How much memory will I need? This is a question you might have asked yourself (or others) when building a solution, creating a data structure or choosing an algorithm. Will this graph of mine fit in my 3G heap if it contains 1,000,000 edges and I use a HashMap to store it? Can I use the standard Collections API while building my custom caching solution or is the overhead posed by them too much?

Shallow size – size of the object itself including only references to other objects, without objects which are referenced. Retained size – size of the object including references to other objects. Sep 02, 2017 Shallow size – size of the object itself including only references to other objects, without objects which are referenced. Retained size – size of the object including references to other objects. Septic Tank Shape, Size & Dimensions: Septic tank is a devise which deals with the sewage from water closets. In other words, it is a watertight single-storeyed, underground tank in which sewage is retained sufficiently long to permit sedimentation of suspended solids and practical digestion of settled sludge by anaerobic bacterial action. The grain size characteristics of soils that are predominantly coarse grained are evaluated by a sieve analysis. A nest of sieves is prepared by stacking test sieves one above the other with the largest opening at the top followed by sieves of successively smaller openings and a catch pan at the bottom. Jun 08, 2015 The shallow size is the size of memory held by an object itself (generally, arrays and strings have larger shallow sizes). See also Object sizes. Retained size column displays the maximum retained size among the same set of objects. The size of memory that can be freed once an object is deleted (and this its dependents made no longer reachable.

Apparently, the answer to the simple question is a bit more complex. In this post we’ll take a first peek at it and see how deep the rabbit hole actually is.

The answer to the question in the headline comes in several parts. At first we need to understand whether you are interested in shallow or retained heap sizes.

The shallow heap is easy – it consists of only the heap occupied by the object itself. There are some nuances to how to calculate it, but for the scope of this article we leave it as is. Stay tuned for future posts on the same topic.

The retained heap is in many ways more interesting. Only rarely are you interested in the shallow heap, in most cases your actual question can be translated to “If I remove this object from the memory, how much memory can now be freed by the garbage collector”.

Did you know that 20% of Java applications have memory leaks? Adobe premiere pro cc 2017 windows 7 32 bit. Don’t kill your application – instead find and fix leaks with Plumbr in minutes.

Now, as we all remember, all Java garbage collection (GC) algorithms follow this logic:

Shallow Size Vs Retained Size Chart

There are some objects which are considered “important” by the GC. These are called GC roots and are (almost) never discarded. They are, for example, currently executing method’s local variables and input parameters, application threads, references from native code and similar “global” objects.
Any objects referenced from those GC roots are assumed to be in use and hence not discarded by the GC. One object can reference another in different ways in Java, in the most common case an object A is stored in a field of an object B. In such case we say “B references A”.
The process is repeated until all objects that can be transitively reached from GC roots are visited and marked as “in use”.
Everything else is unused and can be thrown away.

Now to illustrate how to calculate the retained heap, let’s follow the aforementioned algorithm with the following example objects:

Shallow Size Vs Retained Size Calculator

To simplify the sample, let’s estimate that all the objects O1-O4 have the shallow heap of 1024B = 1kB. Lets start calculating the retained sizes of those objects.

Chrome Inspect Shallow Size Vs Retained Size

O4 has no references to other objects, so its retained size is equal to its shallow size of 1kB.
O3 has a reference to O4. Garbage collecting O3 would thus mean O4 would also be eligible for garbage collection and so we can say that O3 retained heap is2kB.
O2 has a reference to O3. But it is now important to note that removing the pointer from O2 to O3 does not make O3 eligible for GC, as O1 still has got a pointer to it. So O2 retained heap is only 1kB.
O1 on the other hand is the object keeping all the references in this small graph, so if we would remove O1, everything on this graph would be garbage collected. So O1 retained heap is 4kB.

Which implications does this have in practice? In fact, understanding the differences between shallow and retained heap sizes makes it possible to work with tools such as memory profilers and heap dump analyzers – for example digging into Eclipse MAT might prove to be impossible if you don’t know how to distinguish these two types of heap size measurements.

Full disclosure: this post was inspired by Patrick Dubroy’s talk on Google I/O, which you can watch in full length here.