In order to use Spark, environment has to bee set up according to Using Spark in UFAL Environment.
When Spark computation starts, it uses environment variable MASTER to determine the mode of computation. The following values are possible:
local: Run locally using single thread.local[N] (e.g., local[2] or local[4]): Run locally using N threads.local[*] (default if MASTER variable does not exist): Run locally using as many threads as there are processor cores.spark://master_address:master_port: Run in a distributed fashion using specified master.
Spark computations can be started both on desktop machines and on cluster machines, either by specifying MASTER to one of local modes, or by not specifying MASTER at all (local[*] is used then).
Note that when you use sbatch or srun to run a cluster locally, your job is by default expected to use just a single core, so you should specify MASTER=local. If you do not, Spark will use all cores on the machine, even though Slurm gave you only one.
Spark cluster can be started using Slurm. The cluster is user-specific, but it can be used for several consecutive Spark computations.
The Spark cluster can be started using one of the following two commands:
spark-sbatch: start a Spark cluster via an sbatch spark-sbatch [sbatch args] workers memory_per_workerG[:python_memoryG] command [arguments...]
spark-srun: start a Spark cluster via an srun spark-srun [salloc args] workers memory_per_workerG[:python_memoryG] [command arguments...]
Both spark-sbatch and spark-srun commands start a Spark cluster with the specified number of workers, each with the given amount of memory. Then they set MASTER and SPARK_ADDRESS to the address of the Spark master and SPARK_WEBUI to the URL of the master web interface. Both these values are also written on standard output, and the SPARK_WEBUI is added to the Slurm job Comment. Finally, the specified command is started; when spark-srun is used, the command may be empty, in which case bash is opened.
TL;DR: Good default is 2G.
The memory for each worker is specified using the following format:
spark_memory_per_workerG[:memory_per_Python_processG]
The Spark memory limits the Java heap, and half of it is reserved for memory storage of cached RDDs. The second value sets a memory limit of every Python process and is by default set to 2G.
Start Spark cluster with 10 workers 2GB RAM each and then run interactive shell. The cluster stops after the shell is exited.
spark-srun 10 2G
Start Spark cluster with 20 workers 4GB RAM each in the cpu-ms partition, and run screen in it, so that several computations can be performed using this cluster. The cluster has to be stopped manually (either by quitting the scree or calling scancel).
spark-sbatch -p cpu-ms 20 4G screen -D -m