Github user ilganeli commented on a diff in the pull request:
https://github.com/apache/spark/pull/5074#discussion_r27055193
--- Diff: docs/programming-guide.md ---
@@ -1086,6 +1086,62 @@ for details.
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+### Shuffle operations
+
+Certain operations within Spark trigger an event known as the shuffle. The
shuffle is Spark's
+mechanism for re-distributing data so that is grouped differently across
partitions. This typically
+involves re-arranging and copying data across executors and machines,
making shuffle a complex and
+costly operation.
+
+#### Background
+
+To understand what happens during the shuffle we can consider the example
of the
+[`reduceByKey`](#ReduceByLink) operation. The `reduceByKey` operation
generates a new RDD where all
+values for a single key are combined into a tuple - the key and the result
of executing a reduce
+function against all values associated with that key. The challenge is
that not all values for a
+single key necessarily reside on the same partition, or even the same
machine, but they must be
+co-located to present a single array per key.
+
+In Spark, data is generally not distributed across partitions to be in the
necessary place for a
+specific operation. During computations, a single task will operate on a
single partition - thus, to
+organize all the data for a single `reduceByKey` reduce task to execute,
Spark needs to perform an
+all-to-all operation. It must read from all partitions to find all the
values for all keys, and then
+organize those such that all values for any key lie within the same
partition - this is called the
+**shuffle**.
+
+Although the set of elements in each partition of newly shuffled data will
be deterministic, the
+ordering of these elements is not. If one desires predictably ordered data
following shuffle
+operations, [`mapPartitions`](#MapPartLink) can be used to sort each
partition or `sortBy` can be
+used to perform a global sort. A similar operation,
+[`repartitionAndSortWithinPartitions`](#Repartition2Link`) coupled with
`mapPartitions`,
+may be used to enact a Hadoop style shuffle.
+
+Operations which can cause a shuffle include **repartition** operations
like
+[`repartition`](#RepartitionLink), and [`coalesce`](#CoalesceLink),
**'byKey** operations
+(except for counting) like [`groupByKey`](#GroupByLink) and
[`reduceByKey`](#ReduceByLink), and
+**join** operations like [`cogroup`](#CogroupLink) and [`join`](#JoinLink).
+
+#### Performance Impact
+**Shuffle** is an expensive operation since it involves disk I/O, data
serialization, and
+network I/O. To organize data for the shuffle, Spark generates two sets of
tasks - map tasks to
+organize the data, and a set of reduce tasks to aggregate it. Internally,
results from individual
+map jobs are kept in memory until they can't fit. Then, these are sorted
based on the target reduce
+task and written to a single file. On the reduce side, tasks read the
relevant sorted blocks.
+
+Certain shuffle operations can consume significant amounts of heap memory
since they generate hash
+tables in memory. Specifically, `reduceByKey` and `aggregateByKey` on the
map-side and `'byKey`
+operations on the reduce-side. When data does not fit in memory Spark will
spill these tables to
+disk, incurring the additional overhead of disk I/O and increased garbage
collection.
+
+Shuffle also generates a large number of intermediate files on disk. As of
Spark 1.3, these files
+are not cleaned up from Spark's temporary storage until Spark is stopped,
which means that
+long-running Spark jobs may consume available disk space. This is done so
the shuffle doesn't need
+to be re-computed if the lineage is re-computed. The temporary storage
directory is specified by the
+`spark.local.dir` configuration parameter when configuring the Spark
context.
+
+Shuffle behavior can be fine-tuned by adjusting a variety of configuration
parameters. See the
--- End diff --
Sandy - are there any examples of cross-document links? I'm not sure how to
do this. Thanks.
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