The Memory Management facility is used to assist the client in avoiding a situation where a space server gets into an out-of-memory failure scenario. Based on the configured cache policy, the memory manager protects the space (and the application, in the case it is running collocated with the space) from consuming memory beyond a defined threshold.
The client/Application is expected to have some business logic that will handle org.openspaces.core.SpaceMemoryShortageException that might be thrown (when using the openspaces API). When the legacy IJSpace interface is used, com.j_spaces.core.MemoryShortageException will be thrown instead. Without such business logic, the space server or a client local cache may eventually exhaust all their parent process available memory resources.
Most of the considerations described in this topic are also relevant for the client application when running a Local Cache running with a LRU Cache policy.
The space memory can be managed using the following mechanisms:
Eviction policy - You can set the policy to run ALL IN CACHE or LRU (Least Recently Used).
Memory Manager - Provides options for controlling the JVM that is hosting the space memory utilization. It allows you to define thresholds for situations where the memory becomes over-utilized.
Cache Eviction Policies
The space supports two cache eviction policies: LRU-Cache Policy (code 0) and ALL IN CACHE-Cache Policy (code 1) defined via the the space-config.engine.cache_policy property. See below example how you can configure it:
LRU-Cache Policy - Assumes the JVM hosting the space instance does not have enough heap to hold all data in memory.
By default ALL IN CACHE policy is used for an in-memory data grid,and LRU-Cache Policy is used for a persistent space with an external data source.
Calculating the Available Memory
Both the ALL_IN_CACHE and LRU cache policies calculate the JVM's available memory to determine if there is a need to throw SpaceMemoryShortageException or to start to evict objects. Calculating the available memory is performed when the following operations are called: abort, changeReplicationState, clear, commit, count, getReplicationStatus, getRuntimeInfo, getSpacePump, getTemplatesInfo, joinReplicationGroup, leaveReplicationGroup, notify, prepare, prepareAndCommit, execute, read, readMultiple, replace, spaceCopy, update, updateMultiple, write.
Before throwing SpaceMemoryShortageException or MemoryShortageException the local cache/local view/space performs an explicit garbage collection call (System.gc()), allowing the JVM to reclaim any unused heap memory. This activity may happen at the client side when running a local cache or a local view, or at the space side (JVM hosting the GSC).
The explicit garbage collection call reduces the probability of throwing SpaceMemoryShortageException or MemoryShortageException in case the JVM does have some available memory left. Still, such a call might impact the client side (when running local cache/view) or space-side responsiveness since during the garbage collection activity, no JVM thread will manage to perform its activity. With a client or space using a large heap size, this might introduce a long pause.
To avoid this behavior, you can add one of the following into your client or space side JVM parameters list:
Only do so if you've determined it necessary, as this disables a feature designed to protect your application.
Handling Large JVM Heap Sizes
When configuring the JVM to use large heap sizes (over 10GB), it is recommended to use the following values:
These values represent 400MB difference between the high_watermark_percentage and the low_watermark_percentage when having 10GB max heap size. The above values will make sure the memory manager will not waste memory, but throw MemoryShortageException when running in ALL_IN_CACHE or evict objects when running in LRU cache policy mode when the absolute amount of JVM available memory is low.
With large JVM heap size it is recommended to use the CMS Garbage collector. This will avoid long Garbage collector pauses.
Memory Manager Activity when Initializing the Space
In this phase of the space life cycle, the space checks for the amount of available memory. This is relevant when the space performs a warm start, such as ExternalDataSource.initialLoad().
Memory Manager and Transient Objects
Transient Objects are specified using the @SpaceClass (persist=false) decoration. You may specify transient decoration at the class or object level (field method level decoration).
When using transient objects, note that they are:
Included in the free heap size calculation.
Included in the count of total objects (for max cache size).
Not evicted when running in LRU cache policy mode.
You may use the transient object option to prevent the space from evicting objects when running in LRU cache policy mode.
Memory Manager's Synchronous Eviction
Since LRU eviction can be costly, it is done in asynchronously by the memory manager. However, when the amount of used memory reaches a threshold, LRU eviction by the memory manager is done synchronously and API calls to the space are blocked. The synchronous eviction watermark can be set by the space-config.engine.memory_usage.synchronous_eviction_watermark_percentage memory manager parameter..
Explicit Eviction of Objects from the Space
Objects can be evicted explicitly from the space by calling the takeMultiple or clear operations on the GigaSpace interface combined with the TakeModifiers.EVICT_ONLY modifier. The clear operation only returns the number of objects actually evicted from the space. The takeMultiple operation returns the actual objects that were evicted. Here's usage example:
GigaSpace gigaSpace = ...;
User template = new User();
// Using clear - evicts all the objects of type User from the space
int numEvicted = gigaSpace.clear(template, TakeModifiers.EVICT_ONLY);
GigaSpace gigaSpace = ...;
User template = new User();
// Using takeMultiple - evicts all the objects of type User from the space
User evictedUsers = gigaSpace.takeMultiple(template, Integer.MAX_VALUE, TakeModifiers.EVICT_ONLY);
It's important to note the following about the TakeModifiers.EVICT_ONLY modifier:
It can be used with any take operation - take, takeById, takeMultiple, etc.
It can be used only with LRU policy.
When using this modifier, the timeout argument in operations that allow to specify a timeout is ignored. The operations will always return immediately.
When using this modifier, the take or clear calls will never be propagated to the underlying database (EDS layer) when running in synchronous or asynchronous persistency mode. A take operation for example might return a null while a matching object exists in the underlying database.
The TakeModifiers.EVICT_ONLY is ignored when used in a transactional operation - A take or clear in the context of a transaction will not result eviction.
Exceeding Physical Memory Capacity
The overall space capacity is not necessarily limited to the capacity of its physical memory. Currently there are two options for exceeding this limit:
Using an LRU andExternal Data Source - in this mode, all the space data is kept in the database and therefore the space capacity is dependent on the database capacity rather than the memory capacity. The space maintains in memory, a partial image of the persistent view in an LRU basis.
UsingPartitioned Space - in this mode, the space utilizes the physical memory of multiple JVMs. This means the application using the space is able to access all the space instances transparently, as if they were a single space with higher memory capacity.
Memory Manager Parameters
The following properties used to control the memory manager.
Supported with Cache Policy
Defines the maximum size of the space cache. This is the total number of space objects across all space class instances, within a single space. This parameter is ignored when running an ALL_IN_CACHE cache policy.
Specifies the recommended lower threshold for the JVM heap size that should be occupied by the space container. When the system reaches the high_watermark_percentage, it evicts objects on an LRU basis, and attempts to reach this low_watermark_percentage. This process continues until there are no more objects to be evicted, or until memory use reaches the low_watermark_percentage.
Specifies an upper threshold for checking only write-type operations. Above this level, all operations are checked.
Number of retries to lower the memory level below the Low_watermark_percentage. If after all retries, the memory level is still above the space-config.engine.memory_usage.write_only_block_percentage, a com.j_spaces.core.MemoryShortageException is thrown for that write request.
If true, the garbage collector is called explicitly before trying to evict.
When using the LRU cache policy, space-config.engine.memory_usage.explicit-gc=false means that the garbage collector might evict fewer objects than the defined minimum (low watermark percentage). This tag is false by default, because setting the garbage collector explicitly consumes a large amount of CPU, thus affecting performance. Therefore, it is recommended that you set this to true only if you want to ensure that the minimum amount of objects are evicted from the space (and not less than the minimum).
Time (in milliseconds) to wait after evicting a batch of objects, before measuring the current memory utilization.
When a persistent space running in LRU cache policy mode is started/deployed, it loads data from the underlying data source before being available for clients to access. The default behavior is to load data up to 50% of the space-config.engine.cache_size value. See the Reloading Data section for details.
LRU touch activity kicks-in when the percentage of objects within the space exceeds space-config.engine.memory_usage.lruTouchThreshold where the space-config.engine.cache_size is the max amount. This avoid the overhead involved with the LRU activity. A 0 value means always touch, 100 means no touch at all.
The default value of the space-config.engine.memory_usage.lruTouchThreshold is 50 which means the LRU touch activity will kick-in when the amount of objects within the space will cross half of the amount specified by the space-config.engine.cache_size value.
A com.j_spaces.core.MemoryShortageException or an org.openspaces.core.SpaceMemoryShortageException are thrown only when the JVM garbage collection and the eviction mechanism do not evict enough memory. This can happen if the space-config.engine.memory_usage.low_watermark_percentage value is too high.