Re: Serizlize You Cannot Use for Object Size
Lasse Reichstein Nielsen wrote:
Patricia Shanahan <pats@acm.org> writes:
Lasse Reichstein Nielsen wrote:
In Java, you don't care about the exact size of an object. You care
about how many objects you create and how long they live, but
whether an object has 4 or 8 bytes of overhead is completely
irrelevant.
Huh? Here's a specific problem. Suppose I have an application that uses
a lot of memory. The size of a problem can be expressed in terms of a
few parameters. I know the numbers of certain types of objects that will
be created, as functions of those problem size parameters.
For simplicity, let's assume a single basic size parameter N. However,
for real problems there may be more size parameters.
I would like to run a problem with N=10,000. I know, by experiment, that
it does not run on any machine to which I currently have access. If I
ask my academic adviser (or my manager if I were working in industry)
for access to a bigger memory, the inevitable question is "How big?".
I admit I was overgeneralizing. However, I don't think it was by much :)
So the "How big?" question is a good one, but hard to answer.
If you know that on the current version of the VM, OS and hardware you
plan to use, the objects takes exactly 24 bytes, how much memory will
you need then? What if it was 32 bytes?
The technique I would use to answer the question is to measure, on the
JVM I intend to use but on a system to which I have access, the in-use
memory (totalMemory() - freeMemory() immediately after a System.gc()
call) with controlled numbers of objects in existence. Given that
result, and the relationships between problem size and object creation,
I can project out the memory for larger problem sizes.
If you are cutting it so close that the overhead of the object
implementation counts, then I would go for a language with manual
memory management instead of Java. I wouldn't rely on automatic,
garbage collected, memory management for something with so much
simultaneous live data.
I'm certain not everybody agrees with such heresy :)
There is too much that is convenient about Java for me to throw it out
just because memory size estimation requires some effort. Note that I
have seen far more consistency than you seem to expect in things like
array and object overhead. Of course, switching to a 64 bit JVM does
make a difference.
Patricia