官方原文如下
Printing elements of an RDD
Another common idiom is attempting to print out the elements of an RDD using
rdd.foreach(println)
or
rdd.map(println)
. On a single machine, this will generate the expected output and print all the RDD’s elements. However, in
cluster
mode, the output to
stdout
being called by the executors is now writing to the executor’s
stdout
instead, not the one on the driver, so
stdout
on the driver won’t show these! To print all elements on the driver, one can use the
collect()
method to first bring the RDD to the driver node thus:
rdd.collect().foreach(println)
. This can cause the driver to run out of memory, though, because
collect()
fetches the entire RDD to a single machine; if you only need to print a few elements of the RDD, a safer approach is to use the
take()
:
rdd.take(100).foreach(println)
.
主要意思是:
列印一個彈性分布式資料集元素,使用時要注意不要導緻記憶體溢出!
建議使用
take()
:
rdd.take(100).foreach(println),
而不使用rdd.collect().foreach(println)。
因為後者會導緻記憶體溢出!!