JVM gc只新生代串行GC

1.代码一：

public class JVMTest {
public static void main(String args[]) throws InterruptedException {
byte [] bytes1 = new byte[1024*1024*2];
byte [] bytes2 = new byte[1024*1024*2];
byte [] bytes3 = new byte[1024*1024*4];
Thread.sleep(2000);
byte [] bytes4 = new byte[1024*1024*4];
Thread.sleep(2000);
}
}

执行：java -Xms20M -Xmx20M -Xmn10M -XX:+UseSerialGC -XX:+PrintHeapAtGC -Xloggc:gc.log JVMTest

gc两次：

{Heap before GC invocations=0 (full 0):
def new generation   total 9216K, used 5287K [7f9a00000, 7fa400000, 7fa400000)
eden space 8192K,  64% used [7f9a00000, 7f9f29c78, 7fa200000)
from space 1024K,   0% used [7fa200000, 7fa200000, 7fa300000)
to   space 1024K,   0% used [7fa300000, 7fa300000, 7fa400000)
tenured generation   total 10240K, used 0K [7fa400000, 7fae00000, 7fae00000)
the space 10240K,   0% used [7fa400000, 7fa400000, 7fa400200, 7fae00000)
compacting perm gen  total 21248K, used 4813K [7fae00000, 7fc2c0000, 800000000)
the space 21248K,  22% used [7fae00000, 7fb2b3628, 7fb2b3800, 7fc2c0000)
No shared spaces configured.
0.904: [GC 5287K->4447K(19456K), 0.0315833 secs]
Heap after GC invocations=1 (full 0):
def new generation   total 9216K, used 351K [7f9a00000, 7fa400000, 7fa400000)
eden space 8192K,   0% used [7f9a00000, 7f9a00000, 7fa200000)
from space 1024K,  34% used [7fa300000, 7fa357ed8, 7fa400000)
to   space 1024K,   0% used [7fa200000, 7fa200000, 7fa300000)
tenured generation   total 10240K, used 4096K [7fa400000, 7fae00000, 7fae00000)
the space 10240K,  40% used [7fa400000, 7fa800020, 7fa800200, 7fae00000)
compacting perm gen  total 21248K, used 4813K [7fae00000, 7fc2c0000, 800000000)
the space 21248K,  22% used [7fae00000, 7fb2b3628, 7fb2b3800, 7fc2c0000)
No shared spaces configured.
}
{Heap before GC invocations=1 (full 0):
def new generation   total 9216K, used 4711K [7f9a00000, 7fa400000, 7fa400000)
eden space 8192K,  53% used [7f9a00000, 7f9e41eb0, 7fa200000)
from space 1024K,  34% used [7fa300000, 7fa357ed8, 7fa400000)
to   space 1024K,   0% used [7fa200000, 7fa200000, 7fa300000)
tenured generation   total 10240K, used 4096K [7fa400000, 7fae00000, 7fae00000)
the space 10240K,  40% used [7fa400000, 7fa800020, 7fa800200, 7fae00000)
compacting perm gen  total 21248K, used 4817K [7fae00000, 7fc2c0000, 800000000)
the space 21248K,  22% used [7fae00000, 7fb2b4498, 7fb2b4600, 7fc2c0000)
No shared spaces configured.
2.940: [GC 8807K->8537K(19456K), 0.0193472 secs]
Heap after GC invocations=2 (full 0):
def new generation   total 9216K, used 345K [7f9a00000, 7fa400000, 7fa400000)
eden space 8192K,   0% used [7f9a00000, 7f9a00000, 7fa200000)
from space 1024K,  33% used [7fa200000, 7fa2566e8, 7fa300000)
to   space 1024K,   0% used [7fa300000, 7fa300000, 7fa400000)
tenured generation   total 10240K, used 8192K [7fa400000, 7fae00000, 7fae00000)
the space 10240K,  80% used [7fa400000, 7fac00030, 7fac00200, 7fae00000)
compacting perm gen  total 21248K, used 4817K [7fae00000, 7fc2c0000, 800000000)
the space 21248K,  22% used [7fae00000, 7fb2b4498, 7fb2b4600, 7fc2c0000)
No shared spaces configured.
}

2.代码2：

public class JVMTest {
public static void main(String args[]) throws InterruptedException {
byte [] bytes1 = new byte[1024*1024*2];
byte [] bytes2 = new byte[1024*1024*2];
byte [] bytes3 = new byte[1024*1024*2];
System.out.println("step 1");
byte [] bytes4 = new byte[1024*1024*2];
Thread.sleep(2000);
System.out.println("step 2");
byte [] bytes5 = new byte[1024*1024*2];
byte [] bytes6 = new byte[1024*1024*2];
System.out.println("step 3");
byte [] bytes7 = new byte[1024*1024*2];
Thread.sleep(2000);
}
}

java -Xms20M -Xmx20M -Xmn10M -XX:+UseSerialGC -XX:+PrintHeapAtGC -Xloggc:gc.log JVMTest

1次minor gc 一次full gc：原因 minor gc晋升到旧生代的大小大于旧生代的剩余空间。

{Heap before GC invocations=0 (full 0):
def new generation   total 9216K, used 7335K [7f9a00000, 7fa400000, 7fa400000)
eden space 8192K,  89% used [7f9a00000, 7fa129c88, 7fa200000)
from space 1024K,   0% used [7fa200000, 7fa200000, 7fa300000)
to   space 1024K,   0% used [7fa300000, 7fa300000, 7fa400000)
tenured generation   total 10240K, used 0K [7fa400000, 7fae00000, 7fae00000)
the space 10240K,   0% used [7fa400000, 7fa400000, 7fa400200, 7fae00000)
compacting perm gen  total 21248K, used 4814K [7fae00000, 7fc2c0000, 800000000)
the space 21248K,  22% used [7fae00000, 7fb2b38c0, 7fb2b3a00, 7fc2c0000)
No shared spaces configured.
1.019: [GC 7335K->6495K(19456K), 0.0418591 secs]
Heap after GC invocations=1 (full 0):
def new generation   total 9216K, used 351K [7f9a00000, 7fa400000, 7fa400000)
eden space 8192K,   0% used [7f9a00000, 7f9a00000, 7fa200000)
from space 1024K,  34% used [7fa300000, 7fa357ed8, 7fa400000)
to   space 1024K,   0% used [7fa200000, 7fa200000, 7fa300000)
tenured generation   total 10240K, used 6144K [7fa400000, 7fae00000, 7fae00000)
the space 10240K,  60% used [7fa400000, 7faa00030, 7faa00200, 7fae00000)
compacting perm gen  total 21248K, used 4814K [7fae00000, 7fc2c0000, 800000000)
the space 21248K,  22% used [7fae00000, 7fb2b38c0, 7fb2b3a00, 7fc2c0000)
No shared spaces configured.
}
{Heap before GC invocations=1 (full 0):
def new generation   total 9216K, used 6754K [7f9a00000, 7fa400000, 7fa400000)
eden space 8192K,  78% used [7f9a00000, 7fa040af0, 7fa200000)
from space 1024K,  34% used [7fa300000, 7fa357ed8, 7fa400000)
to   space 1024K,   0% used [7fa200000, 7fa200000, 7fa300000)
tenured generation   total 10240K, used 6144K [7fa400000, 7fae00000, 7fae00000)
the space 10240K,  60% used [7fa400000, 7faa00030, 7faa00200, 7fae00000)
compacting perm gen  total 21248K, used 4817K [7fae00000, 7fc2c0000, 800000000)
the space 21248K,  22% used [7fae00000, 7fb2b47b0, 7fb2b4800, 7fc2c0000)
No shared spaces configured.
3.073: [Full GC 12898K->12633K(19456K), 0.0712905 secs]
Heap after GC invocations=2 (full 1):
def new generation   total 9216K, used 4441K [7f9a00000, 7fa400000, 7fa400000)
eden space 8192K,  54% used [7f9a00000, 7f9e56680, 7fa200000)
from space 1024K,   0% used [7fa300000, 7fa300000, 7fa400000)
to   space 1024K,   0% used [7fa200000, 7fa200000, 7fa300000)
tenured generation   total 10240K, used 8192K [7fa400000, 7fae00000, 7fae00000)
the space 10240K,  80% used [7fa400000, 7fac000c8, 7fac00200, 7fae00000)
compacting perm gen  total 21248K, used 4817K [7fae00000, 7fc2c0000, 800000000)
the space 21248K,  22% used [7fae00000, 7fb2b47b0, 7fb2b4800, 7fc2c0000)
No shared spaces configured.
}

新生代对象晋升到旧生代的规则：经过多次minor gc之后还存在的对象或者是to space放不下的直接晋升到old generation