reduce: function takes accumulated value and next value to find some aggregation.
reduceByKey: is also the same operation with specified key.
reduceGroups: is apply specified operation to the grouped data.
I don't know how memory managed for these operations. For example, how data is taken while using reduce function(e.g all data loaded to the memory?)? I want to know how data is managed for reduce operations. I also want to know what is the difference between these operations according to the data management.
Reduce is one of the cheapest operations in Spark,since that the only thing it does is actually grouping similar data to the same node.The only cost of a reduce operation is the reading of the tuple and a decision of where it should be grouped.
This means that the simple reduce,in contrast to the reduceByKey or reduceGroups is more expensive because Spark does not know how to make the grouping and searches for correlations among tuples.
Reduce can also ignore a tuple if it does not meet any requirement.
Related
I have come across several sources of information, such as the one found here, which explain "predicate pushdown" as :
… if you can “push down” parts of the query to where the data is stored, and thus filter out most of the data, then you can greatly reduce network traffic.
However, I have also seen the term "projection pushdown" in other documentation such as here, which appears to be the same thing but I am not sure in my understanding.
Is there a specific difference between the two terms?
Predicate refers to the where/filter clause which effects the amount of rows returned.
Projection refers to the selected columns.
For example:
If your filters pass only 5% of the rows, only 5% of the table will be passed from the storage to Spark instead of the full table.
If your projection selects only 3 columns out of 10, then less columns will be passed from the storage to Spark and if your storage is columnar (e.g. Parquet, not Avro) and the non selected columns are not a part of the filter, then these columns won't even have to be read.
In set and bag relational algebra alike, predicate pushdown eliminates tuples.
In bag relational algebra, projection pushdown eliminates attributes ("columns"), but in case of column based storage, it doesn't matter much, becasue columns that are not used higher up aren't being carried to begin with. Even a row based database may or may not benefit from projection pushdown (even SQL doesn't specify a physical access plan). Projection in bag RA is a very nominal operation that can be physically done at just the metadata level (flag some columns as inaccessible).
In set relational algebra, projection pushdown generally eliminates tuples as well, so this is where it has significance. Set RA projection is not an inexpensive operation, due to the need for deduplication. It's like a GROUP BY with no aggregated fields. Still, it's often worth doing the projection before a join, due to a possible vast decrease of tuple count.
Bag algebra tools eg. SQL also have ways for set RA projection, such as SELECT DISTINCT.
Neither predicate pushdown nor projection pushdown lead to more optimal execution in all cases. Depends on their selectivity and a lot of other things. Still, esp. predicate pushdown is a good heuristic as joins tend to be most expensive.
If the pushed-down projection has a sort index for the retained fields, or it needs to scan the table, there may be join algorithms with which the projection can be fused, avoiding a double reading of table/index structures.
I'm learning Spark and start understanding how Spark distributes the data and combines the results.
I came to the conclusion that using the operation map followed by reduce has an advantage on using just the operation aggregate. This is (at least I believe so) because aggregate uses a sequential operation, which hurts parallelism, while map and reduce can benefit from full parallelism.
So when having a choice, isn't it better to use map and reduce than aggregate ? Are there cases where aggregate is preferred ? Or maybe when aggregate can't be replaced by the combination map and reduce ?
As an example - I want to find the string with the max length:
val z = sc.parallelize(List("123","12","345","4567"))
// instead of this aggregate ....
z.aggregate(0)((x, y) => math.max(x, y.length), (x, y) => math.max(x, y))
// .... shouldn't I rather use this map - reduce combination ?
z.map(_.length).reduce((x, y) => math.max(x, y))
A little example will can be better than long explanations.
Imagine you have a class Toto with an age field. You have many Toto and you desire to compute sum of ages of every Toto.
final case class Toto(val age: Int)
val rdd = sc.parallelize(0 until n).map(Toto(_))
// map/reduce style
val sum1 = rdd
// O(n) operations to go througth every Toto's age
.map(_.age)
// another O(n) to access data then O(n) operations to sum the n values
.reduce(_ + _)
// You get the result with 2 pass over your data plus O(n) additions
// aggregate style
val sum2 = rdd.aggregate(0)((agg, e) => agg + e.age, _ + _)
// With one pass over the data, and O(n) additions you obtain the same result
It's a bit more complicate if you take into account access and each operations.
Because aggregate still access then sum the age into the aggregate wich represent O(2.n) operations, O(n) access plus O(n) additions, plus negligeable merged operation between aggregates.
On the other side with map/reduce style, first the map represent O(n) access, then again O(n) access to data to reduce them with an overhead of O(n) addition operations for a total of O(3.n) operations.
Without forgetting the fact that Spark is lazy and all of your transformation will be leverage by a final action.
I presume that using aggregate will save some operations and then will improve application running time. But depending on what you're doing it could be more usefull to express successive map followed by a reduce for readability compare to an aggregate or combineByKey (generalization of aggregateByKey). So i will suppose that it depends on which goals you desire to reach depending the use case.
I believe I can partially answer my own question. I was wrongly assuming that, because a sequential operation is used, aggregate might be hurt in its parallelism. The data can still be parallelized and the sequential op will be executed on each chunk. This doesn't seem less performing than the map operation. So then the question that remains is: why would you use aggregate as opposed to the map-reduce combination ?
Aggregate operation allows to specify a combiner function (to reduce the amount of data sent through the shuffle), which is different to reducer, with map-reduce combination the same function is used to combine and reduce. I know used old Map Reduce terminology but conceptually all shared nothing shuffle based frameworks do this and if you google for mapreduce combiner you will find a lot of explanations of the concept.
I recently played around with UDAFs and looked into the sourcecode of the built-in aggregation function collect_list, I was suprised to see that collect_list does not have a merge method implemented, although I think this is really straight-farward (just concatenate two Arrays). Code taken from org.apache.spark.sql.catalyst.expressions.aggregate.collect.Collect
override def merge(buffer: InternalRow, input: InternalRow): Unit = {
sys.error("Collect cannot be used in partial aggregations.")
}
It is no longer the case, as SPARK-1893 but I'd assume that the initial design had mostly collect_list in mind.
Because collect_list is logically equivalent to groupByKey the motivation would be exactly the same to avoid long GC pauses. In particular map side combine in groupByKey has been disabled with Spark SPARK-772:
Map side combine in group by key case does not reduce the amount of data shuffled. Instead, it forces a lot more objects to go into old gen, and leads to worse GC.
So to address you comment
I think this is really straight-farward (just concatenate two Arrays).
It might be simple but it doesn't add much value (unless there is another reducing operation on top of it) and sequence concatenation is expensive.
What is the difference between reduce and reduceByKey in Apache Spark in terms of their functionalities?
Why reduceByKey is a transformation and reduce is an action?
This is close to a duplicate of my answer explaining reduceByKey, but I will elaborate to the specific part that makes the two different. However refer to my answer for a bit more specifics on the internals of reduceByKey.
Basically, reduce must pull the entire dataset down into a single location because it is reducing to one final value. reduceByKey on the other hand is one value for each key. And since this action can be run on each machine locally first then it can remain an RDD and have further transformations done on its dataset.
Note, however that there is a reduceByKeyLocally you can use to automatically pull down the Map to a single location also.
Please go through this official documentation link .
reduce is an action which Aggregate the elements of the dataset using a function func (which takes two arguments and returns one),also we can use reduce for single RDDs (for more info Please click HERE).
reduceByKey When called on a dataset of (K, V) pairs, returns a dataset of (K, V) pairs where the values for each key are aggregated using the given reduce function func, which must be of type (V,V) => V. (for more info Please click HERE)
this is the qt assistant :
reduce(f): Reduces the elements of this RDD using the specified
commutative and associative binary operator. Currently reduces
partitions locally.
reduceByKey(func, numPartitions=None, partitionFunc=) :
Merge the values for each key using an associative and commutative reduce
function.
I just discovered the RDD.zip() method and I cannot imagine what its contract could possibly be.
I understand what it does, of course. However, it has always been my understanding that
the order of elements in an RDD is a meaningless concept
the number of partitions and their sizes is an implementation detail only available to the user for performance tuning
In other words, an RDD is a (multi)set, not a sequence (and, of course, in, e.g., Python one gets AttributeError: 'set' object has no attribute 'zip')
What is wrong with my understanding above?
What was the rationale behind this method?
Is it legal outside the trivial context like a.map(f).zip(a)?
EDIT 1:
Another crazy method is zipWithIndex(), as well as well as the various zipPartitions() variants.
Note that first() and take() are not crazy because they are just (non-random) samples of the RDD.
collect() is also okay - it just converts a set to a sequence which is perfectly legit.
EDIT 2: The reply says:
when you compute one RDD from another the order of elements in the new RDD may not correspond to that in the old one.
This appears to imply that even the trivial a.map(f).zip(a) is not guaranteed to be equivalent to a.map(x => (f(x),x)). What is the situation when zip() results are reproducible?
It is not true that RDDs are always unordered. An RDD has a guaranteed order if it is the result of a sortBy operation, for example. An RDD is not a set; it can contain duplicates. Partitioning is not opaque to the caller, and can be controlled and queried. Many operations do preserve both partitioning and order, like map. That said I find it a little easy to accidentally violate the assumptions that zip depends on, since they're a little subtle, but it certainly has a purpose.
The mental model I use (and recommend) is that the elements of an RDD are ordered, but when you compute one RDD from another the order of elements in the new RDD may not correspond to that in the old one.
For those who want to be aware of partitions, I'd say that:
The partitions of an RDD have an order.
The elements within a partition have an order.
If you think of "concatenating" the partitions (say laying them "end to end" in order) using the order of elements within them, the overall ordering you end up with corresponds to the order of elements if you ignore partitions.
But again, if you compute one RDD from another, all bets about the order relationships of the two RDDs are off.
Several members of the RDD class (I'm referring to the Scala API) strongly suggest an order concept (as does their documentation):
collect()
first()
partitions
take()
zipWithIndex()
as does Partition.index as well as SparkContext.parallelize() and SparkContext.makeRDD() (which both take a Seq[T]).
In my experience these ways of "observing" order give results that are consistent with each other, and the ones that translate back and forth between RDDs and ordered Scala collections behave as you would expect -- they preserve the overall order of elements. This is why I say that, in practice, RDDs have a meaningful order concept.
Furthermore, while there are obviously many situations where computing an RDD from another must change the order, in my experience order tends to be preserved where it is possible/reasonable to do so. Operations that don't re-partition and don't fundamentally change the set of elements especially tend to preserve order.
But this brings me to your question about "contract", and indeed the documentation has a problem in this regard. I have not seen a single place where an operation's effect on element order is made clear. (The OrderedRDDFunctions class doesn't count, because it refers to an ordering based on the data, which may differ from the raw order of elements within the RDD. Likewise the RangePartitioner class.) I can see how this might lead you to conclude that there is no concept of element order, but the examples I've given above make that model unsatisfying to me.