Friday, 25 September 2015

Memory based eviction

Eviction Today

Infinispan since its inception has supported a way to help users control how much memory the in memory cache entries consume in the JVM.  This has always been limited to a number of entries.  In the past users have had to estimate the average amount of bytes their entries used on the heap.  With this average you can easily calculate how many entries could safely be stored in memory without running into issues.  For users who have keys and values that are relatively similar this can work well.  However when the case requires entries that vary in size this can be problematic and you end up calculating the average size based on the worse case.

Enter Memory Based Eviction

Infinispan 8 introduces memory based eviction counting.  That is Infinispan will automatically keep track of how large the key, value and overhead if possible.  It can use these values then to try to limit the number of entries instead to a memory count such as 1 Gigabyte.

Key/Value limitations

Unfortunately this is currently limited to only using keys and values stored as primitives, primitive wrappers (ie. Integer), java.lang.String(s) and any of the previously mentioned stored in an array.  This means this feature cannot be used with any custom classes.  If enough feedback is provided we could provide a SPI to allow the user to plug in their own counter for their own classes, but this is not planned currently.

There are a couple ways to easily get around this.  One is to use storeAsBinary, which will store your keys and/or values as byte arrays for you automatically, satisfying this limitation.  A second way is when you are using the client such as HotRod, in this case the data is stored in the serialized (byte[]) form.  Note that compatibility mode will prevent this from occurring and you are unable to use these configurations together.

Eviction Type limitation

Due to the complexity of LIRS, memory based eviction is only supported with LRU at this time. See the types here.  This could be enhanced at a later point, but is also not planned.

How to enable

You can enable memory based eviction either through programmatic or declarative configuration.  Note that Infinispan added long support (limited to 2^48) for the size value which directly helps memory based eviction if users want to utilize caches larger than 2 GB.



Supported JVMs

This was tested and written specifically for Oracle and OpenJDK JVMs.  In testing these JVMs showed memory accuracy within 1% of desired value. Other JVMs may shown incorrect values.

The algorithm takes into account JVM options, such as compressed pointers and 32 bit JVM vs 64 bit JVM.  Keep in mind this is only for the data container and doesn't take into account additional overhead such as created threads or other runtime objects.

Other JVMs are not handled such as the IBM JVM which was briefly tested and showed incorrect numbers greater than 10% of the desired amount.  Support for other JVMs can be added later as interest is shown for them.

Closing Notes

I hope this feature helps people to better handle their memory constraints while using Infinispan!  Let us know if you have any feedback or concerns.


 - Will

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