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Diffstat (limited to 'docs/scala-programming-guide.md')
| -rw-r--r-- | docs/scala-programming-guide.md | 127 |
1 files changed, 93 insertions, 34 deletions
diff --git a/docs/scala-programming-guide.md b/docs/scala-programming-guide.md index 99412733d4..77373890ee 100644 --- a/docs/scala-programming-guide.md +++ b/docs/scala-programming-guide.md @@ -23,7 +23,7 @@ To write a Spark application, you need to add a dependency on Spark. If you use groupId = org.apache.spark artifactId = spark-core_{{site.SCALA_BINARY_VERSION}} - version = {{site.SPARK_VERSION}} + version = {{site.SPARK_VERSION}} In addition, if you wish to access an HDFS cluster, you need to add a dependency on `hadoop-client` for your version of HDFS: @@ -73,14 +73,14 @@ The master URL passed to Spark can be in one of the following formats: <table class="table"> <tr><th>Master URL</th><th>Meaning</th></tr> <tr><td> local </td><td> Run Spark locally with one worker thread (i.e. no parallelism at all). </td></tr> -<tr><td> local[K] </td><td> Run Spark locally with K worker threads (ideally, set this to the number of cores on your machine). +<tr><td> local[K] </td><td> Run Spark locally with K worker threads (ideally, set this to the number of cores on your machine). </td></tr> -<tr><td> spark://HOST:PORT </td><td> Connect to the given <a href="spark-standalone.html">Spark standalone - cluster</a> master. The port must be whichever one your master is configured to use, which is 7077 by default. +<tr><td> spark://HOST:PORT </td><td> Connect to the given <a href="spark-standalone.html">Spark standalone + cluster</a> master. The port must be whichever one your master is configured to use, which is 7077 by default. </td></tr> -<tr><td> mesos://HOST:PORT </td><td> Connect to the given <a href="running-on-mesos.html">Mesos</a> cluster. - The host parameter is the hostname of the Mesos master. The port must be whichever one the master is configured to use, - which is 5050 by default. +<tr><td> mesos://HOST:PORT </td><td> Connect to the given <a href="running-on-mesos.html">Mesos</a> cluster. + The host parameter is the hostname of the Mesos master. The port must be whichever one the master is configured to use, + which is 5050 by default. </td></tr> </table> @@ -265,11 +265,25 @@ A complete list of actions is available in the [RDD API doc](api/core/index.html ## RDD Persistence -One of the most important capabilities in Spark is *persisting* (or *caching*) a dataset in memory across operations. When you persist an RDD, each node stores any slices of it that it computes in memory and reuses them in other actions on that dataset (or datasets derived from it). This allows future actions to be much faster (often by more than 10x). Caching is a key tool for building iterative algorithms with Spark and for interactive use from the interpreter. - -You can mark an RDD to be persisted using the `persist()` or `cache()` methods on it. The first time it is computed in an action, it will be kept in memory on the nodes. The cache is fault-tolerant -- if any partition of an RDD is lost, it will automatically be recomputed using the transformations that originally created it. - -In addition, each RDD can be stored using a different *storage level*, allowing you, for example, to persist the dataset on disk, or persist it in memory but as serialized Java objects (to save space), or even replicate it across nodes. These levels are chosen by passing a [`org.apache.spark.storage.StorageLevel`](api/core/index.html#org.apache.spark.storage.StorageLevel) object to `persist()`. The `cache()` method is a shorthand for using the default storage level, which is `StorageLevel.MEMORY_ONLY` (store deserialized objects in memory). The complete set of available storage levels is: +One of the most important capabilities in Spark is *persisting* (or *caching*) a dataset in memory +across operations. When you persist an RDD, each node stores any slices of it that it computes in +memory and reuses them in other actions on that dataset (or datasets derived from it). This allows +future actions to be much faster (often by more than 10x). Caching is a key tool for building +iterative algorithms with Spark and for interactive use from the interpreter. + +You can mark an RDD to be persisted using the `persist()` or `cache()` methods on it. The first time +it is computed in an action, it will be kept in memory on the nodes. The cache is fault-tolerant -- +if any partition of an RDD is lost, it will automatically be recomputed using the transformations +that originally created it. + +In addition, each RDD can be stored using a different *storage level*, allowing you, for example, to +persist the dataset on disk, or persist it in memory but as serialized Java objects (to save space), +or replicate it across nodes, or store the data in off-heap memory in [Tachyon](http://tachyon-project.org/). +These levels are chosen by passing a +[`org.apache.spark.storage.StorageLevel`](api/core/index.html#org.apache.spark.storage.StorageLevel) +object to `persist()`. The `cache()` method is a shorthand for using the default storage level, +which is `StorageLevel.MEMORY_ONLY` (store deserialized objects in memory). The complete set of +available storage levels is: <table class="table"> <tr><th style="width:23%">Storage Level</th><th>Meaning</th></tr> @@ -292,8 +306,16 @@ In addition, each RDD can be stored using a different *storage level*, allowing </tr> <tr> <td> MEMORY_AND_DISK_SER </td> - <td> Similar to MEMORY_ONLY_SER, but spill partitions that don't fit in memory to disk instead of recomputing them - on the fly each time they're needed. </td> + <td> Similar to MEMORY_ONLY_SER, but spill partitions that don't fit in memory to disk instead of + recomputing them on the fly each time they're needed. </td> +</tr> +<tr> + <td> OFF_HEAP </td> + <td> Store RDD in a <i>serialized</i> format in Tachyon. + This is generally more space-efficient than deserialized objects, especially when using a + <a href="tuning.html">fast serializer</a>, but more CPU-intensive to read. + This also significantly reduces the overheads of GC. + </td> </tr> <tr> <td> DISK_ONLY </td> @@ -307,30 +329,59 @@ In addition, each RDD can be stored using a different *storage level*, allowing ### Which Storage Level to Choose? -Spark's storage levels are meant to provide different tradeoffs between memory usage and CPU efficiency. -We recommend going through the following process to select one: - -* If your RDDs fit comfortably with the default storage level (`MEMORY_ONLY`), leave them that way. This is the most - CPU-efficient option, allowing operations on the RDDs to run as fast as possible. -* If not, try using `MEMORY_ONLY_SER` and [selecting a fast serialization library](tuning.html) to make the objects - much more space-efficient, but still reasonably fast to access. -* Don't spill to disk unless the functions that computed your datasets are expensive, or they filter a large - amount of the data. Otherwise, recomputing a partition is about as fast as reading it from disk. -* Use the replicated storage levels if you want fast fault recovery (e.g. if using Spark to serve requests from a web - application). *All* the storage levels provide full fault tolerance by recomputing lost data, but the replicated ones - let you continue running tasks on the RDD without waiting to recompute a lost partition. - -If you want to define your own storage level (say, with replication factor of 3 instead of 2), then use the function factor method `apply()` of the [`StorageLevel`](api/core/index.html#org.apache.spark.storage.StorageLevel$) singleton object. +Spark's storage levels are meant to provide different trade-offs between memory usage and CPU +efficiency. It allows uses to choose memory, disk, or Tachyon for storing data. We recommend going +through the following process to select one: + +* If your RDDs fit comfortably with the default storage level (`MEMORY_ONLY`), leave them that way. + This is the most CPU-efficient option, allowing operations on the RDDs to run as fast as possible. + +* If not, try using `MEMORY_ONLY_SER` and [selecting a fast serialization library](tuning.html) to +make the objects much more space-efficient, but still reasonably fast to access. You can also use +`OFF_HEAP` mode to store the data off the heap in [Tachyon](http://tachyon-project.org/). This will +significantly reduce JVM GC overhead. + +* Don't spill to disk unless the functions that computed your datasets are expensive, or they filter +a large amount of the data. Otherwise, recomputing a partition is about as fast as reading it from +disk. + +* Use the replicated storage levels if you want fast fault recovery (e.g. if using Spark to serve +requests from a web application). *All* the storage levels provide full fault tolerance by +recomputing lost data, but the replicated ones let you continue running tasks on the RDD without +waiting to recompute a lost partition. + +If you want to define your own storage level (say, with replication factor of 3 instead of 2), then +use the function factor method `apply()` of the +[`StorageLevel`](api/core/index.html#org.apache.spark.storage.StorageLevel$) singleton object. + +Spark has a block manager inside the Executors that let you chose memory, disk, or off-heap. The +latter is for storing RDDs off-heap outside the Executor JVM on top of the memory management system +[Tachyon](http://tachyon-project.org/). This mode has the following advantages: + +* Cached data will not be lost if individual executors crash. +* Executors can have a smaller memory footprint, allowing you to run more executors on the same +machine as the bulk of the memory will be inside Tachyon. +* Reduced GC overhead since data is stored in Tachyon. # Shared Variables -Normally, when a function passed to a Spark operation (such as `map` or `reduce`) is executed on a remote cluster node, it works on separate copies of all the variables used in the function. These variables are copied to each machine, and no updates to the variables on the remote machine are propagated back to the driver program. Supporting general, read-write shared variables across tasks would be inefficient. However, Spark does provide two limited types of *shared variables* for two common usage patterns: broadcast variables and accumulators. +Normally, when a function passed to a Spark operation (such as `map` or `reduce`) is executed on a +remote cluster node, it works on separate copies of all the variables used in the function. These +variables are copied to each machine, and no updates to the variables on the remote machine are +propagated back to the driver program. Supporting general, read-write shared variables across tasks +would be inefficient. However, Spark does provide two limited types of *shared variables* for two +common usage patterns: broadcast variables and accumulators. ## Broadcast Variables -Broadcast variables allow the programmer to keep a read-only variable cached on each machine rather than shipping a copy of it with tasks. They can be used, for example, to give every node a copy of a large input dataset in an efficient manner. Spark also attempts to distribute broadcast variables using efficient broadcast algorithms to reduce communication cost. +Broadcast variables allow the programmer to keep a read-only variable cached on each machine rather +than shipping a copy of it with tasks. They can be used, for example, to give every node a copy of a +large input dataset in an efficient manner. Spark also attempts to distribute broadcast variables +using efficient broadcast algorithms to reduce communication cost. -Broadcast variables are created from a variable `v` by calling `SparkContext.broadcast(v)`. The broadcast variable is a wrapper around `v`, and its value can be accessed by calling the `value` method. The interpreter session below shows this: +Broadcast variables are created from a variable `v` by calling `SparkContext.broadcast(v)`. The +broadcast variable is a wrapper around `v`, and its value can be accessed by calling the `value` +method. The interpreter session below shows this: {% highlight scala %} scala> val broadcastVar = sc.broadcast(Array(1, 2, 3)) @@ -340,13 +391,21 @@ scala> broadcastVar.value res0: Array[Int] = Array(1, 2, 3) {% endhighlight %} -After the broadcast variable is created, it should be used instead of the value `v` in any functions run on the cluster so that `v` is not shipped to the nodes more than once. In addition, the object `v` should not be modified after it is broadcast in order to ensure that all nodes get the same value of the broadcast variable (e.g. if the variable is shipped to a new node later). +After the broadcast variable is created, it should be used instead of the value `v` in any functions +run on the cluster so that `v` is not shipped to the nodes more than once. In addition, the object +`v` should not be modified after it is broadcast in order to ensure that all nodes get the same +value of the broadcast variable (e.g. if the variable is shipped to a new node later). ## Accumulators -Accumulators are variables that are only "added" to through an associative operation and can therefore be efficiently supported in parallel. They can be used to implement counters (as in MapReduce) or sums. Spark natively supports accumulators of numeric value types and standard mutable collections, and programmers can add support for new types. +Accumulators are variables that are only "added" to through an associative operation and can +therefore be efficiently supported in parallel. They can be used to implement counters (as in +MapReduce) or sums. Spark natively supports accumulators of numeric value types and standard mutable +collections, and programmers can add support for new types. -An accumulator is created from an initial value `v` by calling `SparkContext.accumulator(v)`. Tasks running on the cluster can then add to it using the `+=` operator. However, they cannot read its value. Only the driver program can read the accumulator's value, using its `value` method. +An accumulator is created from an initial value `v` by calling `SparkContext.accumulator(v)`. Tasks +running on the cluster can then add to it using the `+=` operator. However, they cannot read its +value. Only the driver program can read the accumulator's value, using its `value` method. The interpreter session below shows an accumulator being used to add up the elements of an array: |