Wierd spurt in used memory

[Deepak Narayan]

I recently wrote a small program that extensively uses Collections. And while I was profiling this for memory utilization on my local machine, I found something wierd happening. To measure memory util, I gave sysouts with totalMemory()-freeMemory() (invoked on the Runtime instance).

    
HashSet vphValue = new HashSet();
System.out.println(run.totalMemory()+" totmem b4 ph loop");
for(int i = 0 ; i < size ; i++){
vphValue.addAll((ArrayList)map.get(origKeys[i]));
 
System.out.println(run.totalMemory()-run.freeMemory()+ " A addAll"+i+" "+run.totalMemory());
 
initSize += origKeys[i].length()+vphValue.toString().length();
 
System.out.println(run.totalMemory()-run.freeMemory()+ " b zeropad"+i+" "+run.totalMemory());
 
if(vphValue.size() == 0){
if(!(emptyKey.length()==0)){
emptyKey.append(',');
}
emptyKey.append(origKeys[i]);
}

if(i==0 || (""+(i+1)).length()>(""+i).length()){
for(int j = (""+(i+1)).length() ; j < (""+size).length() ; j++){
zeroPad.append('0');
}
}//End of Padding
 
System.out.println(run.totalMemory()-run.freeMemory()+ " A zeropad"+i+" "+run.totalMemory());
 
keyPlaceHolder.put(zeroPad.toString()+(i+1),origKeys[i]);
 
System.out.println(run.totalMemory()-run.freeMemory()+ " A kphPut"+i+" "+run.totalMemory());
 
valuePlaceHolder.put(zeroPad.toString()+(i+1),vphValue);
 
System.out.println(run.totalMemory()-run.freeMemory()+ " A vphPut"+i+" "+run.totalMemory());
 
vphValue.clear();
}

At the end of a run, I get the following statements on console:

  1438456 B vphValue0 62455808
1438456 A vphValue0 62455808
1438456 A addAll0 62455808
1438456 b zeropad0 62455808
1438456 A zeropad0 62455808
1438456 A kphPut0 62455808
1438456 A vphPut0 62455808
1438456 B vphValue1 62455808
1438456 A vphValue1 62455808
1438456 A addAll1 62455808
1438456 b zeropad1 62455808
1438456 A zeropad1 62455808
1438456 A kphPut1 62455808
1438456 A vphPut1 62455808
1438456 B vphValue2 62455808
1438456 A vphValue2 62455808
1438456 A addAll2 62455808
1438456 b zeropad2 62455808
1438456 A zeropad2 62455808
1438456 A kphPut2 62455808
1438456 A vphPut2 62455808
1438456 B vphValue3 62455808
1438456 A vphValue3 62455808
[B]1438456 A addAll3 62455808
1515792 b zeropad3 62455808[/B]
1515792 A zeropad3 62455808
1515792 A kphPut3 62455808
1515792 A vphPut3 62455808
1515792 B vphValue4 62455808
1515792 A vphValue4 62455808
1515792 A addAll4 62455808
1515792 b zeropad4 62455808
1515792 A zeropad4 62455808
1515792 A kphPut4 62455808
1515792 A vphPut4 62455808
1515792 B vphValue5 62455808
1515792 A vphValue5 62455808
1515792 A addAll5 62455808
1515792 b zeropad5 62455808
1515792 A zeropad5 62455808
1515792 A kphPut5 62455808
1515792 A vphPut5 62455808
1515792 B vphValue6 62455808
1515792 A vphValue6 62455808
1515792 A addAll6 62455808
1515792 b zeropad6 62455808
1515792 A zeropad6 62455808
1515792 A kphPut6 62455808
1515792 A vphPut6 62455808  

There were 7 iterations of this loop in total.
Notice the output in bold.. The memory used upto this point was constant and then there's a sudden spurt of close to 80 Bytes. I have two questions..
1. Why is the memory usage constant over the first 4 iterations, till the section in bold? I mean, shouldnt there be a progressive increase if any?

2. Why the spurt?

[Steve Luke]

You would have to look a little deeper to see what is taking the memory. My bet would be this is typical behavior for collections. Both HashMaps and ArrayLists are initialized to hold some number of records (capacity) and only grow in memory usage when that capacity is reached (the rate and timing of growth for the ArrayList is left as implementation detail, while for HashMap you can control it with the load factor).

What you are seeing in the growth spurt is probably one or both of the collections you are using reaching its capacity and allocating more space for itself.

[Deepak Narayan]

Thank You!