[英]Why is this form of block synchronizing faster than the other?
因此,我剛剛完成了有關同步的任務,並且對獲得的結果感到好奇,因為我不太了解為什么一種形式比另一種形式更快。
有一個帳戶類,定義如下:
public class Account {
/*
* ------------
* Data members
* ------------
*/
/**
* Attribute presents an account number
*/
private int acc;
/**
* Attribute that presents an customer name
*/
private String name;
/**
* Attribute that presents an account balance
*/
private double balance;
/*
* ------------
* Constructors
* ------------
*/
/**
* Assigns account number, name and balance.
*
* @param acc A unique integer that represents account number
* @param name A string indicating human-readable customer's name
* @param balance A double indicating account balance
*/
public Account(int acc, String name, double balance) {
super();
this.acc = acc;
this.name = name;
this.balance = balance;
}
@Override
/**
* equals method works as == operator
* it checks if two accounts are identical
*/
public boolean equals(Object obj) {
if (this == obj)
return true;
if (obj == null)
return false;
if (getClass() != obj.getClass())
return false;
Account other = (Account) obj;
if (acc != other.acc)
return false;
if (Double.doubleToLongBits(balance) != Double
.doubleToLongBits(other.balance))
return false;
if (name == null) {
if (other.name != null)
return false;
} else if (!name.equals(other.name))
return false;
if (acc!=other.acc)return false;
return true;
}
/**
* Accessor for account no
* @return account no
*/
public int getAcc() {
return acc;
}
/**
* Mutator for account no
* @param acc A unique int for acoount number
*/
public void setAcc(int acc) {
this.acc = acc;
}
/**
* Accessor for a customer's name
* @return a customer's name
*/
public String getName() {
return name;
}
/**
* Mutator for a customer name
* @param name A string that represents a customer name
*/
public void setName(String name) {
this.name = name;
}
/**
* Accessor for account balance
* @return an account balance
*/
public double getBalance() {
return balance;
}
/**
* Mutator for account balance
* @param balance A double that represents an account balance
*/
public void setBalance(double balance) {
this.balance = balance;
}
/**
* A method to print this account
*/
public String toString(){
return "Account: "+acc+" \tName: "+name+" \tBalance:\t"+balance;
}
/**
* A method that allows a customer to deposit money into this account
* @param amount A double that represents a deposit amount
*/
public void debosit(double amount){
// Waste some time doing fake computations
// do not remove or modify any of the following 3 statements
double k = 999999999;
for(int i=0;i<100;i++)
k = k / 2;
balance = balance + amount;
// Waste some time doing fake computations
// do not remove or modify any of the following 3 statements
k = 999999999;
for(int i=0;i<100;i++)
k = k / 2;
}
/**
* A method that allows a customer to withdraw money from this account
* @param amount A double that represents a withdrawal amount
*/
public void withdraw(double amount){
// Waste some time doing fake computations
// do not remove or modify any of the following 3 statements
double k = 999999999;
for(int i=0;i<100;i++)
k = k / 2;
balance = balance - amount;
// Waste some time doing fake computations
// do not remove or modify any of the following 3 statements
k = 999999999;
for(int i=0;i<100;i++)
k = k / 2;
}
}
public void debosit(double amount){
// Waste some time doing fake computations
// do not remove or modify any of the following 3 statements
double k = 999999999;
for(int i=0;i<100;i++)
k = k / 2;
balance = balance + amount;
// Waste some time doing fake computations
// do not remove or modify any of the following 3 statements
k = 999999999;
for(int i=0;i<100;i++)
k = k / 2;
}
/**
* A method that allows a customer to withdraw money from this account
* @param amount A double that represents a withdrawal amount
*/
public void withdraw(double amount){
// Waste some time doing fake computations
// do not remove or modify any of the following 3 statements
double k = 999999999;
for(int i=0;i<100;i++)
k = k / 2;
balance = balance - amount;
// Waste some time doing fake computations
// do not remove or modify any of the following 3 statements
k = 999999999;
for(int i=0;i<100;i++)
k = k / 2;
}
存款人類如下所示:
public class Depositor extends Thread {
private Account account ;
public Depositor(Account account){
this.account = account;
}
public void run(){
synchronized(account){
for (int i=0;i<10000000;i++)
{
account.debosit(10);
}
/*
try {
sleep(10);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}*/
}
}
}
而且提款器類類似於存款,只是運行提款方法而不是存款一種。
具有main方法的類如下:
public class AccountManager {
public static void main(String[] args) {
// TODO Auto-generated method stub
Account [] account = new Account[10];
Depositor [] deposit = new Depositor[10];
Withdrawer [] withdraw = new Withdrawer[10];
// The birth of 10 accounts
account[0] = new Account(1234,"Mike",1000);
account[1] = new Account(2345,"Adam",2000);
account[2] = new Account(3456,"Linda",3000);
account[3] = new Account(4567,"John",4000);
account[4] = new Account(5678,"Rami",5000);
account[5] = new Account(6789,"Lee",6000);
account[6] = new Account(7890,"Tom",7000);
account[7] = new Account(8901,"Lisa",8000);
account[8] = new Account(9012,"Sam",9000);
account[9] = new Account(4321,"Ted",10000);
// The birth of 10 depositors
deposit[0] = new Depositor(account[0]);
deposit[1] = new Depositor(account[1]);
deposit[2] = new Depositor(account[2]);
deposit[3] = new Depositor(account[3]);
deposit[4] = new Depositor(account[4]);
deposit[5] = new Depositor(account[5]);
deposit[6] = new Depositor(account[6]);
deposit[7] = new Depositor(account[7]);
deposit[8] = new Depositor(account[8]);
deposit[9] = new Depositor(account[9]);
// The birth of 10 withdraws
withdraw[0] = new Withdrawer(account[0]);
withdraw[1] = new Withdrawer(account[1]);
withdraw[2] = new Withdrawer(account[2]);
withdraw[3] = new Withdrawer(account[3]);
withdraw[4] = new Withdrawer(account[4]);
withdraw[5] = new Withdrawer(account[5]);
withdraw[6] = new Withdrawer(account[6]);
withdraw[7] = new Withdrawer(account[7]);
withdraw[8] = new Withdrawer(account[8]);
withdraw[9] = new Withdrawer(account[9]);
System.out.println("Print initial account balances");
// Print initial account balances
for(int i=0;i<10;i++)
System.out.println(account[i]);
// Get start time in milliseconds
long start = System.currentTimeMillis();
System.out.println("Depositor and Withdrawal threads have been created");
/*
* Interleave all threads
*/
for(int i=0; i<10; i++){
deposit[i].start();
withdraw[i].start();
}
for(int i=0; i<10; i++){
try {
deposit[i].join();
withdraw[i].join();
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
// Get elapsed time in milliseconds
long elapsedTimeMillis = System.currentTimeMillis()-start;
System.out.println("Print final account balances after all the child thread terminated...");
// Print final account balances after all the child thread terminated...
for(int i=0;i<10;i++)
System.out.println(account[i]);
// Get elapsed time in seconds
float elapsedTimeSec = elapsedTimeMillis/1000F;
System.out.println("Elapsed time in milliseconds "+elapsedTimeMillis);
System.out.println("Elapsed time in seconds is "+elapsedTimeSec);
// Get elapsed time in minutes
float elapsedTimeMin = elapsedTimeMillis/(60*1000F);
// Get elapsed time in hours
float elapsedTimeHour = elapsedTimeMillis/(60*60*1000F);
// Get elapsed time in days
float elapsedTimeDay = elapsedTimeMillis/(24*60*60*1000F);
}
}
抱歉,有很多代碼,但是最后我的問題是:
如果使用塊同步,則可以將其直接放在帳戶類中,如下所示:
public void withdraw(double amount){
// Waste some time doing fake computations
// do not remove or modify any of the following 3 statements
double k = 999999999;
for(int i=0;i<100;i++)
k = k / 2;
synchronized(this)
balance = balance - amount;
// Waste some time doing fake computations
// do not remove or modify any of the following 3 statements
k = 999999999;
for(int i=0;i<100;i++)
k = k / 2;
}
兩種解決方案都完成后,總共需要1秒鍾才能運行。 但是,如果將同步轉移到提款機和存款人類,則看起來像這樣:
public void run(){
// Withdraw 10 CAD into instance variable account
for (int i=0;i<10000000;i++)
{
synchronized(account)
account.withdraw(10);
}
}
這大約需要運行時間的1/10。
為什么是這樣? 他們不應該花相同的時間嗎?
鎖定有開銷。 如果鎖定次數增加了100倍,則鎖定開銷應增加100倍。
之所以看不到100倍的差異,而只有10倍的原因,是因為代碼在做其他事情,但是如果您只有鎖定,您會注意到更大的差異。 除非JIT編譯器會注意到循環中的同步塊並將其吊起,否則似乎在測試中沒有這樣做。
如果同步塊位於withdraw()
方法上,則一次只能有一個線程執行該方法,這意味着當一個線程正在執行“偽計算”時,其他線程必須等待。 看起來像這樣:
獲取鎖->偽造計算器->修改余額->偽造計算器->釋放鎖
但是,如果僅同步更改方法中的一行,而1個線程擁有鎖,則其他線程可以執行“偽計算”,這樣可以節省一些時間。 看起來像這樣
假計算器->獲取鎖->修改余額->釋放鎖->假計算器
注意,在第二種同步方式中,線程可以在另一個線程修改余額並釋放鎖之后立即獲取鎖,因此它不必等待偽計算。
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