4
根據以下文檔http://www.oracle.com/technetwork/java/javase/gc-tuning-6-140523.html#par_gc.ergonomics.default_size，使用公式MIN（內存/ 4,1GB）選擇並行GC的默認最大堆大小。從公式中可以明顯看出，但文檔仍然指出「不管計算機上安裝了多少內存，默認的最大堆大小不會超過1GB」。要驗證我寫了下面的程序熱點默認最大堆大小
public class Allocate{
public static void main(String[] args) throws Exception {
long megabytes = Long.valueOf(args[0]);
long bytes = megabytes * 1024 * 1024;
int longs = (int) (bytes/8);
long[] arr = new long[longs];
Thread.sleep(Long.MAX_VALUE);
System.out.println(arr.length);
}
}
我在一個16Gb的RAM上執行這個程序。
[email protected] allocation]$ /usr/java/jdk1.6.0_26/bin/java Allocate 2048 &
[1] 9291
[[email protected] allocation]$ /usr/java/jdk1.6.0_26/bin/jmap -heap 9291
Attaching to process ID 9291, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 20.1-b02
using thread-local object allocation.
Parallel GC with 8 thread(s)
Heap Configuration:
MinHeapFreeRatio = 40
MaxHeapFreeRatio = 70
MaxHeapSize = 4208984064 (4014.0MB)
NewSize = 1310720 (1.25MB)
MaxNewSize = 17592186044415 MB
OldSize = 5439488 (5.1875MB)
NewRatio = 2
SurvivorRatio = 8
PermSize = 21757952 (20.75MB)
MaxPermSize = 85983232 (82.0MB)
Heap Usage:
PS Young Generation
Eden Space:
capacity = 65798144 (62.75MB)
used = 1315976 (1.2550125122070312MB)
free = 64482168 (61.49498748779297MB)
2.0000199397721614% used
From Space:
capacity = 10944512 (10.4375MB)
used = 0 (0.0MB)
free = 10944512 (10.4375MB)
0.0% used
To Space:
capacity = 10944512 (10.4375MB)
used = 0 (0.0MB)
free = 10944512 (10.4375MB)
0.0% used
PS Old Generation
capacity = 2322923520 (2215.3125MB)
used = 2147483664 (2048.000015258789MB)
free = 175439856 (167.31248474121094MB)
92.44745449045176% used
PS Perm Generation
capacity = 21757952 (20.75MB)
used = 2606752 (2.485992431640625MB)
free = 19151200 (18.264007568359375MB)
11.980686417545181% used
[[email protected] allocation]$
2GB陣列被分配到老一代。 MaxHeapSize是4GB，是系統內存的1/4。爲什麼JVM爲堆預留4GB？