I am working on a Spark application that is using of a large (~3G) broadcast variable as a lookup table. The application refines the data in this lookup table in an iterative manner. So this large variable is broadcast many times during the lifetime of the application process.
From what I have observed perhaps 60% of the execution time is spent waiting for the variable to broadcast in each iteration. My reading of a Spark performance article suggests that the time spent broadcasting will increase with the number of nodes I add.
My question for the group - what would you suggest as an alternative to broadcasting a large variable like this?
One approach I have considered is segmenting my RDD and adding a copy of the lookup table for each X number of values to process. So, for example, if I have a list of 1 million entries to process (eg, RDD[Entry]), I could split this into segments of 100K entries, with a copy of the lookup table, and make that an RDD[(Lookup, Array[Entry]).
Another solution I am looking at it is making the lookup table an RDD instead of a broadcast variable. Perhaps I could use an IndexedRDD to improve performance. One issue with this approach is that I would have to rewrite my application code to use two RDDs so that I do not reference the lookup RDD in the from within the closure of another RDD.
Any other recommendations?