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| -rw-r--r-- | docs/configuration.md | 13 | ||||
| -rw-r--r-- | docs/tuning.md | 54 |
2 files changed, 44 insertions, 23 deletions
diff --git a/docs/configuration.md b/docs/configuration.md index d961f43acf..c496146e3e 100644 --- a/docs/configuration.md +++ b/docs/configuration.md @@ -722,17 +722,20 @@ Apart from these, the following properties are also available, and may be useful Fraction of the heap space used for execution and storage. The lower this is, the more frequently spills and cached data eviction occur. The purpose of this config is to set aside memory for internal metadata, user data structures, and imprecise size estimation - in the case of sparse, unusually large records. + in the case of sparse, unusually large records. Leaving this at the default value is + recommended. For more detail, see <a href="tuning.html#memory-management-overview"> + this description</a>. </td> </tr> <tr> <td><code>spark.memory.storageFraction</code></td> <td>0.5</td> <td> - The size of the storage region within the space set aside by - <code>spark.memory.fraction</code>. This region is not statically reserved, but dynamically - allocated as cache requests come in. Cached data may be evicted only if total storage exceeds - this region. + Amount of storage memory immune to eviction, expressed as a fraction of the size of the + region set aside by <code>spark.memory.fraction</code>. The higher this is, the less + working memory may be available to execution and tasks may spill to disk more often. + Leaving this at the default value is recommended. For more detail, see + <a href="tuning.html#memory-management-overview">this description</a>. </td> </tr> <tr> diff --git a/docs/tuning.md b/docs/tuning.md index 879340a015..a8fe7a4532 100644 --- a/docs/tuning.md +++ b/docs/tuning.md @@ -88,9 +88,39 @@ than the "raw" data inside their fields. This is due to several reasons: but also pointers (typically 8 bytes each) to the next object in the list. * Collections of primitive types often store them as "boxed" objects such as `java.lang.Integer`. -This section will discuss how to determine the memory usage of your objects, and how to improve -it -- either by changing your data structures, or by storing data in a serialized format. -We will then cover tuning Spark's cache size and the Java garbage collector. +This section will start with an overview of memory management in Spark, then discuss specific +strategies the user can take to make more efficient use of memory in his/her application. In +particular, we will describe how to determine the memory usage of your objects, and how to +improve it -- either by changing your data structures, or by storing data in a serialized +format. We will then cover tuning Spark's cache size and the Java garbage collector. + +## Memory Management Overview + +Memory usage in Spark largely falls under one of two categories: execution and storage. +Execution memory refers to that used for computation in shuffles, joins, sorts and aggregations, +while storage memory refers to that used for caching and propagating internal data across the +cluster. In Spark, execution and storage share a unified region (M). When no execution memory is +used, storage can acquire all the available memory and vice versa. Execution may evict storage +if necessary, but only until total storage memory usage falls under a certain threshold (R). +In other words, `R` describes a subregion within `M` where cached blocks are never evicted. +Storage may not evict execution due to complexities in implementation. + +This design ensures several desirable properties. First, applications that do not use caching +can use the entire space for execution, obviating unnecessary disk spills. Second, applications +that do use caching can reserve a minimum storage space (R) where their data blocks are immune +to being evicted. Lastly, this approach provides reasonable out-of-the-box performance for a +variety of workloads without requiring user expertise of how memory is divided internally. + +Although there are two relevant configurations, the typical user should not need to adjust them +as the default values are applicable to most workloads: + +* `spark.memory.fraction` expresses the size of `M` as a fraction of the total JVM heap space +(default 0.75). The rest of the space (25%) is reserved for user data structures, internal +metadata in Spark, and safeguarding against OOM errors in the case of sparse and unusually +large records. +* `spark.memory.storageFraction` expresses the size of `R` as a fraction of `M` (default 0.5). +`R` is the storage space within `M` where cached blocks immune to being evicted by execution. + ## Determining Memory Consumption @@ -151,18 +181,6 @@ time spent GC. This can be done by adding `-verbose:gc -XX:+PrintGCDetails -XX:+ each time a garbage collection occurs. Note these logs will be on your cluster's worker nodes (in the `stdout` files in their work directories), *not* on your driver program. -**Cache Size Tuning** - -One important configuration parameter for GC is the amount of memory that should be used for caching RDDs. -By default, Spark uses 60% of the configured executor memory (`spark.executor.memory`) to -cache RDDs. This means that 40% of memory is available for any objects created during task execution. - -In case your tasks slow down and you find that your JVM is garbage-collecting frequently or running out of -memory, lowering this value will help reduce the memory consumption. To change this to, say, 50%, you can call -`conf.set("spark.storage.memoryFraction", "0.5")` on your SparkConf. Combined with the use of serialized caching, -using a smaller cache should be sufficient to mitigate most of the garbage collection problems. -In case you are interested in further tuning the Java GC, continue reading below. - **Advanced GC Tuning** To further tune garbage collection, we first need to understand some basic information about memory management in the JVM: @@ -183,9 +201,9 @@ temporary objects created during task execution. Some steps which may be useful * Check if there are too many garbage collections by collecting GC stats. If a full GC is invoked multiple times for before a task completes, it means that there isn't enough memory available for executing tasks. -* In the GC stats that are printed, if the OldGen is close to being full, reduce the amount of memory used for caching. - This can be done using the `spark.storage.memoryFraction` property. It is better to cache fewer objects than to slow - down task execution! +* In the GC stats that are printed, if the OldGen is close to being full, reduce the amount of + memory used for caching by lowering `spark.memory.storageFraction`; it is better to cache fewer + objects than to slow down task execution! * If there are too many minor collections but not many major GCs, allocating more memory for Eden would help. You can set the size of the Eden to be an over-estimate of how much memory each task will need. If the size of Eden |