JVM参数:MaxTenuringThreshold和TargetSurvivorRatio说明
-XX:MaxTenuringThreshold
在新生代中对象存活次数(经过Minor GC的次数)后仍然存活，就会晋升到旧生代。
-XX:TargetSurvivorRatio
一个计算期望存活大小Desired survivor size的参数。默认50
计算公式： (survivor_capacity * TargetSurvivorRatio) / 100 * sizeof(a pointer)：survivor_capacity（一个survivor space的大小）乘以TargetSurvivorRatio，表明所有age的survivor space对象的大小如果超过Desired survivor size，则重新计算threshold，以age和MaxTenuringThreshold的最小值为准，否则以MaxTenuringThreshold为准.
public class GCTenuringThreshold {
public static void main(String[] args) throws Exception {
GCMemoryObject object1 = new GCMemoryObject(2);
GCMemoryObject object2 = new GCMemoryObject(8);
GCMemoryObject object3 = new GCMemoryObject(8);
GCMemoryObject object4 = new GCMemoryObject(8);
object2 = null;
object3 = null;
GCMemoryObject object5 = new GCMemoryObject(8);
Thread.sleep(4000);
object2 = new GCMemoryObject(8);
object3 = new GCMemoryObject(8);
object2 = null;
object3 = null;
object5 = null;
GCMemoryObject object6 = new GCMemoryObject(8);
Thread.sleep(5000);
GCMemoryObject object7 = new GCMemoryObject(8);
GCMemoryObject object8 = new GCMemoryObject(8);
GCMemoryObject object9 = new GCMemoryObject(8);
GCMemoryObject object10 = new GCMemoryObject(8);
//object6 = null;
Thread.sleep(5000);
System.out.println("ok");
}
}
class GCMemoryObject {
private byte[] bytes = null;
public GCMemoryObject(int multi) {
bytes = new byte[1024 * 256 * multi];
}
} 以-Xms20M –Xmx20M –Xmn10M –XX:+UseSerialGC参数执行以上代码，
输出如下：
[GC [DefNew
Desired survivor size 524288 bytes, new threshold 1 (max 15)
- age 1: 677968 bytes, 677968 total
: 6999K->662K(9216K), 0.0028753 secs] 6999K->2710K(19456K), 0.0029048 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
[GC [DefNew
Desired survivor size 524288 bytes, new threshold 15 (max 15)
- age 1: 16 bytes, 16 total
: 6891K->0K(9216K), 0.0025312 secs] 8939K->2709K(19456K), 0.0026029 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
[GC [DefNew
Desired survivor size 524288 bytes, new threshold 15 (max 15)
- age 1: 48 bytes, 48 total
- age 2: 16 bytes, 64 total
: 6200K->0K(9216K), 0.0095025 secs] 8910K->8853K(19456K), 0.0095770 secs] [Times: user=0.00 sys=0.00, real=0.01 secs]
ok
Heap
def new generation total 9216K, used 4297K [0x32310000, 0x32d10000, 0x32d10000)
eden space 8192K, 52% used [0x32310000, 0x32742508, 0x32b10000)
from space 1024K, 0% used [0x32c10000, 0x32c10040, 0x32d10000)
to space 1024K, 0% used [0x32b10000, 0x32b10000, 0x32c10000)
tenured generation total 10240K, used 8853K [0x32d10000, 0x33710000, 0x33710000)
the space 10240K, 86% used [0x32d10000, 0x335b57a8, 0x335b5800, 0x33710000)
compacting perm gen total 12288K, used 369K [0x33710000, 0x34310000, 0x37710000)
the space 12288K, 3% used [0x33710000, 0x3376c438, 0x3376c600, 0x34310000)
ro space 10240K, 51% used [0x37710000, 0x37c3b700, 0x37c3b800, 0x38110000)
rw space 12288K, 54% used [0x38110000, 0x387a76c0, 0x387a7800, 0x38d10000) 可以看出，每次都重新计算threshhold,或者说从survivor space晋升到旧生代后，重新计算threshold。
SurvivorRatio过大，都应该是age为准，因为触发gc的条件没有达到，但survivor space/2的条件优先达到。
所以调小SurvivorRatio，可以达到MaxTenuringThreshold次数还存活的对象。代码如下：
public class GCTenuringThreshold1 {
//private GCMemoryObject1 ng = new GCMemoryObject1(15);
public static void main(String[] args) throws Exception {
GCMemoryObject1 object2 = new GCMemoryObject1(0.1f);
GCMemoryObject1 object1 = new GCMemoryObject1(2f);
Thread.sleep(4000);
object1 = null;
GCMemoryObject1 object4 = new GCMemoryObject1(0.1f);
GCMemoryObject1 object3 = new GCMemoryObject1(2f);
Thread.sleep(4000);
object3 = null;
GCMemoryObject1 object6 = new GCMemoryObject1(0.3f);
GCMemoryObject1 object5 = new GCMemoryObject1(2f);
Thread.sleep(4000);
object5 = null;
GCMemoryObject1 object8 = new GCMemoryObject1(0.3f);
GCMemoryObject1 object7 = new GCMemoryObject1(2f);
Thread.sleep(4000);
object7 = null;
GCMemoryObject1 object10 = new GCMemoryObject1(0.3f);
GCMemoryObject1 object9 = new GCMemoryObject1(2f);
System.out.println("ok");
}
}
class GCMemoryObject1 {
private byte[] bytes = null;
public GCMemoryObject1(float multi) {
bytes = new byte[(int)(1024 * 1024*multi)];
}
} 以-Xms20M -Xmx20M -Xmn10M -XX:SurvivorRatio=1 -XX:+UseSerialGC -XX:MaxTenuringThreshold=3 -XX:+PrintTenuringDistribution -XX:+PrintGCDetails
运行上述代码。
[GC [DefNew
Desired survivor size 1736704 bytes, new threshold 3 (max 3)
- age 1: 363408 bytes, 363408 total
: 2529K->354K(6848K), 0.0017645 secs] 2529K->354K(17088K), 0.0018275 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
[GC [DefNew
Desired survivor size 1736704 bytes, new threshold 3 (max 3)
- age 1: 314616 bytes, 314616 total
- age 2: 363176 bytes, 677792 total
: 2748K->661K(6848K), 0.0009114 secs] 2748K->661K(17088K), 0.0009399 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
[GC [DefNew
Desired survivor size 1736704 bytes, new threshold 3 (max 3)
- age 1: 314616 bytes, 314616 total
- age 2: 314600 bytes, 629216 total
- age 3: 363176 bytes, 992392 total
: 3042K->969K(6848K), 0.0010587 secs] 3042K->969K(17088K), 0.0010872 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
[GC [DefNew
Desired survivor size 1736704 bytes, new threshold 3 (max 3)
- age 1: 314616 bytes, 314616 total
- age 2: 314600 bytes, 629216 total
- age 3: 314600 bytes, 943816 total
: 3341K->921K(6848K), 0.0012151 secs] 3341K->1276K(17088K), 0.0012443 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
ok
Heap
def new generation total 6848K, used 3049K [0x32310000, 0x32d10000, 0x32d10000)
eden space 3456K, 61% used [0x32310000, 0x32524128, 0x32670000)
from space 3392K, 27% used [0x32670000, 0x327566c8, 0x329c0000)
to space 3392K, 0% used [0x329c0000, 0x329c0000, 0x32d10000)
tenured generation total 10240K, used 354K [0x32d10000, 0x33710000, 0x33710000)
the space 10240K, 3% used [0x32d10000, 0x32d68aa8, 0x32d68c00, 0x33710000)
compacting perm gen total 12288K, used 369K [0x33710000, 0x34310000, 0x37710000)
the space 12288K, 3% used [0x33710000, 0x3376c430, 0x3376c600, 0x34310000)
ro space 10240K, 51% used [0x37710000, 0x37c3b700, 0x37c3b800, 0x38110000)
rw space 12288K, 54% used [0x38110000, 0x387a76c0, 0x387a7800, 0x38d10000)