Chapter 5. Configuring JVM memory usage

When you configure Data Grid eviction you specify:

You can either perform eviction manually or configure Data Grid to do one of the following:

Refer to the schema reference for more details about the eviction strategies.

Limit the size of Data Grid caches to a total number of entries.

Maximum count eviction

In the following example, Data Grid removes an entry when the cache contains a total of 500 entries and a new entry is created:

<distributed-cache>
  <memory max-count="500" when-full="REMOVE"/>
</distributed-cache>

{
  "distributed-cache" : {
    "memory" : {
      "max-count" : "500",
      "when-full" : "REMOVE"
    }
  }
}

distributedCache:
  memory:
    maxCount: "500"
    whenFull: "REMOVE"

ConfigurationBuilder builder = new ConfigurationBuilder();
builder.memory().maxCount(500).whenFull(EvictionStrategy.REMOVE);

Limit the size of Data Grid caches to a maximum amount of memory.

Maximum size eviction

In the following example, Data Grid removes an entry when the size of the cache reaches 1.5 GB (gigabytes) and a new entry is created:

<distributed-cache>
  <encoding media-type="application/x-protostream"/>
  <memory max-size="1.5GB" when-full="REMOVE"/>
</distributed-cache>

{
  "distributed-cache" : {
    "encoding" : {
      "media-type" : "application/x-protostream"
    },
    "memory" : {
      "max-size" : "1.5GB",
      "when-full" : "REMOVE"
    }
  }
}

distributedCache:
  encoding:
    mediaType: "application/x-protostream"
  memory:
    maxSize: "1.5GB"
    whenFull: "REMOVE"

ConfigurationBuilder builder = new ConfigurationBuilder();
builder.encoding().mediaType("application/x-protostream")
       .memory()
         .maxSize("1.5GB")
         .whenFull(EvictionStrategy.REMOVE);

If you choose the manual eviction strategy, Data Grid does not perform eviction. You must do so manually with the evict() method.

You should use manual eviction with embedded caches only. For remote caches, you should always configure Data Grid with the REMOVE or EXCEPTION eviction strategy.

<distributed-cache>
  <memory max-count="500" when-full="MANUAL"/>
</distributed-cache>

{
  "distributed-cache" : {
    "memory" : {
      "max-count" : "500",
      "when-full" : "MANUAL"
    }
  }
 }

distributedCache:
  memory:
    maxCount: "500"
    whenFull: "MANUAL"

ConfigurationBuilder builder = new ConfigurationBuilder();
builder.encoding().mediaType("application/x-protostream")
       .memory()
         .maxSize("1.5GB")
         .whenFull(EvictionStrategy.REMOVE);

Passivation persists data to cache stores when Data Grid evicts entries. You should always enable eviction if you enable passivation, as in the following examples:

<distributed-cache>
  <persistence passivation="true">
    <!-- Persistent storage configuration. -->
  </persistence>
  <memory max-count="100"/>
</distributed-cache>

{
  "distributed-cache": {
    "memory" : {
      "max-count" : "100"
    },
    "persistence" : {
      "passivation" : true
    }
  }
}

distributedCache:
  memory:
    maxCount: "100"
  persistence:
    passivation: "true"

ConfigurationBuilder builder = new ConfigurationBuilder();
builder.memory().maxCount(100);
builder.persistence().passivation(true); //Persistent storage configuration

When you configure expiration, Data Grid stores keys with metadata that determines when entries expire.

Data Grid checks if lifespan or maximum idle metadata is set and then compares the values with the current time.

If (creation + lifespan < currentTime) or (lastUsed + maxIdle < currentTime) then Data Grid detects that the entry is expired.

Expiration occurs whenever entries are accessed or found by the expiration reaper.

For example, k1 reaches the maximum idle time and a client makes a Cache.get(k1) request. In this case, Data Grid detects that the entry is expired and removes it from the data container. The Cache.get(k1) request returns null.

Data Grid also expires entries from cache stores, but only with lifespan expiration. Maximum idle expiration does not work with cache stores. In the case of cache loaders, Data Grid cannot expire entries because loaders can only read from external storage.

Data Grid uses a reaper thread that runs periodically to detect and remove expired entries. The expiration reaper ensures that expired entries that are no longer accessed are removed.

The Data Grid ExpirationManager interface handles the expiration reaper and exposes the processExpiration() method.

In some cases, you can disable the expiration reaper and manually expire entries by calling processExpiration(); for instance, if you are using local cache mode with a custom application where a maintenance thread runs periodically.

Because maximum idle expiration relies on the last access time for cache entries, it has some limitations with clustered cache modes.

With lifespan expiration, the creation time for cache entries provides a value that is consistent across clustered caches. For example, the creation time for k1 is always the same on all nodes.

For maximum idle expiration with clustered caches, last access time for entries is not always the same on all nodes. To ensure that entries have the same relative access times across clusters, Data Grid sends touch commands to all owners when keys are accessed.

The touch commands that Data Grid send have the following considerations:

Set lifespan and maximum idle times for all entries in a cache.

Expiration for Data Grid caches

In the following example, Data Grid expires all cache entries after 5 seconds or 1 second after the last access time, whichever happens first:

<replicated-cache>
  <expiration lifespan="5000" max-idle="1000" />
</replicated-cache>

{
  "replicated-cache" : {
    "expiration" : {
      "lifespan" : "5000",
      "max-idle" : "1000"
    }
  }
}

replicatedCache:
  expiration:
    lifespan: "5000"
    maxIdle: "1000"

ConfigurationBuilder builder = new ConfigurationBuilder();
builder.expiration().lifespan(5000, TimeUnit.MILLISECONDS)
                    .maxIdle(1000, TimeUnit.MILLISECONDS);

Specify lifespan and maximum idle times for individual entries. When you add lifespan and maximum idle times to entries, those values take priority over expiration configuration for caches.

When you add entries to off-heap caches, Data Grid dynamically allocates native memory to your data.

Data Grid hashes the serialized byte[] for each key into buckets that are similar to a standard Java HashMap. Buckets include address pointers that Data Grid uses to locate entries that you store in off-heap memory.

Configure Data Grid to store cache entries in native memory outside the JVM heap space.

Off-heap storage

Data Grid stores cache entries as bytes in native memory. Eviction happens when there are 100 entries in the data container and Data Grid gets a request to create a new entry:

<replicated-cache>
  <memory storage="OFF_HEAP" max-count="500"/>
</replicated-cache>

{
  "replicated-cache" : {
    "memory" : {
      "storage" : "OBJECT",
      "max-count" : "500"
    }
  }
}

replicatedCache:
  memory:
    storage: "OFF_HEAP"
    maxCount: "500"

ConfigurationBuilder builder = new ConfigurationBuilder();
builder.memory().storage(StorageType.OFF_HEAP).maxCount(500);