JVM foundation - > what is STW?

What is? STW？

STW: Stop-The-World: It's in the garbage collection algorithm ⾏ In the process , take JVM Memory freezes, application stalls ⼀ States .

• stay STW State, ,JAVA Of All threads are stopped ⽌ Of board ⾏ Of -> GC Except for threads
• once Stop-the-world happen , except GC Out of thread required , Other threads will stop working , Thread interrupted until GC The end of the mission to continue their mission .
• STW Is inevitable , Garbage collection algorithm ⾏ There will be STW, All we have to do is reduce the pause time
• GC The focus of various algorithm optimization , Namely Reduce STW( Pause ), At the same time, it's also JVM Focus of tuning .

When to enter STW state ?

• Reachability analysis algorithm Enumerate root nodes in （GC Roots） It will lead to all Java Execution thread pause , Get into STW state

Why do we have to STW Reason for pause ?

• Analytical work Must be in a way that ensures consistency In a snapshot of
• Consistency means that the whole execution system appears to be frozen at a certain point in time during the whole analysis period
• If during the analysis Object reference relationships are still changing , Of the analysis results Accuracy cannot be guaranteed
• By STW Interrupted application threads It will be finished in GC Then recover , Frequent interruptions can make users feel stuck
• So we need to reduce STW Happen , It is equivalent to trying to find a way Reduce GC The frequency of garbage collection
• STW Status and which one to use GC Collector independent , be-all GC Collectors have this state , Because to ensure consistency .
• But OK GC The collector can reduce the pause time
• Reduce STW( Pause ) and Reduce GC The frequency of garbage collection Is the focus of tuning

If the system gets stuck obviously , The big probability is Frequent execution GC Garbage collection , Enter frequently STW The reason why the state stops

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