问题
Can any one please correct my understanding on persisting by Spark.
If we have performed a cache() on an RDD its value is cached only on those nodes where actually RDD was computed initially. Meaning, If there is a cluster of 100 Nodes, and RDD is computed in partitions of first and second nodes. If we cached this RDD, then Spark is going to cache its value only in first or second worker nodes. So when this Spark application is trying to use this RDD in later stages, then Spark driver has to get the value from first/second nodes.
Am I correct?
(OR)
Is it something that the RDD value is persisted in driver memory and not on nodes ?
回答1:
Change this:
then Spark is going to cache its value only in first or second worker nodes.
to this:
then Spark is going to cache its value only in first and second worker nodes.
and...Yes correct!
Spark tries to minimize the memory usage (and we love it for that!), so it won't make any unnecessary memory loads, since it evaluates every statement lazily, i.e. it won't do any actual work on any transformation, it will wait for an action to happen, which leaves no choice to Spark, than to do the actual work (read the file, communicate the data to the network, do the computation, collect the result back to the driver, for example..).
You see, we don't want to cache everything, unless we really can to (that is that the memory capacity allows for it (yes, we can ask for more memory in the executors or/and the driver, but sometimes our cluster just doesn't have the resources, really common when we handle big data) and it really makes sense, i.e. that the cached RDD is going to be used again and again (so caching it will speedup the execution of our job).
That's why you want to unpersist() your RDD, when you no longer need it...! :)
Check this image, is from one of my jobs, where I had requested 100 executors, however the Executors tab displayed 101, i.e. 100 slaves/workers and one master/driver:
回答2:
RDD.cache is a lazy operation. it does nothing until unless you call an action like count. Once you call the action the operation will use the cache. It will just take the data from the cache and do the operation.
RDD.cache- Persists the RDD with default storage level (Memory only). Spark RDD API
2.Is it something that the RDD value is persisted in driver memory and not on nodes ?
RDD can be persisted to disk and Memory as well . Click on the link to Spark document for all the option Spark Rdd Persist
回答3:
# no actual caching at the end of this statement
rdd1=sc.read('myfile.json').rdd.map(lambda row: myfunc(row)).cache()
# again, no actual caching yet, because Spark is lazy, and won't evaluate anything unless
# a reduction op
rdd2=rdd2.map(mysecondfunc)
# caching is done on this reduce operation. Result of rdd1 will be cached in the memory of each worker node
n=rdd1.count()
So to answer your question
If we have performed a cache() on an RDD its value is cached only on those nodes where actually RDD was computed initially
The only possibility of caching something is on worker nodes, and not on driver nodes.
cache function can only be applied to an RDD (refer), and since RDD only exists on the worker node's memory (Resilient Distributed Datasets!), it's results are cached in the respective worker node memory. Once you apply an operation like count which brings back the result to the driver, it's not really an RDD anymore, it's merely a result of computation done RDD by the worker nodes in their respective memories
Since cache in the above example was called on rdd2 which is still on multiple worker nodes, the caching only happens on the worker node's memory.
In the above example, when do some map-red op on rdd1 again, it won't read the JSON again, because it was cached
FYI, I am using the word memory based on the assumption that the caching level is set to MEMORY_ONLY. Of course, if that level is changed to others, Spark will cache to either memory or storage based on the setting
回答4:
Here is an excellent answer on caching
(Why) do we need to call cache or persist on a RDD
Basically caching stores the RDD in the memory / disk (based on persistence level set) of that node, so that the when this RDD is called again it does not need to recompute its lineage (lineage - Set of prior transformations executed to be in the current state).
来源：https://stackoverflow.com/questions/39196748/is-spark-rdd-cached-on-worker-node-or-driver-node-or-both