GC.md

Golang GC

• v1.1 STW
• v1.3 Mark STW && Sweep Parallels
• v1.5 Tri-colour marking （Mark Sweep && STW）
• v1.8 hybrid write barrier

Mark && Sweep

• phase 1 : mark

• phase 2 : sweep

    When GC Start, execute STW(Stop the world)
         0. Stop the world 
         1. Find process , check all reachable objects and mark them 
         2. Sweep unmark objects 
         3. Start the world
  
    Encounter Problem 
        1.Scan all heap for marking
        2.Sweep will generate heap fragments
  
  
    So Tri-colour marking Comming...
  

Tri-colour marking

• phase 1 : The program create all objects marked white

• phase 2 : Scan all reachable objects marked gray (When GC Start)

• phase 3 : Mark objects of gray (
        step 1. like quote object marked gray
        step 2. mark self with black
        step 3. monitor memory updating && goto step 1
        step 4. when gray objects disappear, sweep white objects
        step 5. finally , make black to white (current gc finished,next begin)
  )

    Solve Problem 
        1.Sweep && User mode logic will make currency.
    Encounter Problem 
        1.User logic will update quote object.
        2.How to solve Process generating new object ?
        3.How to make mark and User mode logic currency ?
       
  
  
    So Write Barrier Comming...
  

Write Barrier && Hybrid GC

1.Write operator before && after compared.
2.It is first perceived by other components of the system.
(means gc processing:
    monitor memory update && remark object(stop the world)
)
3.When process generate new object,mark gray



When Black Object quoted White Object
        1.Write Barrier trigger
        2.Send Signal to GC
        3.GC rescan objects && mark gray
        
So, when GC start, whatever objects with created or quoted
once updating, mark gray.