The Python Standard Library - Py

GC

https://juejin.im/post/5b34b117f265da59a50b2fbe
https://testerhome.com/topics/16556
https://blog.csdn.net/tab_space/article/details/51939855
最后一篇文章解释清楚了
总的来说如下：
先用引用计数，当引用计数为0，内存回收；
之后用分代回收，设置threshold比如(700,10,10)
700是分配内存的次数减去释放内存的次数
10是0代回收的次数
10是1代回收的次数
接着用match sweep算法标记因为循环引用的：
第二个步骤是将比当代处理的“代”更年轻的“代”的链表合并到当前“代”中
https://blog.csdn.net/xiongchengluo1129/article/details/80462651

官方指南

https://docs.python.org/3/library/gc.html#gc.collect

Set the garbage collection thresholds (the collection frequency). 
Setting *threshold0* to zero disables collection.
The GC classifies objects into three generations depending on how many collection
sweeps they have survived. New objects are placed in the youngest generation
(generation `0`). If an object survives a collection it is moved into the next older
generation. Since generation `2` is the oldest generation, objects in that generation
remain there after a collection. In order to decide when to run, the collector keeps track of
the number object allocations and deallocations since the last collection. When the
number of allocations minus the number of deallocations exceeds *threshold0*, collection
starts. Initially only generation `0` is examined. If generation `0` has been examined more
than *threshold1* times since generation `1` has been examined, then generation `1` is
examined as well. Similarly, *threshold2* controls the number of collections of
generation `1` before collecting generation `2`.