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What is the principle of synchronized lock escalation in multithreading?

2022-07-27 02:25:00 【Current affairs reader hjj】

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In a multithreaded synchronized What is the principle of lock upgrade ？
About thread context switching
- - - Context switch
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In a multithreaded synchronized What is the principle of lock upgrade ？

synchronized Lock upgrade principle

There is a... In the object header of the lock object threadid Field , On the first visit threadid It's empty ,jvm Let it hold biased lock , And will threadid Set as its thread id, It will be judged when entering again threadid Is it with its thread id Agreement , If consistent, you can use this object directly , If it's not consistent , Upgrade biased lock to lightweight lock , Get the lock by spinning the loop a certain number of times , After a certain number of execution , If the object to be used has not been acquired normally , The lock will be upgraded from lightweight to heavyweight , This process constitutes synchronized Lock upgrade .

Purpose of lock upgrade

Lock upgrade is to reduce the performance consumption of locks . stay Java 6 Optimization after synchronized How to implement , We use the way of upgrading biased lock to lightweight lock and then to heavyweight lock , Thus, the performance consumption of the lock is reduced .

Four necessary conditions for deadlock

1)  mutual exclusion ： A resource can only be accessed by one process at a time .

2)  Request and hold ：  When a process is blocked by a request for resources , Keep the acquired resources .

3)  Inalienable ： Resources obtained by the process , Before use , No forcible deprivation of .

4)  Loop waiting for ： A circular waiting resource relationship is formed between several processes .

 Understand the cause of deadlock , In particular, there are four necessary conditions for deadlock , It can be avoided as much as possible 、 Prevention and 
 Deadlock Relieving . therefore , In system design 、 Pay attention to how to keep these four necessary conditions from holding , How to make sure 
 A reasonable allocation algorithm of fixed resources , Avoid processes permanently occupying system resources . Besides , Also prevent the process from waiting 
 Occupy resources in case of . therefore , Reasonable planning should be made for the allocation of resources .

About thread context switching

Context switch

In multithreading programming, the number of general threads is greater than CPU The number of cores , And one CPU The core can only be used by one thread at any time , In order for these threads to be executed effectively ,CPU The strategy is to allocate time slices for each thread and rotate them . When one thread runs out of time slices, it will be ready again for other threads to use , This process is a context switch .

Generally speaking, it is ： The current task is finished CPU Time slice will save its state before switching to another task , In order to switch this task next time , You can reload the status of this task . The process from save to reload is a context switch .

Because of the limited funds , The purchased ECS is 1 nucleus 2G Of , The core is 1 That is, it can only be used by one thread at any time , Running multiple threads will cause context switching , Therefore, the effect of parallelism cannot be achieved , Running multiple threads with a single core may lead to thread context switching, which may lead to the execution efficiency not as high as that of a single thread , Therefore, it is not recommended to use multithreading in projects .