Object address, size and heap space / JVMTI and Serviceability agent cooperation

Crazy Java crazyjavahacking at gmail.com
Mon Mar 5 02:42:53 PST 2012


I am working on a project of visualizing the HotSpot heap. It is basically
a GCSpy rewrite and I am trying o write a HotSpot driver.

Just very quickly what the tool is providing - set of tiles per every heap
space (young, old and permanent generation), where every tile has a well
defined size (let's say 64kB). The visualization is then defined by the
color of every tile (the white means tile is absolutely free of objects and
100% red tile means the whole tile is used by objects).

To do that, I need 3 information:

   1. object size - how many bytes every object consumes in memory (not
   recursive/deep size)
   2. object address - to compute the heap fragmentation, I need to know
   the address of object (when using together with size, I should know the
   exact part of heap occupied by every object)
   3. heap spaces - what heap spaces are used and their addresses as well
   (to know what object belongs to what heap space)

I spend quite a lot of time trying to figure out how can we get those
information and there is a way to get all of them, however I need some
interactions of APIs:

   1. object size - can by determined by Java Agent, native agent, JVMTI
   and serviceability agent as well
   2. object address - the only way I found is by using the Serviceability
   3. heap spaces - again just Serviceability agent seems like having such
   a capability

Now the idea is to use JVMTI and Serviceability agent together. JVMTI will
call callbacks every time object is allocated (for "new" object allocation
bytecode instrumentationwill be used), deallocated or GC was finished and
the size and address information will be determined by Serviceability
Agent. For performance reasons it will be super inefficient to iterate
through the whole heap every time JVMTI generates callback. Well, the
problem is that even if I get notifications from JVMTI, I cannot determine
the address of object from JVMTI, so I don't know how to map the identity
of object from JVMTI to Serviceability agent.

So I should be able to just update the part of heap that is really needed
to be updated instead of whole heap and I have no idea how this could be

Do you have any idea how could I get the object address and heap spaces
from JVMTI, or the callbacks (JVMTI callbacks like objectAlloc(),
GCFinished(), ...) from Serviceability agent, or how to connect those APIs
to work together?

Thanks very much,
Martin Skurla
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