.I merge \[em]
one or more timetable datasets are joined, by simply concatenating them.
This step allows building one dataset to compare multiple experiments in
the same figure.
.PP
.I rPlot \[em]
one ot more figures are generated by a single R script
.[
r cookbook
.]
which takes as input the previously generated dataset.
The path of the dataset is recorded in the figure as well, which
contains enough information to determine all the stages in the execution
and postprocess pipelines.
.SH 1
Appendix A: Current setup
.LP
As of this moment, the
.I build
machine which contains the nix store is
.I xeon07
and the
.I "target"
machine used to run the experiments is Mare Nostrum 4 with the
.I output
directory placed at
.CW /gpfs/projects/bsc15/garlic .
By default, the experiment results are never deleted from the
.I target
so you may want to remove the ones already stored in the nix store to
free space.