我正在使用gc,当gc(我使用并行gc)没有涉及时,我发现了一些有趣的情况,这是我的代码
public static void main(String[] args) {
System.out.println(Runtime.getRuntime().freeMemory() / (1024 * 1024) + " free");
String[] strings = new String[(40 * 1024 * 1024) / Integer.BYTES];
System.out.println(strings.length);
System.out.println(Runtime.getRuntime().freeMemory() / (1024 * 1024) + " free");
strings = new String[(40 * 1024 * 1024) / Integer.BYTES];
System.out.println(strings.length);
System.out.println(Runtime.getRuntime().freeMemory() / (1024 * 1024) + " free");
}
我的Java版本是:
java version "1.8.0_191"
Java(TM) SE Runtime Environment (build 1.8.0_191-b12)
Java HotSpot(TM) 64-Bit Server VM (build 25.191-b12, mixed mode)
该程序涉及以下argc:
-Xmx64m -XX:+PrintGCDetails -XX:+PrintGCDateStamps -XX:+PrintGCTimeStamps
这是输出:
59 free
10485760
19 free
2019-06-09T14:53:18.868+0600: 0.105: [GC (Allocation Failure) [PSYoungGen: 1640K->480K(18944K)] 42600K->41448K(62976K), 0.0018199 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
2019-06-09T14:53:18.870+0600: 0.107: [Full GC (Ergonomics) [PSYoungGen: 480K->0K(18944K)] [ParOldGen: 40968K->41339K(44032K)] 41448K->41339K(62976K), [Metaspace: 3033K->3033K(1056768K)], 0.0174681 secs] [Times: user=0.04 sys=0.00, real=0.02 secs]
2019-06-09T14:53:18.888+0600: 0.125: [GC (Allocation Failure) [PSYoungGen: 0K->0K(18944K)] 41339K->41339K(62976K), 0.0017270 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
2019-06-09T14:53:18.889+0600: 0.126: [Full GC (Allocation Failure) [PSYoungGen: 0K->0K(18944K)] [ParOldGen: 41339K->41321K(44032K)] 41339K->41321K(62976K), [Metaspace: 3033K->3033K(1056768K)], 0.0140842 secs] [Times: user=0.03 sys=0.01, real=0.01 secs]
Heap
PSYoungGen total 18944K, used 491K [0x00000000feb00000, 0x0000000100000000, 0x0000000100000000)
eden space 16384K, 3% used [0x00000000feb00000,0x00000000feb7afa0,0x00000000ffb00000)
from space 2560K, 0% used [0x00000000ffd80000,0x00000000ffd80000,0x0000000100000000)
to space 2560K, 0% used [0x00000000ffb00000,0x00000000ffb00000,0x00000000ffd80000)
ParOldGen total 44032K, used 41321K [0x00000000fc000000, 0x00000000feb00000, 0x00000000feb00000)
object space 44032K, 93% used [0x00000000fc000000,0x00000000fe85a6d0,0x00000000feb00000)
Metaspace used 3064K, capacity 4496K, committed 4864K, reserved 1056768K
class space used 336K, capacity 388K, committed 512K, reserved 1048576K
Exception in thread "main" java.lang.OutOfMemoryError: Java heap space
at lightbox.GcWIthoutComp.main(GcWIthoutComp.java:11)
根据日志,JVM不能为字符串第二次分配内存,但是如果我添加JVM arg -Xcomp
,它将工作并提供以下输出
59 free
10485760
19 free
2019-06-09T15:01:06.593+0600: 0.830: [GC (Allocation Failure) [PSYoungGen: 1982K->512K(18944K)] 42942K->41480K(62976K), 0.0008554 secs] [Times: user=0.01 sys=0.00, real=0.00 secs]
2019-06-09T15:01:06.594+0600: 0.831: [Full GC (Ergonomics) [PSYoungGen: 512K->0K(18944K)] [ParOldGen: 40968K->411K(37376K)] 41480K->411K(56320K), [Metaspace: 3377K->3377K(1056768K)], 0.0100865 secs] [Times: user=0.01 sys=0.00, real=0.02 secs]
10485760
20 free
Heap
PSYoungGen total 18944K, used 1147K [0x00000000feb00000, 0x0000000100000000, 0x0000000100000000)
eden space 16384K, 7% used [0x00000000feb00000,0x00000000fec1ed48,0x00000000ffb00000)
from space 2560K, 0% used [0x00000000ffb00000,0x00000000ffb00000,0x00000000ffd80000)
to space 2560K, 0% used [0x00000000ffd80000,0x00000000ffd80000,0x0000000100000000)
ParOldGen total 44032K, used 41371K [0x00000000fc000000, 0x00000000feb00000, 0x00000000feb00000)
object space 44032K, 93% used [0x00000000fc000000,0x00000000fe866c90,0x00000000feb00000)
Metaspace used 3399K, capacity 4500K, committed 4864K, reserved 1056768K
class space used 334K, capacity 388K, committed 512K, reserved 1048576K
据我所知,在检测未使用的内存方面,编译后的代码对于GC来说要舒服得多。此外,如果我删除-Xcomp并将数组创建移到单独的方法中,它不会抛出OOM异常:
public static void main(String[] args) {
System.out.println(Runtime.getRuntime().freeMemory() / (1024 * 1024) + " free");
allocate();
System.out.println(Runtime.getRuntime().freeMemory() / (1024 * 1024) + " free");
allocate();
System.out.println(Runtime.getRuntime().freeMemory() / (1024 * 1024) + " free");
}
private static void allocate() {
String[] strings = new String[(40 * 1024 * 1024) / Integer.BYTES];
System.out.println(strings.length);
}
我的问题是:
如果你有有用的链接,请在评论中提供。
发布于 2019-06-09 17:55:16
注释只放在一行中,很难解释其中的内容。重要的区别似乎在于,使用-Xcomp时,已经可以收集本地变量字符串。所以你会得到:
2019-06-09T15:01:06.593+0600: 0.830: [GC (Allocation Failure) [PSYoungGen: 1982K->512K(18944K)] 42942K->41480K(62976K), 0.0008554 secs] [Times: user=0.01 sys=0.00, real=0.00 secs]
2019-06-09T15:01:06.594+0600: 0.831: [Full GC (Ergonomics) [PSYoungGen: 512K->0K(18944K)] [ParOldGen: 40968K->411K(37376K)] 41480K->411K(56320K), [Metaspace: 3377K->3377K(1056768K)], 0.0100865 secs] [Times: user=0.01 sys=0.00, real=0.02 secs]
所以你会得到一个分配失败,但是GC可以释放足够的内存。
如果没有-Xcomp,则无法收集loal变量字符串,因此您得到的消息类似于:
2019-06-09T14:53:18.868+0600: 0.105: [GC (Allocation Failure) [PSYoungGen: 1640K->480K(18944K)] 42600K->41448K(62976K), 0.0018199 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
2019-06-09T14:53:18.870+0600: 0.107: [Full GC (Ergonomics) [PSYoungGen: 480K->0K(18944K)] [ParOldGen: 40968K->41339K(44032K)] 41448K->41339K(62976K), [Metaspace: 3033K->3033K(1056768K)], 0.0174681 secs] [Times: user=0.04 sys=0.00, real=0.02 secs]
2019-06-09T14:53:18.888+0600: 0.125: [GC (Allocation Failure) [PSYoungGen: 0K->0K(18944K)] 41339K->41339K(62976K), 0.0017270 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
2019-06-09T14:53:18.889+0600: 0.126: [Full GC (Allocation Failure) [PSYoungGen: 0K->0K(18944K)] [ParOldGen: 41339K->41321K(44032K)] 41339K->41321K(62976K), [Metaspace: 3033K->3033K(1056768K)], 0.0140842 secs] [Times: user=0.03 sys=0.01, real=0.01 secs]
这表明GC试图释放内存,但无法完成。
(看看PSYoungGen:它显示了内存是如何关闭的。当它在没有-Xcomp的情况下失败时,几乎没有什么是可能的。使用-Xcomp,它从1982K下降到512K。另请参阅https://dzone.com/articles/understanding-garbage-collection-log)
我找不到-Xcomp正在做的事情。所以,很抱歉,我现在只能给你们解释一下日志。因此,这就像在分配新生成的数组之前将字符串设置为null一样。因此,如果在第二次为数组赋值之前添加一个字符串= null;:即使没有-Xcomp,您也会得到大致相同的gc消息。
关于在方法中使用它:你的方法中有一个局部变量。因此,你赋值数组,然后方法结束,垃圾收集器可以收集局部变量。
https://stackoverflow.com/questions/56513329
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