Update paper.md

Author: jan-janssen

docs/paper/paper.md

@@ -43,6 +43,8 @@ To address this limitation while at the same time leveraging the powerful and no
 
 Rather than implementing its own job scheduler, Executorlib instead leverages existing job schedulers to request and manage Python processes and associated computing resources. Further, instead of defining a new syntax and concepts, Executorlib extends the existing syntax of the Executor class in the Python standard library. Currently, Executorlib supports five different job schedulers implement as different Executor classes. The first is the `SingleNodeExecutor` for rapid prototyping on a laptop or local workstation in a way that is functionally similar to the standard `ProcessPoolExecutor`. The second, `SlurmClusterExecutor` submits Python functions as individual jobs to a SLURM job scheduler using the `sbatch` command, which can be useful for long-running tasks, e.g., that call a compute intensive legacy code. The third is the `SlurmJobExecutor` which distributes Python functions in an existing SLURM job using the `srun` command. In analogy, the `FluxClusterExecutor` submits Python functions as individual jobs to a flux job scheduler and the `FluxJobExecutor` distributes Python functions in a flux job. Given the hierarchial approach of the flux scheduler there is no limit to the number of `FluxJobExecutor` instances which can be nested inside each other to construct hierarchical workflows. 
 
+To assign dedicated computing resources to individual Python functions, the Executorlib Executor classes extend the submission function `submit()` to support not only the Python function and its inputs, but also a Python dictionary specifying the requested computing resources `resource_dict`. The resource dictionary can define the number of compute cores, number of threads, number of GPUs, as well as job scheduler specific parameters like the working directory, maximum run time or the maximum memory. With this hierarchical approach, Executorlib allows the user to finely control the execution of each individual Python function, using parallel communication libraries like the Message Passing Interface (MPI) for Python [@mpi4py] or GPU-optimized libraries to aggressively optimize complex compute intensive tasks of heterogenous HPC that are best solved by tightly-coupled parallelization approaches, while offering a simple and easy to maintain approach to the orchestration of many such weakly-coupled tasks. This ability to seamlessly combine different programming models again accelerates the rapid prototyping of heterogenous HPC workflows without sacrificing performance of critical code components.
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 # Usage To-Date 
 While initially developed in the US DOE Exascale Computing Project’s Exascale Atomistic Capability for Accuracy, Length and Time (EXAALT) to accelerate the development of computational materials science simulation workflows for the Exascale, Executorlib has since been generalized to support a wide-range of backends and HPC clusters at different scales. Based on this generalization, it is also been implemented in the pyiron workflow framework [@pyiron] as primary task scheduling interface.