A DESCRIPTION OF THE REQUEST :
I wrote a little benchmark for testing the G1 collector with the LinkedHashMap class. I never leave more than 2 entries in the map. When using the default collector, performance is quite good, but with G1 the GC pauses frequently and the VM consumes huge amounts of memory.

JUSTIFICATION :
Many users of the new G1 collector might run similar benchmarks and conclude that the G1 collector performs poorly overall. I suspect that in a real application G1 will behave much better.

I almost filed this as a bug, because it seems that the collector shouldn't have to work so hard to collect (what I believe) is nothing but garbage. Since the collector is actually working (slowly), I consider this an RFE.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
I expect the performance of G1 in this simple test to be as good as the default collector.
ACTUAL -
Performance is extremely poor, with frequent long GC pauses.

---------- BEGIN SOURCE ----------
import java.util.LinkedHashMap;
import java.util.Map;

public class CacheTest {
    public static void main(String[] args) throws Exception {
        Map<Object, Object> cache = new LinkedHashMap<Object, Object>();
        long i = 0;
        while (true) {
            cache.put(i, new String());
            cache.remove(i - 1);
            i++;
            if (i % 100000 == 0) {
                System.out.println(i);
            }
        }
    }
}

C:\>"\Program Files\Java\jre6\bin\java.exe" -version
java version "1.6.0_14-ea"
Java(TM) SE Runtime Environment (build 1.6.0_14-ea-b04)
Java HotSpot(TM) Client VM (build 14.0-b13, mixed mode, sharing)


C:\>"\Program Files\Java\jre6\bin\java.exe" -XX:+UnlockExperimentalVMOptions -XX:+UseG1GC -verbose:gc CacheTest
[GC pause (young) 1024K->124K(8192K), 0.0017905 secs]
[GC pause (young) 2172K->1872K(8192K), 0.0206060 secs]
[GC pause (young) 2896K->2744K(16M), 0.0214125 secs]
[GC pause (young) 5816K->5352K(32M), 0.0337724 secs]
100000
200000
300000
400000
500000
600000
[GC pause (young) 58M->50M(105M), 0.5629917 secs]
700000
800000
900000
1000000
1100000
1200000
[GC pause (young)-- 116M->117M(122M), 1.1623899 secs]
1300000
[Full GC 121M->116K(8192K), 0.0933021 secs]
1400000
1500000
1600000
1700000
[GC pause (young) 43M->36M(83M), 0.4415740 secs]
...




---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
Use default collector instead.