The meaning and trigger conditions of gc

The necessity of jvm gc

When the heap memory of the jvm is not enough, you need to perform gc to release the memory

Conditions for gc execution

Before the jvm executes gc, all threads must be stopped and gc executed

What object is recycled by gc

The JVM uses the reachability algorithm to recover unreferenced objects. The reference counting method cannot count the applications between objects.

When gc, whether the object is recycled or not depends on what java reference type the object is

• FinalReference = "will never be recycled
• SoftReference = "Reclaimed before memory leaks
• WeakReference = "Will be recycled during garbage collection
• PhantomReference=>anytime

gc classification and execution environment

A simple analysis of the structure of the heap

Before jvm1.8, the heap memory was divided into Eden, S0, S1, Old Meomory, Perm (abolished after 1.8), each newly instantiated object will be put into young gen, when the Eden+s0 area is full,Perform minor gc. After 15 times of gc, the objects that still exist are upgraded to the old age (or the object is relatively large, and if the object exceeds the threshold, it will be directly placed in the old age). When the entire heap memory is not enough, the entire heap memory will be gc.is major gc.

Introduction to commonly used gc algorithms

• Mark-Sweep Algorithm: Mark useless objects, and then clean up and recycle.Disadvantages: Inefficient, generates garbage debris.
• Copying algorithm: Divide two memory areas of equal size according to capacity. When one block is used up, copy the live objects to another block, and then clean up the used memory space at one time.Disadvantage: The memory usage is not high, only half of the original.
• Marking-cleaning algorithm: mark useless objects, move all surviving objects to one end, and then directly clear the memory beyond the end boundary.
• Generation algorithm: Divide the memory into several blocks according to the life cycle of the object, generally the new generation and the old generation, the new generation basically adopts the copy algorithm, and the old generation adopts the mark sorting algorithm

gc collector classification and advantages and disadvantages

Gc collector	Algorithms used	Action area	Features	Benefits	Disadvantages
Serial Collector	Copy Algorithm	New Generation	Markup and cleanup are single-threaded	Simple and efficient	The jvm must be paused, the pause time is long
Serial Old Collector	Markup-Organization	Old Age	Markup and cleanup are single-threaded	Simple and efficient	The jvm must be paused, the pause time is long
ParNew Collector	Copy Algorithm	New Generation	Parallel	Good throughput	The jvm must be paused, the pause time is long
ParallelScavenge Collector	Copy Algorithm	New Generation	Parallel	Precise control of gc time	It is difficult to control the time of gc well
Parold Collector	Copy Algorithm	Old Age	Parallel	Good throughput	The jvm must be paused, the pause time is long
CMS collector	Markup Clear	Old Age	Parallel	Minimum pause time	Gc takes longer to handle floating garbage
G1 Collector	Tag Collation	Entire heap	Parallel	Efficient, no fragmentation, GC pause time can be specified	G1 requires additional memory to record data partitions
ZGC collector	Tag Collation	Entire heap	Parallel	Efficient and low-latency processing of large memory gc	Generate floating garbage

Definition of Floating Garbage: The marking process marks living objects. Judging from the reachability of GCRoot, the objects are alive, but the "dead" objects cannot be marked.If it is marked as alive in the initial marking stage, and "dies" during concurrent operation, the remark process cannot be corrected, so it becomes floating garbage and needs to wait for the arrival of the next gc