RE: does calling cache()/persist() on a RDD trigger its immediate evaluation?

[Kapil Malik]

Hi Pengcheng YIN,
RDD cache / persist calls do not trigger evaluation.
Unpersist call is blocking (it does have an async flavor but am not sure what 
are the SLAs on behavior).

val rdd = sc.textFile().map()
rdd.persist() // This does not trigger actual storage
while(true){
    val count = rdd.filter().count // this will trigger storage of RDD, so far 
so good
    if(count == 0)
        break
    
    newRdd = /* some codes that use `rdd` several times, and produce an new RDD 
*/
    rdd.unpersist() // This is immediate !!, if newRDD has not been evaluated + 
stored yet, it's not good
    rdd = newRdd.persist() // this will do nothing till next iteration of loop 
(at count).
}

IMHO, last 3 lines can be replaced with -

    newRdd = /* some codes that use `rdd` several times, and produce an new RDD 
*/
ADDED -->   newRdd.persist( ) // mark for storage
ADDED  -->   newRdd.filter( ... ).count // trigger storage
    rdd.unpersist()
    rdd = newRdd

Although, others can correct me if I am mistaken. You can also verify this with 
small dataset.

Thanks,

Kapil 
-----Original Message-----
From: Pengcheng YIN [mailto:pcyin1...@gmail.com] 
Sent: 04 January 2015 12:53
To: user@spark.apache.org
Subject: does calling cache()/persist() on a RDD trigger its immediate 
evaluation?

Hi Pro,

I have a question regarding calling cache()/persist() on an RDD. All RDDs in 
Spark are lazily evaluated, but does calling cache()/persist() on a RDD trigger 
its immediate evaluation?

My spark app is something like this:

val rdd = sc.textFile().map()
rdd.persist()
while(true){
    val count = rdd.filter().count
    if(count == 0)
        break
    
    newRdd = /* some codes that use `rdd` several times, and produce an new RDD 
*/
    rdd.unpersist()
    rdd = newRdd.persist()
}

In each iteration, I persist `rdd`, and unpersist it at the end of the 
iteration, replace `rdd` with persisted `newRdd`. My concern is that, if RDD is 
not evaluated and persisted when persist() is called, I need to change the 
position of persist()/unpersist() called to make it more efficient.

Thanks,
Pengcheng




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