Java Memory aware cache

I am looking for some ideas, and maybe already some concrete implemenatation if somebody knows any, but I am willing to code the wanted cache on my own.

I want to have a cache that caches only as many gigs as I configure. In comparision to the rest of the app the cache part will use nearly 100% of memory, so we can generalize the used memory of the app beeing the cache size(+ garbage).

Are there methods for getting a guess of how much memory is used? Or is it better to rely on soft pointers? Soft pointer and running always at the top of the jvm memory limit might be very inefficent with lots of cpu cycles for memory cleaning? Can I do some analysis on existing objects, like a myObject.getMemoryUsage()?

The LinkedHashMap has enough cache hits for my purpose so I don't hav开发者_如何学Ce to code some strategic caching monster, but I don't know how to solve this momory issue properly. Any ideas? I don't want OOME flying anywhere.

What is best pratice?

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SoftReference are not a great idea as they tend to be clearer all at once. This means when you get a performance hit from a GC, you also get a hit having to re-build your cache.

You can use Instrumentation.getObjectSize() to get the shallow size of an Object and use reflection to obtain a deep size. However, doing this relatively expensive and not something you want to get doing very often.

Why can't you limit the size to a number of object? In fact, I would start with the simplest cache you can and only add what you really need.

LRU cache in Java.

EDIT: One way to track how much memory you are using is to Serialize the value and store it as a byte[]. This can give you fairly precise control however can slow down your solution by up to 1000x times. (Nothing comes for free ;)

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I would recommend using the Java Caching System. Though if you wanted to roll your own, I'm not aware of any way to get an objects size in memory. Your best bet would be to extend AbstractMap and wrap the values in SoftReferences. Then you could set the java heap size to the maximum size you wanted. Though, your implementation would also have to find and clean out stale data. It's probably easier just to use JCS.

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The problem with SoftReferences is that they give more work to the garbage collector. Although it doesn't meet your requirements, HBase has a very interesting strategy in order to prevent the cache from contributing to the garbage collection pauses : they store the cache in native memory :

• https://issues.apache.org/jira/browse/HBASE-4027
• https://issues.apache.org/jira/secure/attachment/12488272/HBase-4027+%281%29.pdf

A good start for your use-case would be to store all your data on disk. It might seem naive, but thanks to the I/O cache, frequently accessed data will reside in memory. I highly recommend reading these architecture notes from the Varnish caching system :

• https://www.varnish-cache.org/trac/wiki/ArchitectNotes

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The best practice I find is to delegate the caching functionality outside of Java if possible. Java may be good in managing memory, but at dedicated caching system should be used for anything more than a simple LRU cache.

There is a large cost with GC when it kicks in.

EHCache is one of the more popular ones I know of. Java Caching System from another answer is good as well.

However, I generally offload that work to an underlying function (usually the JPA persistence layer by the application server, I let it get handled there so I don't have to deal with it on the application tier).

If you are caching other data such as web requests, http://hc.apache.org/httpclient-3.x/ is also another good candidate.

However, just remember you also have "a file system" there's absolutely nothing wrong with writing to the file system data you have retrieved. I've used the technique several times to fix out of memory errors due to improper use of ByteArrayOutputStreams