java - GC和YGC的发生以及下一步的行动当然

Question

我们有一个 java 应用程序，我可以说它也使用了相当多的字符串变量。此外，我们启动时将最大和初始内存设置为 256Mb。此后运行了一段时间，直到最近我们发现首先是一次GC，近两周后又是一次GC。由此我们发现FC的平均值为0.2145s，YGC的平均值为0.0511s。我们不确定这些值是否合适或会导致其他结果？我们决定将内存增加到 512Mb，并研究一些其他 GC 设置，例如 CMS 设置？还应该优化代码，例如字符串变量的使用吗？通常什么对象或变量类型会移动到旧对象中？

Timestamp     S0      S1     E     O      P      YGC    YGCT      FGC FGCT   GCT
   2752472.6  69.74   0.00   3.67  51.72  44.17   4614  235.630   2   0.429  236.059
   2752477.6  69.74   0.00   5.38  51.72  44.17   4614  235.630   2   0.429  236.059
   2752482.6  69.74   0.00   7.51  51.72  44.17   4614  235.630   2   0.429  236.059
   2752487.6  69.74   0.00   9.52  51.72  44.17   4614  235.630   2   0.429  236.059
   2752492.6  69.74   0.00  10.55  51.72  44.17   4614  235.630   2   0.429  236.059

代码片段

BoneCP connectionPool = null;
  class ConnectionHandler implements Runnable {


    private Socket receivedSocketConn1;
    ConnectionHandler(Socket receivedSocketConn1) {
      this.receivedSocketConn1=receivedSocketConn1;
    }
    Connection dbconn = null;

    public void run() { // etc
     BufferedWriter writeBuffer = null;
     BufferedReader readBuffer = null;
     String capturedMessage="";

     try{
        dbconn = connectionPool.getConnection();
        dbconn.setAutoCommit(false);

        while ((nextChar=readBuffer.read()) != -1){          
          capturedMessage += (char) nextChar;

          if (nextChar == '*')
          {
           try{

                //all queries here.
                //for insert query this how i do
                Statement stmt1 = null;
                stmt1 = dbconn.createStatement();
                String insertQuery3 =........
                count = stmt9.executeUpdate(insertQuery3);
                try{
                  if ( stmt1!= null ){  stmt1.close();
                }
                else{
                System.out.println("No stm1 exist");

                }
                }catch(SQLException ex){   
                  System.out.println("SQLException has been caught for stmt1");
                  ex.printStackTrace(System.out);
                }

                ///For select we do this  

                Statement stmt2 = null;
                stmt2 = dbconn.createStatement(); 
                String selectQuery2= .........
                ResultSet rs2 = stmt2.executeQuery(selectQuery2);

                if(rs2.next())
                {

                }
                try{
                 if ( rs2!= null ){  
                     rs2.close();
                 }   else{
                 System.out.println("No rs2 exist");
                 }

                 if ( stmt2!= null ){ 
                      stmt2.close();
                 }   else{
                 System.out.println("No stm2 exist");

                 }
                }catch(SQLException ex)
                {   
                System.out.println("SQLException has been caught for stmt2");
                ex.printStackTrace(System.out);
                }



               dbconn.commit
            }
           catch (SQLException ex){
                ex.printStackTrace(System.out);
                try{  
              dbconn.rollback();
           }
           catch (Exception rollback){  
              rollback.printStackTrace(System.out);
              }
       }
       catch (Exception e){
           e.printStackTrace(System.out);
           try{  
              dbconn.rollback();
           }
           catch (Exception rollback){  
              rollback.printStackTrace(System.out);
             }
       }
       finally
     {

     }


        }
     catch (SocketTimeoutException ex){
           ex.printStackTrace();
     }
     catch (IOException ex){
           ex.printStackTrace();
     }
     catch (Exception ex){
           ex.printStackTrace(System.out);
     }    
      finally{
        try{
         if ( dbconn != null ){
           dbconn.close();
         }
         else{
          System.out.println("dbConn is null in finally close");
         }
        }
        catch(SQLException ex){
            ex.printStackTrace();
        }
        try{
          if ( writeBuffer != null ){
            writeBuffer.close();
         }
         else{
          System.out.println("w is null in finally close");
         }
        }
        catch(IOException ex){
            ex.printStackTrace(System.out);
        }
       }
      }
    }

终结器分析来自sun.misc.Launcher$ExtClassLoader @ 0xe003a648 (50%)大小:11.2 MB 类:196 个对象:201.7k 类加载器:7

共有91个对象实现了finalize方法。

 Histogram of Objects with Finalize Method
Class Name                         Objects Shallow Heap 
com.mysql.jdbc.JDBC4Connection
First 10 of 38 objects             38      42,560 
java.net.SocksSocketImpl
First 10 of 39 objects             39      4,368 
java.util.jar.JarFile
All 6 objects                      6        384 
java.util.zip.Inflater
All 6 objects                      6        192 
java.util.concurrent.ScheduledThreadPoolExecutor
All 1 objects                      1        80 
java.util.concurrent.ThreadPoolExecutor
All 1 objects                      1        72 
Total: 6 entries
 91 47,656 

终结器分析

(46%)大小:13.6 MB 类:578 个对象:304.8k 类加载器:4

共有 19,341 个对象实现了 Finalize 方法。

Class Name                        Objects Shallow Heap 
java.net.SocksSocketImpl
First 10 of 18,354 objects        18,354  2,055,648 
com.sun.mail.smtp.SMTPTransport
First 10 of 162 objects           162     25,920 
java.io.FileOutputStream
First 10 of 742 objects           742     23,744 
com.mysql.jdbc.JDBC4Connection
First 10 of 15 objects            15       16,800 
java.util.jar.JarFile
First 10 of 20 objects            20       1,280 
java.util.zip.Inflater
First 10 of 25 objects            25       800 
java.util.concurrent.ThreadPoolExecutor
All 8 objects                     8        576 
sun.net.www.protocol.jar.URLJarFile
All 5 objects                     5        400 
java.util.concurrent.ScheduledThreadPoolExecutor
All 2 objects                     2        160 
java.lang.ClassLoader$NativeLibrary
All 4 objects                     4        128 
java.io.FileInputStream
All 3 objects                     3        96 
sun.jdbc.odbc.JdbcOdbcDriver
All 1 objects                     1        24 
Total: 12 entries
 19,341 2,125,576 

来自 org.rzo.yajsw.boot.WrapperClassLoader @ 0xe02818c0 的终结器分析 (4%)大小:966.3 KB 类:361 个对象:19.9k 类加载器:3

共有22个对象实现了finalize方法。

 Histogram of Objects with Finalize Method
Class Name                              Objects Shallow Heap 
java.util.jar.JarFile
First 10 of 11 objects                  11      704 
java.util.zip.Inflater
All 10 objects                          10      320 
java.util.concurrent.ThreadPoolExecutor
All 1 objects                           1       72 
Total: 3 entries
 22 1,096 

历史实时结果。

 num     #instances         #bytes  class name
----------------------------------------------
   1:          6001       18078232  [I
   2:         44562        7196072  [B
   3:        103183        6054240  [C
   4:         31832        4786936  <constMethodKlass>
   5:         31832        4340240  <methodKlass>
   6:         31638        3543456  java.net.SocksSocketImpl
   7:         20127        3542352  com.mysql.jdbc.JDBC4ResultSet
   8:         41022        3461576  [Ljava.util.HashMap$Entry;
   9:        104185        3333920  java.lang.String
  10:         20198        3070096  com.mysql.jdbc.StatementImpl
  11:         20323        2926512  com.mysql.jdbc.Field
  12:         60329        2895792  java.util.TreeMap
  13:          2343        2800032  <constantPoolKlass>
  14:         61239        2449560  java.util.TreeMap$Entry
  15:        149690        2395040  java.lang.Object
  16:         41000        1968000  java.util.HashMap
  17:          2343        1895080  <instanceKlassKlass>
  18:          1954        1569536  <constantPoolCacheKlass>
  19:         31623        1517904  java.net.SocketInputStream

套接字的初始启动

71:             2            224  java.net.SocksSocketImpl
176:            1             32  java.net.Socket
202:            1             24  java.net.InetSocketAddress
206:            1             24  java.net.ServerSocket

运行 50 个套接字后。

 19:            52           5824  java.net.SocksSocketImpl
 37:            50           2400  java.net.SocketInputStream
 48:            51           1632  java.net.Socket
 88:             8            384  java.net.SocketOutputStream
 95:             8            320  java.net.SocketTimeoutException
 135:            7            112  java.net.Socket$3
 136:            7            112  java.net.Socket$2
 229:            1             24  java.net.ServerSocket

Answer 1

512 MB 听起来并不算多，而且可能不值得您花时间进一步优化它。 BTW 512 Mb 是 512 兆位。

中长期存在的对象最终会进入老年代。

您可以使用内存分析器优化代码，以减少字符串的使用。您可以使用 StringBuilder 回收对象并减少 GC 压力，这是导致对象错误地最终进入永久空间的常见原因。

极端地说，您可能会发现可以消除所有字符串生成，但我怀疑它是否值得，并且将内存增加到 1 GB 可能会更简单/更便宜。