Go GC garbage collection notes (three color mark)

Go GC Garbage collection notes

Go 1.3

Mark removal method

• Conduct STW, from main Function starts looking for memory usage （ Root reachability algorithm traverses ）.

• Mark all reachable memory usage .

• After marking , Clear these memory occupation .

• end STW, The program continues to run .

Go 1.5 after

Tricolor notation

• From the root （main function ） Start looking for root reachable objects , Mark these reachable objects gray , And put it into the gray object queue .
• Take gray objects from the gray object queue , And mark it black . At the same time, the object it refers to is marked as gray , Put it in the gray object queue .

Repeat the above two steps , Until the gray object queue is empty , All the black objects left at this time are the objects being used , All white objects are garbage objects to be cleaned up .

Tricolor notation + Hybrid write barrier

• initialization GC Mission , Including opening the write barrier and auxiliary GC, Collect root objects . This stage needs to be carried out STW（Go 1.8 After this step STW It was cancelled ）.

• Scan all objects , Including the global pointer and the pointer on the stack ,（ First scan ） Stop the process when scanning the corresponding process stack （ Stop time is short ）, Execute tricolor marking , Empty the gray object queue , Finish marking . This stage is executed in the background .

• Because new objects may be added in the process of three color marking , So we need to write a barrier to record these newly added objects . After three color marking , Rescan global variables 、 Pointer and stack （ Second scan ）, Compare the two scanning results , Judge whether there are garbage objects in the newly added objects , Or whether there are rewritten references in the marked white objects for the first time , If there is , Then re mark it as gray , Prevent it from being cleared . This stage requires STW.

• Recycle all white objects according to the marking results , This stage is executed in the background .