As part of an experiment, I'm working on querying large volumes of data
which is stored offheap.
The content of each record is stored in a chunk of offheap memory. So
instead of having an array of object references, it is an array of records
(no pointer chasing).
My confusion is about some code I'm generating based on the query content.
There are 2 flavors; one flavor where I'm printing if I found something and
the other flavor increments a local long variable and print this at the end
of the loop.
The strange thing is that the first one (printing when the correct entry is
found), is 15x faster than the one where I'm increasing the local counter.
So this is the first:
import java.util.*;
public class FullTableScan_e872b2bd8f274cc18b37ac2a0e3df2ed extends
com.hazelcast.simplemap.impl.FullTableScan{
public void run(){
long offset=slabPointer;
for(long l=0;l<recordIndex;l++){
if((unsafe.getInt(offset+12)==10000) &&
(unsafe.getBoolean(null,offset+16)==true)){
System.out.println("found");
}
offset+=recordDataSize;
}
}
public void init(Map<String, Object> binding){
}
}
And this is the second:
import java.util.*;
public class FullTableScan_e872b2bd8f274cc18b37ac2a0e3df2ed extends
com.hazelcast.simplemap.impl.FullTableScan{
public void run(){
long offset=slabPointer;
long count = 0;
for(long l=0;l<recordIndex;l++){
if((unsafe.getInt(offset+12)==10000) &&
(unsafe.getBoolean(null,offset+16)==true)){
count++;
}
offset+=recordDataSize;
}
System.out.println("count:"+count);
}
public void init(Map<String, Object> binding){
}
}
What could be the reason of this huge performance difference? It isn't a
warmup problem since it was running for 5 minutes. Could there be some data
dependency with the second loop that prevents the loop to be unrolled? I
should analyze the assembler; perhaps this will shed some light on the
situation.
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