Hi, it appears freeMemory is highly not to be trusted ;-): Here is the free memory after each 1000 objects created: #:717000 #:320080 #:1152992 #:512992 #:266208 #:1784800 #:1144800 #:504800 #:389088 #:3894240
The more objects I create the more memory I get ;-). In addition, how can the garbage collecter still collect any of these objects, if I have stored them in an array or is it collecting other objects (??)?
[.... testing testing ....]
ok after some more tests, it appears freeMemory keeps on returning different results because it allocates more and more memory as more is consumed. I ran the taskmanager and looked at the footprint, here is what it gave me:
no objects -> 6 MB
5000 objects-> 9MB (3MB for 5000 objects)
10000 objects -> 12MB (6MB for 10000 objects)
20000 objects -> 18MB (12MB for 20000 objects)
This seems to be pretty accurate, about 629 bytes per object (still seems low for what I am creating ;-)).
I rewrote the testing procedure to see if I could come up with the same, here it is:
System.gc();
int N =10000;
int valid = 0;//count object creations that did not cause additional memory to be allocated by the VM
long last = -1; //free memory during last run
long current = 0; //free memory during current run
long usedup = 0; //total memory used up by all 'valid' allocations
Object workers[] = new Object[N];
for(int i = 0; i < N; i++) {
workers[i] = new MyObject ();
current = Runtime.getRuntime().freeMemory();
if (current < last) { //if allocation is deemed valid update administration
valid++;
usedup += last-current;
}
last = current;
}
System.out.println ("Footprint in bytes:"+usedup/valid);
Result 640 bytes. I hope it is not a coincidence ;-)
greetz Hans
ps here is some more info as well: http://java.sun.com/docs/books/performance/1st_edition/html/JPRAMFootprint.fm.html
At 09:03 AM 8/6/2003 -0400, Shapira, Yoav wrote:
Howdy,
>i've read in a number of locations that you shouldn't put large objects in >session, because of the overhead incurred. >Is this also true for non-serializable objects? Aren't they simply kept in >memory?
Not exactly: I wouldn't put non-serializable objects in the session at all, regardless of their size. You will get errors. Instead, put the ID (or some other key allowing you to retrieve the object) in the session.
>In addition does anybody know how I can accurately estimate the memory >footprint such an object has?
The same way you estimate the memory footprint of any other object.
For example, from a simple command-line program that does nothing else:
int N = 10000; long freeBefore = Runtime.getRuntime().freeMemory();
MyObject[] myArray = new MyObject[N]; for(int i = 0; i < N; i++) { myArray[i] = new MyObject(); }
long freeAfter = Runtime.getRuntime().freeMemory(); long delta = freeAfter - freeBefore; long memPerObject = delta / N;
(Use a large N to reduce impact of overhead, allocation, etc.)
Yoav Shapira
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