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Java Agent for Memory Measurements
Overview ======== Jamm provides MemoryMeter, a java agent to measure actual object memory use including JVM overhead. Building ======== "ant jar"; optionally, "ant test" Use === To use MemoryMeter, start the JVM with "-javaagent:<path to>/jamm.jar" You can then use MemoryMeter in your code like this: MemoryMeter meter = new MemoryMeter(); meter.measure(object); meter.measureDeep(object); meter.countChildren(object); If you would like to use MemoryMeter in a web application, make sure that you do NOT put this jar in WEB-INF/lib, as that may cause problems since your code is accessing a MemoryMeter from a different class loader than the one loaded by the -javaagent and won't see it as initialized. If you want MemoryMeter not to measure or count some specific fields, you can mark them using the Unmetered annotation. If you wish to see the Object tree visited by MemoryMeter for debugging purpose, you can use: MemoryMeter meter = new MemoryMeter().enableDebug(); and MemoryMeter will print the tree to System.out. The Maven coordinates for the latest version of Jamm are ======================================================== groupId: com.github.jbellis artifactId: jamm version: 0.3.1 The fine print ============== MemoryMeter is as accurate as java.lang.instrument.Instrumentation.getObjectSize, which only claims to provide "approximate" results, but in practice seems to work as expected. MemoryMeter uses reflection to crawl the object graph for measureDeep. Reflection is slow: measuring a one-million object Cassandra Memtable (that is, 1 million children from MemoryMeter.countChildren) took about 5 seconds wall clock time. By default, MemoryMeter keeps track of descendants visited by measureDeep with an IdentityHashMap. This prevents both over-counting and infinite loops due to cycles in the object graph. Of course, this tracking imposes a memory cost of its own. You can override this by passing a different tracker provider to the MemoryMeter constructor. Jamm provides AlwaysEmptySet, which allows add() calls but never remembers anything, as one alternative. (Obviously this will break painfully if there actually are cycles present!) A more useful alternative, but out of Jamm's scope, would be a tracker using a Bloom filter to implement a probabilistic set interface -- this would have the potential of _undercounting_ due to false positives, but it would guarantee not to loop over cycles.