255 lines
8.7 KiB
Plaintext
255 lines
8.7 KiB
Plaintext
.TL
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Garlic: the execution pipeline
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.AU
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Rodrigo Arias Mallo
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.AI
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Barcelona Supercomputing Center
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.AB
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.LP
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This document covers the execution of experiments in the Garlic
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benchmark, which are performed under strict conditions. The several
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stages of the execution are documented so the experimenter can have a
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global overview of how the benchmark runs under the hood.
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The results of the experiment are stored in a known path to be used in
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posterior processing steps.
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.AE
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.\"#####################################################################
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.nr GROWPS 3
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.nr PSINCR 1.5p
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.\".nr PD 0.5m
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.nr PI 2m
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\".2C
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.\"#####################################################################
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.NH 1
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Introduction
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.LP
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Every experiment in the Garlic
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benchmark is controlled by a single
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.I nix
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file placed in the
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.CW garlic/exp
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subdirectory.
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Experiments are formed by several
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.I "experimental units"
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or simply
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.I units .
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A unit is the result of each unique configuration of the experiment
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(typically involves the cartesian product of all factors) and
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consists of several shell scripts executed sequentially to setup the
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.I "execution environment" ,
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which finally launch the actual program being analyzed.
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The scripts that prepare the environment and the program itself are
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called the
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.I stages
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of the execution and altogether form the
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.I "execution pipeline"
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or simply the
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.I pipeline .
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The experimenter must know with very good details all the stages
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involved in the pipeline, as they have a large impact on the execution.
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.PP
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Additionally, the execution time is impacted by the target machine in
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which the experiments run. The software used for the benchmark is
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carefully configured and tuned for the hardware used in the execution;
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in particular, the experiments are designed to run in MareNostrum 4
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cluster with the SLURM workload manager and the Omni-Path
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interconnection network. In the future we plan to add
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support for other clusters in order to execute the experiments in other
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machines.
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.\"#####################################################################
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.NH 1
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Isolation
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.LP
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The benchmark is designed so that both the compilation of every software
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package and the execution of the experiment is performed under strict
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conditions. We can ensure that two executions of the same experiment are
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actually running the same program in the same software environment.
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.PP
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All the software used by an experiment is included in the
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.I "nix store"
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which is, by convention, located at the
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.CW /nix
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directory. Unfortunately, it is common for libraries to try to load
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software from other paths like
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.CW /usr
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or
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.CW /lib .
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It is also common that configuration files are loaded from
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.CW /etc
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and from the home directory of the user that runs the experiment.
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Additionally, some environment variables are recognized by the libraries
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used in the experiment, which change their behavior. As we cannot
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control the software and configuration files in those directories, we
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couldn't guarantee that the execution behaves as intended.
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.PP
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In order to avoid this problem, we create a
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.I sandbox
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where only the files in the nix store are available (with some other
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exceptions). Therefore, even if the libraries try to access any path
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outside the nix store, they will find that the files are not there
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anymore. Additionally, the environment variables are cleared before
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entering the environment (with some exceptions as well).
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.\"#####################################################################
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.NH 1
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Execution pipeline
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.LP
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Several predefined stages form the
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.I standard
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execution pipeline and are defined in the
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.I stdPipeline
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array. The standard pipeline prepares the resources and the environment
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to run a program (usually in parallel) in the compute nodes. It is
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divided in two main parts:
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connecting to the target machine to submit a job and executing the job.
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Finally, the complete execution pipeline ends by running the actual
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program, which is not part of the standard pipeline, as should be
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defined differently for each program.
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.NH 2
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Job submission
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.LP
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Some stages are involved in the job submission: the
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.I trebuchet
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stage connects via
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.I ssh
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to the target machine and executes the next stage there. Once in the
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target machine, the
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.I runexp
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stage computes the output path to store the experiment results, using
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the user in the target machine and changes the working directory there.
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In MareNostrum 4 the output path is at
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.CW /gpfs/projects/bsc15/garlic/$user/out .
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Then the
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.I isolate
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stage is executed to enter the sandbox and the
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.I experiment
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stage begins, which creates a directory to store the experiment output,
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and launches several
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.I unit
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stages.
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.PP
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Each unit executes a
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.I sbatch
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stage which runs the
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.I sbatch(1)
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program with a job script that simply calls the next stage. The
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sbatch program internally reads the
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.CW /etc/slurm/slurm.conf
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file from outside the sandbox, so we must explicitly allow this file to
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be available, as well as the
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.I munge
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socket used for authentication by the SLURM daemon. Once the jobs are
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submitted to SLURM, the experiment stage ends and the trebuchet finishes
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the execution. The jobs will be queued for execution without any other
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intervention from the user.
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.PP
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The rationale behind running sbatch from the sandbox is because the
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options provided in environment variables override the options from the
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job script. Therefore, we avoid this problem by running sbatch from the
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sandbox, where the interfering environment variables are removed. The
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sbatch program is also provided in the
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.I "nix store" ,
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with a version compatible with the SLURM daemon running in the target
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machine.
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.NH 2
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Job execution
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.LP
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Once an unit job has been selected for execution, SLURM
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allocates the resources (usually several nodes) and then selects one of
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the nodes to run the job script: it is not executed in parallel yet.
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The job script runs from a child process forked from on of the SLURM
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daemon processes, which are outside the sandbox. Therefore, we first run the
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.I isolate
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stage
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to enter the sandbox again.
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.PP
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The next stage is called
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.I control
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and determines if enough data has been generated by the experiment unit
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or if it should continue repeating the execution. At the current time,
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it is only implemented as a simple loop that runs the next stage a fixed
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amount of times (by default, it is repeated 30 times).
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.PP
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The following stage is
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.I srun
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which launches several copies of the next stage to run in
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parallel (when using more than one task). Runs one copy per task,
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effectively creating one process per task. The CPUs affinity is
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configured by the parameter
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.I --cpu-bind
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and is important to set it correctly (see more details in the
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.I srun(1)
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manual). Appending the
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.I verbose
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value to the cpu bind option causes srun to print the assigned affinity
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of each task, which is very valuable when examining the execution log.
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.PP
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The mechanism by which srun executes multiple processes is the same used
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by sbatch, it forks from a SLURM daemon running in the computing nodes.
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Therefore, the execution begins outside the sandbox. The next stage is
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.I isolate
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which enters again the sandbox in every task. All remaining stages are
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running now in parallel.
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.\" ###################################################################
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.NH 2
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The program
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.LP
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At this point in the execution, the standard pipeline has been
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completely executed, and we are ready to run the actual program that is
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the matter of the experiment. Usually, programs require some arguments
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to be passed in the command line. The
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.I exec
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stage sets the arguments (and optionally some environment variables) and
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executes the last stage, the
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.I program .
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.PP
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The experimenters are required to define these last stages, as they
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define the specific way in which the program must be executed.
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Additional stages may be included before or after the program run, so
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they can perform additional steps.
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.\" ###################################################################
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.NH 2
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Stage overview
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.LP
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The complete execution pipeline using the standard pipeline is shown in
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the Table 1. Some properties are also reflected about the execution
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stages.
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.KF
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.TS
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center;
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lB cB cB cB cB cB
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l c c c c c.
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_
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Stage Target Safe Copies User Std
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_
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trebuchet xeon no no yes yes
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runexp login no no yes yes
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isolate login no no no yes
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experiment login yes no no yes
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unit login yes no no yes
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sbatch login yes no no yes
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_
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isolate comp no no no yes
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control comp yes no no yes
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srun comp yes no no yes
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isolate comp no yes no yes
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_
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exec comp yes yes no no
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program comp yes yes no no
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_
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.TE
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.QS
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.B "Table 1" :
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The stages of a complete execution pipeline. The
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.B target
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column determines where the stage is running,
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.B safe
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states if the stage begins the execution inside the sandbox,
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.B user
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if it can be executed directly by the user,
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.B copies
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if there are several instances running in parallel and
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.B std
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if is part of the standard execution pipeline.
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.QE
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.KE
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