Spark Distributed Analytic Framework¶
Description and Overview¶
Apache Spark is a fast and general engine for large-scale data processing.
How to Use Spark¶
Because of its high memory and I/O bandwidth requirements, we recommend you run your spark jobs on Cori. We recommend that you run Spark inside of Shifter. This will improve performance and usability by utilizing Shifter's per-node file cache for shuffle files and temporary data files. Without this functionality, these files are written either to your scratch directory (which is not optimized for repeated accesses of many small files) or the RAM file system at /tmp (which removes memory from the node doing the calculation and can lead to the node crashing from lack of memory).
Follow the steps below to use spark, note that the order of the commands matters. DO NOT load the spark module until you are inside a batch job.
Submit an interactive batch job with at least 2 nodes. Our setup for Spark puts the master on one node and the workers on the other nodes.
salloc -N 2 -t 30 -C haswell -q interactive --image=nersc/spark-2.3.0:v2 --volume="/global/cscratch1/sd/<user_name>/tmpfiles:/tmp:perNodeCache=size=200G"
This will request a job with the Spark 2.3.0 Shifter image (if you wish to run with an earlier version of spark, replace the version numbers for the above and following commands). It also sets up an xfs file on each node as a per node cache, which will be accessed inside the Shifter image via /tmp. By default, Spark will use this as the directory it caches temporary files. By default Spark will put event logs in $SCRATCH/spark/spark_event_logs, you will need to create this directory the first time you start up Spark.
Wait for the job to start. Once it does you will be on a compute node and you will need to load the Spark module:
export EXEC_CLASSPATH=path_to_any_extra_needed_jars #Only required if you're using external libraries or jarfiles module load spark/2.3.0
You can start Spark with this command:
To connect to the Python Spark Shell, do:
To connect to the Scala Spark Shell, do:
To shutdown the Spark cluster, do:
Below are example batch scripts for Cori. You can change number of nodes/time/queue accordingly (so long as the number of nodes is greater than 1). On Cori you can use the debug queue for short, debugging jobs and the regular queue for long jobs.
Here's an example script for Cori called run.sl:
#!/bin/bash #SBATCH -q regular #SBATCH -N 2 #SBATCH -C haswell #SBATCH -t 00:30:00 #SBATCH -e mysparkjob_%j.err #SBATCH -o mysparkjob_%j.out #SBATCH --image=nersc/spark-2.3.0:v2 #SBATCH --volume="/global/cscratch1/sd/<user_name>/tmpfiles:/tmp:perNodeCache=size=200G" export EXEC_CLASSPATH=path_to_any_extra_needed_jars #Only required if you're using external libraries or jarfiles module load spark/2.3.0 start-all.sh shifter spark-submit $SPARK_EXAMPLES/python/pi.py stop-all.sh
To submit the job:
Example PySpark Script¶
Here's an example pyspark script that calculates pi.
from time import time import numpy as np from random import random from operator import add from pyspark.sql import SparkSession spark = SparkSession.builder.appName('CalculatePi').getOrCreate() sc = spark.sparkContext n = 10000000 def is_point_inside_unit_circle(p): # p is useless here x, y = random(), random() return 1 if x*x + y*y < 1 else 0 t_0 = time() # parallelize creates a spark Resilient Distributed Dataset (RDD) # its values are useless in this case # but allows us to distribute our calculation (inside function) count = sc.parallelize(range(0, n)) \ .map(is_point_inside_unit_circle).reduce(add) print(np.round(time()-t_0, 3), "seconds elapsed for spark approach and n=", n) print("Pi is roughly %f" % (4.0 * count / n))
You can run this script in an interactive or batch session (see above for how to get one of those) with
shifter spark-submit --master $SPARKURL <path_to_python_script>
Guide for Optimizing Your Spark Code¶
Scala or Python¶
When writing code for Spark, historically scala has out performed python (via pySpark).However, as of Spark 2.0, the performance of python code using dataframes has become roughly equivalent for most operations. So we recommend that you write your code in python and dataframes. You will get roughly the same performance and also benefit from the fact that you can use a familiar language and a pandas-like dataframe interface.
Memory Management and Input Data¶
Choose enough nodes that your input data can comfortably fit in the aggregate memory of all the nodes. If you will be working with the same dataframe over and over again, you can "cache" the dataframe to make sure it stays in memory. This will cut down on recalculating time. Just be sure to unpersist it when you're done using it. Whenever possible, store your input data in your $SCRATCH directory.