Spark uses only one core when using forEachRemaining - apache-spark

I'm still new to Spark. I wrote this code to parse large string list.
I had to use forEachRemaining because I had to initialize some non-serializable objects in each partition.
JavaRDD<String> lines=initRetriever();
lines.foreachPartition(iter->{
Obj1 obj1=initObj1()
MyStringParser parser=new MyStringParser(obj1);
iter.forEachRemaining(str->{
try {
parser.parse(str);
} catch (ParsingException e) {
e.printStackTrace();
}
});
System.out.print("all parsed");
obj1.close();
});
I believe Spark is all about parallelism. But this program uses only a single thread on my local machine. Did I do something wrong? Missing configuration? Or maybe the iter doesn't allow it to execute all in parallel.
EDIT
I have no configuration files for Spark.
That's how I initialize Spark
SparkConf conf = new SparkConf()
.setAppName(AbstractSparkImporter.class.getCanonicalName())
.setMaster("local");
I run it on IDE and using mvn:exec command.

As #Alberto-Bonsanto indicated, using local[*] triggers Spark to use all available threads. More info here.
SparkConf conf = new SparkConf()
.setAppName(AbstractSparkImporter.class.getCanonicalName())
.setMaster("local[*]");

Related

SparkContext.setLogLevel("DEBUG") doesn't works in Cluster

I'm trying to control my Spark logs use
sc.setLogLevel("ERROR");
seems like it doesn't work in the cluster environment. Can anyone help?
public static JavaSparkContext getSparkContext(String appName, SparkConf conf) {
SparkSession spark = getSparkSession(appName, conf);
JavaSparkContext sc = new JavaSparkContext(spark.sparkContext());
sc.setLogLevel("WARN");
return sc;
}
To configure log levels, add the following options to your spark submit command:
'--conf "spark.driver.extraJavaOptions=-Dlog4j.configuration=custom-log4j.properties"'
'--conf "spark.executor.extraJavaOptions=-Dlog4j.configuration=custom-log4j.properties"'
This assumes you have a file called custom-log4j.properties on the classpath. This log4j can then control the verbosity of spark's logging.

Register custom classes for Kryo serialization in Beam Spark runner

I have seen that Beam Spark runner uses BeamSparkRunnerRegistrator for kryo registration. Is there a way to register custom user classes as well?
There is a way to do so, but first, may I ask why you want to do this?
Generally speaking, Beam's Spark runner uses Beam coders to serialize user data.
We currently have a bug in which cached DStreams are being serialized using Kryo, and if the user classes are not Kryo serializable this fails. BEAM-2669. We are currently attempting to solve this issue.
If this is the issue you are facing you can currently workaround this by using Kryo's registrator. Is this the issue you are facing? or do you have a different reason for doing this, please let me know.
In any case, here is how you can provide your own custom JavaSparkContext instance to Beam's Spark runner by using SparkContextOptions
SparkConf conf = new SparkConf();
conf.set("spark.serializer", KryoSerializer.class.getName());
conf.set("spark.kryo.registrator", "my.custom.KryoRegistrator");
JavaSparkContext jsc = new JavaSparkContext(..., conf);
SparkContextOptions options = PipelineOptionsFactory.as(SparkContextOptions.class);
options.setRunner(SparkRunner.class);
options.setUsesProvidedSparkContext(true);
options.setProvidedSparkContext(jsc);
Pipeline p = Pipeline.create(options);
For more information see:
Beam Spark runner documentation
Example: ProvidedSparkContextTest.java
Create your own KryoRegistrator with this custom serializer
package Mypackage
class MyRegistrator extends KryoRegistrator {
override def registerClasses(kryo: Kryo) {
kryo.register(classOf[A], new CustomASerializer())
}}
Then, add configuration entry about it with your registrator's fully-qualified name, e.g. Mypackage.MyRegistrator:
val conf = new SparkConf()
conf.set("spark.kryo.registrator", "Mypackage.KryoRegistrator")
See documentation: Data Serialization Spark
If you don’t want to register your classes, Kryo serialization will still work, but it will have to store the full class name with each object, which is wasteful.

Got UnsatisfiedLinkError when start Spark program from Java code

I am using SparkLauncher to launch my spark app from Java.The code looks like
Map<String, String> envMap = new HashMap<>();
envMap.put("HADOOP_CONF_DIR","/etc/hadoop/conf");
envMap.put("JAVA_LIBRARY_PATH", "/opt/cloudera/parcels/CDH-5.5.1-1.cdh5.5.1.p0.11/lib/hadoop/lib/native");
envMap.put("LD_LIBRARY_PATH", "/opt/cloudera/parcels/CDH-5.5.1-1.cdh5.5.1.p0.11/lib/hadoop/lib/native");
envMap.put("SPARK_HOME","/opt/cloudera/parcels/CDH-5.5.1-1.cdh5.5.1.p0.11/lib/spark");
envMap.put("DEFAULT_HADOOP_HOME","/opt/cloudera/parcels/CDH-5.5.1-1.cdh5.5.1.p0.11/lib/hadoop");
envMap.put("SPARK_DIST_CLASSPATH","all jars under /opt/cloudera/parcels/CDH-5.5.1-1.cdh5.5.1.p0.11/jars");
envMap.put("HADOOP_HOME","/opt/cloudera/parcels/CDH-5.5.1-1.cdh5.5.1.p0.11/lib/hadoop");
SparkLauncher sparklauncher = new SparkLauncher(envMap)
.setAppResource("myapp.jar")
.setSparkHome("/opt/cloudera/parcels/CDH-5.5.1-1.cdh5.5.1.p0.11/lib/spark/")
.setMainClass("spark.HelloSpark")
.setMaster("yarn-cluster")
.setConf(SparkLauncher.DRIVER_MEMORY, "2g")
.setConf("spark.driver.userClassPathFirst", "true")
.setConf("spark.executor.userClassPathFirst", "true").launch();
Every time, I got
User class threw exception: java.lang.UnsatisfiedLinkError:
org.xerial.snappy.SnappyNative.maxCompressedLength(I)I
It looks like your jar is including Spark/Hadoop libraries that have conflict with other libraries in the cluster. Check that your Spark and Hadoop dependencies are marked as provided.

Access SparkConf from the worker

Is there a way to get spark configurations from the worker (i.e. inside the closure of a map function). I tried using
SparkEnv.get().conf()
but it seems to not contain all the custom spark configs that I've set prior to creating SparkContext
EDIT:
Through SparkEnv I'm able to get default configurations set via spark-defaults.config but all confs I set explicitly through the setter method
SparkConf conf = new SparkConf()
conf.set("my.configuration.key", "myConfigValue")
SparkContext sc = new SparkContext(conf)
are not present in the SparkConf object I get through SparkEnv.get().conf()
SparkEnv is a part of the developer API and is not intended for external use.
You can simply create a broadcast variable, though.
val confBd = sc.broadcast(sc.getConf.getAll.toMap)
rdd.foreachPartition(_ => println(confBd.value.get("spark.driver.host")))

Idiomatic way to create a spark context with a default master

Is there an idiomatic way to create a spark context, that if no other master is provided will default to some fall back master?
e.g.
new SparkContext(defaultMaster = "local[4]")
If I run this with let's say, spark-submit and specify a master as a CLI param, or via an env variable, it will use that, but if I run it without specifying anything, it will default to what I provided above.
Is there a built in way to achieve this? (I have workarounds but I was wondering if there is a common pattern for this behavior)
You can use the following:
val conf = new SparkConf()
conf.setIfMissing("spark.master", "local[4]")
val sc = new SparkContext(conf)
you can set the default master url in conf/spark-defaults.conf in the Spark directory
or
Use :
val conf = new SparkConf()
conf.setMaster("local[4]")
val sc = new SparkContext(conf)
And whenever you set master url using --master it overrides the default values.

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