-XX:PretenureSizeThreshold

`-XX:PretenureSizeThreshold`

HotSpot虚拟机提供了-XX：PretenureSizeThreshold参数，指定大于该设置值的对象直接在老年代分配，这样做的目的就是避免在Eden区及两个Survivor区之间来回复制，产生大量的内存复制操作。

		Map<Integer, byte[]> m = new HashMap<Integer, byte[]>();for (int i = 0; i < 5 * 1024; i++) {byte[] b = new byte[1024];m.put(i, b);}

-Xmx30m -Xms30m -XX:+UseSerialGC -XX:+PrintGCDetails -XX:PretenureSizeThreshold=1000Heapdef new generation   total 9216K, used 6949K [0x00000000fe200000, 0x00000000fec00000, 0x00000000fec00000)eden space 8192K,  84% used [0x00000000fe200000, 0x00000000fe8c96f0, 0x00000000fea00000)from space 1024K,   0% used [0x00000000fea00000, 0x00000000fea00000, 0x00000000feb00000)to   space 1024K,   0% used [0x00000000feb00000, 0x00000000feb00000, 0x00000000fec00000)tenured generation   total 20480K, used 0K [0x00000000fec00000, 0x0000000100000000, 0x0000000100000000)the space 20480K,   0% used [0x00000000fec00000, 0x00000000fec00000, 0x00000000fec00200, 0x0000000100000000)Metaspace       used 2794K, capacity 4486K, committed 4864K, reserved 1056768Kclass space    used 298K, capacity 386K, committed 512K, reserved 1048576K

5M空间还是在新生代，这里不会分配到老年代，原因是，虚拟机面对体量不大的对象，会优先分配到TLAB区域中，因此失去了分配到老年代的机会

下面禁用TLAB

-Xmx30m -Xms30m -XX:+UseSerialGC -XX:+PrintGCDetails -XX:PretenureSizeThreshold=1000 -XX:-UseTLABHeapdef new generation   total 9216K, used 995K [0x00000000fe200000, 0x00000000fec00000, 0x00000000fec00000)eden space 8192K,  12% used [0x00000000fe200000, 0x00000000fe2f8f50, 0x00000000fea00000)from space 1024K,   0% used [0x00000000fea00000, 0x00000000fea00000, 0x00000000feb00000)to   space 1024K,   0% used [0x00000000feb00000, 0x00000000feb00000, 0x00000000fec00000)tenured generation   total 20480K, used 5473K [0x00000000fec00000, 0x0000000100000000, 0x0000000100000000)the space 20480K,  26% used [0x00000000fec00000, 0x00000000ff158460, 0x00000000ff158600, 0x0000000100000000)Metaspace       used 2794K, capacity 4486K, committed 4864K, reserved 1056768Kclass space    used 298K, capacity 386K, committed 512K, reserved 1048576K