How to improve performance for slow Spark jobs using DataFrame and JDBC connection?

问题

I am trying to access a mid-size Teradata table (~100 million rows) via JDBC in standalone mode on a single node (local[*]).

I am using Spark 1.4.1. and is setup on a very powerful machine(2 cpu, 24 cores, 126G RAM).

I have tried several memory setup and tuning options to make it work faster, but neither of them made a huge impact.

I am sure there is something I am missing and below is my final try that took about 11 minutes to get this simple counts vs it only took 40 seconds using a JDBC connection through R to get the counts.

bin/pyspark --driver-memory 40g --executor-memory 40g

df = sqlContext.read.jdbc("jdbc:teradata://......)
df.count()

When I tried with BIG table (5B records) then no results returned upon completion of query.

回答1:

All of the aggregation operations are performed after the whole dataset is retrieved into memory into a DataFrame collection. So doing the count in Spark will never be as efficient as it would be directly in TeraData. Sometimes it's worth it to push some computation into the database by creating views and then mapping those views using the JDBC API.

Every time you use the JDBC driver to access a large table you should specify the partitioning strategy otherwise you will create a DataFrame/RDD with a single partition and you will overload the single JDBC connection.

Instead you want to try the following AI (since Spark 1.4.0+):

sqlctx.read.jdbc(
  url = "<URL>",
  table = "<TABLE>",
  columnName = "<INTEGRAL_COLUMN_TO_PARTITION>", 
  lowerBound = minValue,
  upperBound = maxValue,
  numPartitions = 20,
  connectionProperties = new java.util.Properties()
)

There is also an option to push down some filtering.

If you don't have an uniformly distributed integral column you want to create some custom partitions by specifying custom predicates (where statements). For example let's suppose you have a timestamp column and want to partition by date ranges:

    val predicates = 
  Array(
    "2015-06-20" -> "2015-06-30",
    "2015-07-01" -> "2015-07-10",
    "2015-07-11" -> "2015-07-20",
    "2015-07-21" -> "2015-07-31"
  )
  .map {
    case (start, end) => 
      s"cast(DAT_TME as date) >= date '$start'  AND cast(DAT_TME as date) <= date '$end'"
  }

 predicates.foreach(println) 

// Below is the result of how predicates were formed 
//cast(DAT_TME as date) >= date '2015-06-20'  AND cast(DAT_TME as date) <= date '2015-06-30'
//cast(DAT_TME as date) >= date '2015-07-01'  AND cast(DAT_TME as date) <= date '2015-07-10'
//cast(DAT_TME as date) >= date '2015-07-11'  AND cast(DAT_TME as date) <= date //'2015-07-20'
//cast(DAT_TME as date) >= date '2015-07-21'  AND cast(DAT_TME as date) <= date '2015-07-31'


sqlctx.read.jdbc(
  url = "<URL>",
  table = "<TABLE>",
  predicates = predicates,
  connectionProperties = new java.util.Properties()
)

It will generate a DataFrame where each partition will contain the records of each subquery associated to the different predicates.

Check the source code at DataFrameReader.scala

回答2:

Does the unserialized table fit into 40 GB? If it starts swapping on disk performance will decrease drammatically.

Anyway when you use a standard JDBC with ansi SQL syntax you leverage the DB engine, so if teradata ( I don't know teradata ) holds statistics about your table, a classic "select count(*) from table" will be very fast. Instead spark, is loading your 100 million rows in memory with something like "select * from table" and then will perform a count on RDD rows. It's a pretty different workload.

回答3:

One solution that differs from others is to save the data from the oracle table in an avro file (partitioned in many files) saved on hadoop. This way reading those avro files with spark would be a peace of cake since you won't call the db anymore.

来源：https://stackoverflow.com/questions/32188295/how-to-improve-performance-for-slow-spark-jobs-using-dataframe-and-jdbc-connecti

标签

apache-spark

teradata

pyspark

spark-dataframe