众所周知，由于 Full GC 的成本远远高于 Minor GC，因此某些情况下需要尽可能将对象分配在年轻代，这在很多情况下是一个明智的选择。虽然在大部分情况下，JVM 会尝试在 Eden 区分配对象，但是由于空间紧张等问题，很可能不得不将部分年轻对象提前向年老代压缩。因此，在 JVM 参数调优时可以为应用程序分配一个合理的年轻代空间，以最大限度避免新对象直接进入年老代的情况发生。清单 1 所示代码尝试分配 4MB 内存空间，观察一下它的内存使用情况。
清单 1. 相同大小内存分配
public class PutInEden {
public static void main(String[] args){
byte[] b1,b2,b3,b4;//定义变量
b1=new byte[1024*1024];//分配 1MB 堆空间，考察堆空间的使用情况
b2=new byte[1024*1024];
b3=new byte[1024*1024];
b4=new byte[1024*1024];
}
}
Heap
def new generation total 92160K, used 11878K [0x0f010000, 0x15410000, 0x15410000)
eden space 81920K, 2% used [0x0f010000, 0x0f1a9a20, 0x14010000)
from space 10240K, 99% used [0x14a10000, 0x1540fff8, 0x15410000)
to space 10240K, 0% used [0x14010000, 0x14010000, 0x14a10000)
tenured generation total 409600K, used 86434K [0x15410000, 0x2e410000, 0x4d810000)
the space 409600K, 21% used [0x15410000, 0x1a878b18, 0x1a878c00, 0x2e410000)
compacting perm gen total 12288K, used 2062K [0x4d810000, 0x4e410000, 0x51810000)
the space 12288K, 16% used [0x4d810000, 0x4da13b18, 0x4da13c00, 0x4e410000)
No shared spaces configured.
清单 5 输出的日志显示，年轻代分配了 8M，年老代也分配了 8M。我们可以尝试加上-XX:TargetSurvivorRatio=90 参数，这样可以提高 from 区的利用率，使 from 区使用到 90%时，再将对象送入年老代，运行清单 4 代码，输出如清单 6 所示。
清单 6. 修改运行参数后清单 4 输出
Heap
def new generation total 9216K, used 9215K [0x35c10000, 0x36610000, 0x36610000)
eden space 8192K, 100% used [0x35c10000, 0x36410000, 0x36410000)
from space 1024K, 99% used [0x36510000, 0x3660fc50, 0x36610000)
to space 1024K, 0% used [0x36410000, 0x36410000, 0x36510000)
tenured generation total 10240K, used 10239K [0x36610000, 0x37010000, 0x37010000)
the space 10240K, 99% used [0x36610000, 0x3700ff70, 0x37010000, 0x37010000)
compacting perm gen total 12288K, used 371K [0x37010000, 0x37c10000, 0x3b010000)
the space 12288K, 3% used [0x37010000, 0x3706cd90, 0x3706ce00, 0x37c10000)
ro space 10240K, 51% used [0x3b010000, 0x3b543000, 0x3b543000, 0x3ba10000)
rw space 12288K, 55% used [0x3ba10000, 0x3c0ae4f8, 0x3c0ae600, 0x3c610000)
如果将 SurvivorRatio 设置为 2，将 b1 对象预存在年轻代。输出如清单 7 所示。
清单 7. 再次修改运行参数后清单 4 输出
Heap
def new generation total 7680K, used 7679K [0x35c10000, 0x36610000, 0x36610000)
eden space 5120K, 100% used [0x35c10000, 0x36110000, 0x36110000)
from space 2560K, 99% used [0x36110000, 0x3638fff0, 0x36390000)
to space 2560K, 0% used [0x36390000, 0x36390000, 0x36610000)
tenured generation total 10240K, used 10239K [0x36610000, 0x37010000, 0x37010000)
the space 10240K, 99% used [0x36610000, 0x3700fff0, 0x37010000, 0x37010000)
compacting perm gen total 12288K, used 371K [0x37010000, 0x37c10000, 0x3b010000)
the space 12288K, 3% used [0x37010000, 0x3706ce28, 0x3706d000, 0x37c10000)
ro space 10240K, 51% used [0x3b010000, 0x3b543000, 0x3b543000, 0x3ba10000)
rw space 12288K, 55% used [0x3ba10000, 0x3c0ae4f8, 0x3c0ae600, 0x3c610000)
Heap
def new generation total 6144K, used 1378K [0x35c10000, 0x362b0000, 0x362b0000)
eden space 5504K, 25% used [0x35c10000, 0x35d689e8, 0x36170000)
from space 640K, 0% used [0x36170000, 0x36170000, 0x36210000)
to space 640K, 0% used [0x36210000, 0x36210000, 0x362b0000)
tenured generation total 13696K, used 0K [0x362b0000, 0x37010000, 0x37010000)
the space 13696K, 0% used [0x362b0000, 0x362b0000, 0x362b0200, 0x37010000)
compacting perm gen total 12288K, used 374K [0x37010000, 0x37c10000, 0x3b010000)
the space 12288K, 3% used [0x37010000, 0x3706dac8, 0x3706dc00, 0x37c10000)
ro space 10240K, 51% used [0x3b010000, 0x3b543000, 0x3b543000, 0x3ba10000)
rw space 12288K, 55% used [0x3ba10000, 0x3c0ae4f8, 0x3c0ae600, 0x3c610000)
Heap
def new generation total 6144K, used 354K [0x35c10000, 0x362b0000, 0x362b0000)
eden space 5504K, 6% used [0x35c10000, 0x35c689d8, 0x36170000)
from space 640K, 0% used [0x36170000, 0x36170000, 0x36210000)
to space 640K, 0% used [0x36210000, 0x36210000, 0x362b0000)
tenured generation total 13696K, used 1024K [0x362b0000, 0x37010000, 0x37010000)
the space 13696K, 7% used [0x362b0000, 0x363b0010, 0x363b0200, 0x37010000)
compacting perm gen total 12288K, used 374K [0x37010000, 0x37c10000, 0x3b010000)
the space 12288K, 3% used [0x37010000, 0x3706dac8, 0x3706dc00, 0x37c10000)
ro space 10240K, 51% used [0x3b010000, 0x3b543000, 0x3b543000, 0x3ba10000)
rw space 12288K, 55% used [0x3ba10000, 0x3c0ae4f8, 0x3c0ae600, 0x3c610000)
public class MaxTenuringThreshold {
public static void main(String args[]){
byte[] b1,b2,b3;
b1 = new byte[1024*512];
b2 = new byte[1024*1024*2];
b3 = new byte[1024*1024*4];
b3 = null;
b3 = new byte[1024*1024*4];
}
}
[GC [DefNew: 2986K->690K(7680K), 0.0246816 secs] 2986K->2738K(17920K),
0.0247226 secs] [Times: user=0.00 sys=0.02, real=0.03 secs]
[GC [DefNew: 4786K->690K(7680K), 0.0016073 secs] 6834K->2738K(17920K),
0.0016436 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
Heap
def new generation total 7680K, used 4888K [0x35c10000, 0x36610000, 0x36610000)
eden space 5120K, 82% used [0x35c10000, 0x36029a18, 0x36110000)
from space 2560K, 26% used [0x36110000, 0x361bc950, 0x36390000)
to space 2560K, 0% used [0x36390000, 0x36390000, 0x36610000)
tenured generation total 10240K, used 2048K [0x36610000, 0x37010000, 0x37010000)
the space 10240K, 20% used [0x36610000, 0x36810010, 0x36810200, 0x37010000)
compacting perm gen total 12288K, used 374K [0x37010000, 0x37c10000, 0x3b010000)
the space 12288K, 3% used [0x37010000, 0x3706db50, 0x3706dc00, 0x37c10000)
ro space 10240K, 51% used [0x3b010000, 0x3b543000, 0x3b543000, 0x3ba10000)
rw space 12288K, 55% used [0x3ba10000, 0x3c0ae4f8, 0x3c0ae600, 0x3c610000)
[GC [DefNew: 2986K->690K(7680K), 0.0047778 secs] 2986K->2738K(17920K),
0.0048161 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
[GC [DefNew: 4888K->0K(7680K), 0.0016271 secs] 6936K->2738K(17920K),
0.0016630 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
Heap
def new generation total 7680K, used 4198K [0x35c10000, 0x36610000, 0x36610000)
eden space 5120K, 82% used [0x35c10000, 0x36029a18, 0x36110000)
from space 2560K, 0% used [0x36110000, 0x36110088, 0x36390000)
to space 2560K, 0% used [0x36390000, 0x36390000, 0x36610000)
tenured generation total 10240K, used 2738K [0x36610000, 0x37010000, 0x37010000)
the space 10240K, 26% used [0x36610000, 0x368bc890, 0x368bca00, 0x37010000)
compacting perm gen total 12288K, used 374K [0x37010000, 0x37c10000, 0x3b010000)
the space 12288K, 3% used [0x37010000, 0x3706db50, 0x3706dc00, 0x37c10000)
ro space 10240K, 51% used [0x3b010000, 0x3b543000, 0x3b543000, 0x3ba10000)
rw space 12288K, 55% used [0x3ba10000, 0x3c0ae4f8, 0x3c0ae600, 0x3c610000)
import java.util.Vector;
public class HeapSize {
public static void main(String args[]) throws InterruptedException{
Vector v = new Vector();
while(true){
byte[] b = new byte[1024*1024];
v.add(b);
if(v.size() == 10){
v = new Vector();
}
Thread.sleep(1);
}
}
}
