Julian Fang
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# 2019q1 Homework6 (gc) contributed by ` JulianATA ` 另外研究 ` precise GC for c ` in [簡單 OGC 、保守 GC 與精準 GC for C](https://hackmd.io/s/ryXx6FDcV) ## Minimal mark-and-sweep garbage collector ### Naive 運作機制: 包含兩個階段 `Mark` 標記和 `Sweep` 掃除。 #### `Mark` Phase 從 root 開始,找尋 `可到達` 的物件。 ( `Reachable Objects` ) 標記 `可到達` 的物件。 #### `Sweep` Phase 從 heap 中,找尋未被標記的物件( `Unreachable Objects` ),即需要被清除的物件。 將未被標記的物件加入到 `free list` 中,準備清除。 將已被標記的物件重置為未被標記,以利下次的清掃。 ### 如何實作 #### 簡單實作 `Mark` 1. 建立訪問清單 `todo` 將所有 root 中的物件加入。 2. 當 `todo` 中有物件時,執行以下3~4。 3. 從 `todo` 中選任一物件 `v` ,將其除外於 `todo`。 4. 如果 `v` 未被標記,標記 `v` ,將所有 `v` 包含的物件放到 `todo` 中。 #### 簡單實作 `Sweep` 1. 物件 `p` 從 `heap` 的最底層物件開始。 2. 當 `p` 還不是 `top` 的時候,執行以下3~5。 3. 如果 `p` 已被標記,則將其改為未標記。 4. 反之,將 `p` 的記憶體區塊加到 `free list` 中,準備清除。 5. 將 `p` 改為在 heap 中上一層的物件。 ( `using p <- p + sizeof(p)` ) #### 問題點 `1` * 需要清除時,通常表示沒有記憶體空間。 * 但是, Mark 階段需要記憶體空間去創造訪問清單( `todo` ) 。 * 訪問清單( `todo` )無法預測大小,因此無法預先保留。 :::info 解決方法 * 應用 `pointer reversal` ,用 `depth first search` 取代訪問清單。 ::: #### 問題點 `2` * 有些物件沒有指標的空間。 :::info 解決方法 * 使用一個暫存器記憶其 `parent` 位置,就可以不用 `pointer reversal` 也能執行 `depth firsth search` 。 ::: ## Type Imprecision ### 問題與實驗 在 C 或 C++ 的實作中,通常無法精確,因為可能會有 ` unreachable ` 的物件被標為 `reachable` ,原因可能為 type imprecision 或是 liveness imprecision 等等。 以下為實作 type imprecision 情況的程式 Based on [ogc](https://github.com/jserv/ogc) . ```clike #include <assert.h> #include "gc.h" void ImPrecise(long *l) { int* Unreachable; Unreachable = gc_alloc(sizeof(int)); assert(Unreachable); *l = (long)Unreachable; } int main(int argc, char *argv[]) { gc_init(&argc, 1); long *l; l = gc_alloc(sizeof(long)); assert(l); ImPrecise(l); gc_run(); gc_dump_internals(); gc_destroy(); return 0; } ``` 預計輸出 ```bash ## GC internals ## - GC.ptr_map[20]: { ptr: 0x7ffc624006a0, size: 8, marked: 0 } ``` 實際輸出 ```bash ## GC internals ## - GC.ptr_map[20]: { ptr: 0x7fceecc006a0, size: 8, marked: 0 } - GC.ptr_map[58]: { ptr: 0x7fceecc02bd0, size: 4, marked: 0 } ``` 可以看到 `reachable` 的物件,應該只有 type 為 `long` 的 `l` 。 但是 `l` 中儲存的值 `剛好` 等於為一個 `unreachable` 物件的指標。 因此,「保守」的 GC 便將其標為 `reachable` 。 ### 解決方法 ###### tags: ` Cprogramming `

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