jvm-06. Garbage collector

Garbage collector

jvm There are ten kinds of garbage collectors

Generational model

6 Kind of

The younger generation

3 Kind of

ParNew、Serial、Parallel Scavenge.

Old age

3 Kind of

CMS、Serial Old、Parallel Old.

Partition model

3 Kind of

G1、ZGC、Shenandoah.

jdk1.8 Default garbage collector

java -XX:+PrintCommandLineFlags -version

It represents the young generation Parallel Scavenge And the old generation Parallel Old,
abbreviation PS+PO.
Young generation GC It's called minor GC or young GC, Old age GC It's called major GC or old GC, The whole of the younger and older generations GC It's called Full GC.

About jdk Version of , Here's an explanation ,
The reason to use "1.8.0_251" Because my motorcycle happens to be called 251.

Serial

A single threaded garbage collector using the copy method
The blue arrow represents the normal working thread , The yellow arrow represents the garbage collector ,
When the garbage collector is going to work , The worker thread stops at a safe point, Like archiving ,
After the garbage collection is over , The worker thread continues to execute .
stop-the-world,STW： Pause time .

Serial Old

A man who works in an old age , Use markers to remove / A single threaded garbage collector for compression methods .

Parallel Scavenge

Serial Multithreaded version of .

Parallel Old

Serial Old Multithreaded version of .

CMS

concurrent mark sweep
Garbage collection threads and business worker threads are parallel .
It is a very good solution , But it will bring a series of problems caused by concurrency .

ParNew

and Parallel Scavenge be similar , It is designed to cooperate with CMS Produced by use .

Tricolor notation

 white ： The object is not marked ;
 gray , Object has been marked , The member variable is not marked ;
 black ： Both the object itself and the member variables have been marked .

The tricolor marker refers to the garbage collector scanning the reference chain ,
A mark given to an object on a chain , To identify whether it has been scanned .

The scan starts with a gray object , If you encounter a black object, you will not continue , Stop scanning .

There may be problems

Leakage and

A Is such an object , It has a member variable reference to B,B There is a member variable that refers to C.
During garbage collector scanning ：

When the garbage collection thread scans , Business threads are also executing .
As the business goes on , During the execution of the business thread ,
B→D The reference to the lost , For example B Execution method b.setD(null);
When the garbage collector continues ,
because B It's gray. , Then go on to scan B Member variables of D when , Can not find .
This is called missed scanning ,
D It's called floating garbage , But it doesn't matter , The next recycling will D eliminate .

Wrong mark

As the business goes on ,A One more point D References to ,
But the garbage collector sees A It's black , No more scanning for references to its member variables ,
This is the time D May be considered garbage , So as to eliminate .
This is absolutely not allowed , Is the most serious problem .

CMS Solution ：Incremental Update

When a black object is found to have a reference to a white object , Turn black into gray .
It is achieved by writing barriers , When a write operation occurs ,
example a.setD(d) when , If you find that A It's black ,B It's white , Just put A Marked gray .
But this solution creates hidden and deadly problems ：
During the garbage collection process, there may be multiple recycling threads to modify the color mark .

It is equivalent to that there is still no missing mark , It's not practical .

therefore CMS Of remark Stage , amount to STW Of safe point,
It has to be scanned from the beginning ,
Scan points that may be missed , Low efficiency .

G1 Solution ：SATB Snapshot At The Begining