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jvm開發筆記5 – 虛擬機內存管理

作者：王智通

 

一、 前言
ajvm是筆者正在開發中的一個java虛擬機， 想通過編寫這個jvm幫助程序員了解jvm的具體實現細節， 它是國內第一個開源的java虛擬機項目：https://github.com/cloudsec/ajvm， 同時筆者把它的開發筆記也分享到了ata上。 在前麵4篇筆記中， 已經實現了class文件加載器， 反匯編器，jvm的crash信息處理， 同時它已經能運行簡單的java代碼了。 在今天的這篇筆記中， 將開始分享ajvm的內存管理模塊是如何編寫的。

二、內存分配

看下麵一段java代碼：

public class test6 {
        public static void main(String args[]) {
                int[] data, data1;
                int i;
                int num = 0;

                data = new int[2];
                for (i = 0; i < 2; i++) {
                        data[i] = i;
                }

                data1 = new int[3];
        }
}

首先用javac編譯下， 然後用ajvm的反匯編器查看bytecode：

$./wvm -d test/test6.class
Diassember bytecode:

<init>    ()V
stack: 1    local: 1

    0: aload_0
    1: invokespecial #1
    4: return

main    ([Ljava/lang/String;)V
stack: 3    local: 5

    0: iconst_0
    1: istore 4
    3: iconst_2
    4: newarray 10
    6: astore_1
    7: iconst_0
    8: istore_3
    9: iload_3
   10: iconst_2
   11: if_icmpge 13
   14: aload_1
   15: iload_3
   16: iload_3
   17: iastore
   18: iinc 3 1
   21: goto 0xfffffff4
   24: iconst_3
   25: newarray 10
   27: astore_2
   28: return

源碼中data = new int[2];對應的匯編指令為：

    4: newarray 10

根據jvm虛擬機規範的描述， newarray指令的作用是， 從操作數堆棧用取出data數組的元素個數，然後根據newarray後麵的type進行計算要申請的內存大小， type的值在虛擬機規範中如下：

#define T_BOOLEAN                               4
#define T_CHAR                                  5
#define T_FLOAT                                 6
#define T_DOUBLE                                7
#define T_BYTE                                  8
#define T_SHORT                                 9
#define T_INT                                   10
#define T_LONG                                  11

所以10代表這個int類型的數組， 接下來就要給data這個數組從heap中分配內存了。

void *alloc_newarray_memroy(u1 atype, int count)
{
        void *addr = NULL;

        switch (atype) {
        case T_BOOLEAN:
        case T_CHAR:
        case T_BYTE:
                addr = (void *)slab_alloc(jvm_thread_mem, count * sizeof(char));
                break;
        case T_SHORT:
                addr = (void *)slab_alloc(jvm_thread_mem, count * sizeof(short));
                break;
        case T_INT:
        case T_FLOAT:
                addr = (void *)slab_alloc(jvm_thread_mem, count * sizeof(int));
                break;
        case T_LONG:
        case T_DOUBLE:
                addr = (void *)slab_alloc(jvm_thread_mem, count * sizeof(long long));
                break;
        default:
                error("bad atype value.n");
                return NULL;
        }

        return addr;
}

ajvm的內存堆用的是slab算法， slab的內存結構如下：

   -------     ------     ------    ------
   |cache|-->  |slab| --> |slab| -->|slab|
   -------     ------     ------    ------
   |cache|
   -----
   |cache| ...
   -----      ------     ------    ------
   |cache|--> |slab| --> |slab| -->|slab|
   -----      ------     -----     ------
   |cache| ...
   -------    
   |cache|
   -------
   |cache|-->|slab|-->|slab| -->|slab|
   -------   ------   ------    ------

源碼中的slab.c是它完整的實現， 不熟悉slab的同學請自行google。

三、垃圾回收

gc是java程序員普遍關心的問題， 當內存不夠時， 將會觸發jvm的垃圾回收機製。
ajvm使用最原始的引用計數法， 需要建立一個新的數據結構：

typedef struct jvm_object {
        int ref_count;
        CLASS *class;
        void *addr;
        int size;
        struct list_head list;
}JVM_OBJECT;

當數組申請完內存後， 將會建立一個新的JVM_OBJECT與其對應， ref_count被初始化為0, addr指向數組的首地址， size表示數組的大小， JVM_OBJECT將會被加入到jvm_obj_list_head鏈表中， 在這將來的垃圾回收時將會用到。

int jvm_interp_newarray(u2 len, char *symbol, void *base)
{
    ...
        addr = (void *)alloc_newarray_memroy(atype, count);
        if (!addr) {
                error("slab alloc failed.n");
                return -1;
        }
        printf("addr: 0x%xn", addr);

        new_obj = create_new_obj(addr, count);
        if (!new_obj) {
                error("create new obj failed.n");
                return -1;
        }
    ...
}

當數組被引用時， 我們跟數組的地址在JVM_OBJECT鏈表中找到它， 並且把ref_count加1, 表示這個數組在被引用。 比如上麵的：

      17: iastore

這條指令就會對data數組進行引用， 我們隻要在iastore的解釋代碼裏， 對data對應的ref_count加1即可：

int jvm_interp_iastore(u2 len, char *symbol, void *base)
{
        int *addr, index, value;

        if (jvm_arg->disass_class) {
                printf("%sn", symbol);
                return 0;
        }

        pop_operand_stack(int, value)
        pop_operand_stack(int, index)
        pop_operand_stack(int, addr)

        printf("addr: 0x%xtindex: %dt%dn", addr, index, value);
        *(int *)(addr + index) = value;

        if (inc_obj_ref(addr, (&jvm_obj_list_head)) == -1) {
                jvm_error(VM_ERROR_INTERP, "inc jvm obj ref failed.n");
                return -1;
        }

        jvm_pc.pc += len;
        return 0;
}

對於數組data1, 同樣進行了內存分配， 但是始終沒有被引用到， 所以data1將會是gc回收時要釋放的對象。

 

void start_gc(struct list_head *list_head)
{
        JVM_OBJECT *s;
        struct list_head *p, *q;

        list_for_each_safe(p, q, list_head) {
                s = list_entry(p, JVM_OBJECT, list);
                if (s && s->ref_count == 0) {
                        printf("free addr: 0x%xtsize: %dtref_count: %dn",
                                s->addr, s->size, s->ref_count);
                        list_del(p);
                        free_jvm_obj(s);
                }
        }
}

這是ajvm最簡單的gc算法了， 後續將會對其進行優化。

四、演示執行

下麵是ajvm對上述java代碼的解釋和執行過程：

$./wvm -c test test6
jvm pc init at: 0x630510

main    ([Ljava/lang/String;)V
stack: 3    local : 5
code:
0x3 0x36 0x4 0x5 0xbc 0xa 0x4c 0x3 0x3e 0x1d 0x5 0xa2 0x0 0xd 0x2b 0x1d
0x1d 0x4f 0x84 0x3 0x1 0xa7 0xff 0xf4 0x6 0xbc 0xa 0x4d 0xb1
#local at: 0x630540    #stack at: 0x630554

[    1] iconst_0    pc: 0x630510 -> 0x3
#local: 0x0 0x0 0x0 0x0 0x0     #stack: 0x0 0x0 0x0
#local: 0x0 0x0 0x0 0x0 0x0     #stack: 0x0 0x0 0x0
[    2] istore    pc: 0x630511 -> 0x36
#local: 0x0 0x0 0x0 0x0 0x0     #stack: 0x0 0x0 0x0
#local: 0x0 0x0 0x0 0x0 0x0     #stack: 0x0 0x0 0x0
#local: 0x0 0x0 0x0 0x0 0x0     #stack: 0x0 0x0 0x0
#local: 0x0 0x0 0x0 0x0 0x0     #stack: 0x0 0x0 0x0
[    3] iconst_2    pc: 0x630513 -> 0x5
#local: 0x0 0x0 0x0 0x0 0x0     #stack: 0x0 0x0 0x0
#local: 0x0 0x0 0x0 0x0 0x0     #stack: 0x2 0x0 0x0
[    4] newarray    pc: 0x630514 -> 0xbc
#local: 0x0 0x0 0x0 0x0 0x0     #stack: 0x2 0x0 0x0
#local: 0x0 0x0 0x0 0x0 0x0     #stack: 0x0 0x0 0x0
#local: 0x0 0x0 0x0 0x0 0x0     #stack: 0x0 0x0 0x0
#local: 0x0 0x0 0x0 0x0 0x0     #stack: 0x627c20 0x0 0x0
[    5] astore_1    pc: 0x630516 -> 0x4c
#local: 0x0 0x0 0x0 0x0 0x0     #stack: 0x627c20 0x0 0x0
#local: 0x0 0x0 0x0 0x0 0x0     #stack: 0x0 0x0 0x0
#local: 0x0 0x0 0x0 0x0 0x0     #stack: 0x0 0x0 0x0
#local: 0x0 0x627c20 0x0 0x0 0x0     #stack: 0x0 0x0 0x0
[    6] iconst_0    pc: 0x630517 -> 0x3
#local: 0x0 0x627c20 0x0 0x0 0x0     #stack: 0x0 0x0 0x0
#local: 0x0 0x627c20 0x0 0x0 0x0     #stack: 0x0 0x0 0x0
[    7] istore_3    pc: 0x630518 -> 0x3e
#local: 0x0 0x627c20 0x0 0x0 0x0     #stack: 0x0 0x0 0x0
#local: 0x0 0x627c20 0x0 0x0 0x0     #stack: 0x0 0x0 0x0
#local: 0x0 0x627c20 0x0 0x0 0x0     #stack: 0x0 0x0 0x0
#local: 0x0 0x627c20 0x0 0x0 0x0     #stack: 0x0 0x0 0x0
[    8] iload_3    pc: 0x630519 -> 0x1d
#local: 0x0 0x627c20 0x0 0x0 0x0     #stack: 0x0 0x0 0x0
#local: 0x0 0x627c20 0x0 0x0 0x0     #stack: 0x0 0x0 0x0
#local: 0x0 0x627c20 0x0 0x0 0x0     #stack: 0x0 0x0 0x0
#local: 0x0 0x627c20 0x0 0x0 0x0     #stack: 0x0 0x0 0x0
[    9] iconst_2    pc: 0x63051a -> 0x5
#local: 0x0 0x627c20 0x0 0x0 0x0     #stack: 0x0 0x0 0x0
#local: 0x0 0x627c20 0x0 0x0 0x0     #stack: 0x0 0x2 0x0
[   10] if_icmpge    pc: 0x63051b -> 0xa2
#local: 0x0 0x627c20 0x0 0x0 0x0     #stack: 0x0 0x2 0x0
#local: 0x0 0x627c20 0x0 0x0 0x0     #stack: 0x0 0x0 0x0
#local: 0x0 0x627c20 0x0 0x0 0x0     #stack: 0x0 0x0 0x0
#local: 0x0 0x627c20 0x0 0x0 0x0     #stack: 0x0 0x0 0x0
[   11] aload_1    pc: 0x63051e -> 0x2b
#local: 0x0 0x627c20 0x0 0x0 0x0     #stack: 0x0 0x0 0x0
#local: 0x0 0x627c20 0x0 0x0 0x0     #stack: 0x0 0x0 0x0
#local: 0x0 0x627c20 0x0 0x0 0x0     #stack: 0x0 0x0 0x0
#local: 0x0 0x627c20 0x0 0x0 0x0     #stack: 0x627c20 0x0 0x0
[   12] iload_3    pc: 0x63051f -> 0x1d
#local: 0x0 0x627c20 0x0 0x0 0x0     #stack: 0x627c20 0x0 0x0
#local: 0x0 0x627c20 0x0 0x0 0x0     #stack: 0x627c20 0x0 0x0
#local: 0x0 0x627c20 0x0 0x0 0x0     #stack: 0x627c20 0x0 0x0
#local: 0x0 0x627c20 0x0 0x0 0x0     #stack: 0x627c20 0x0 0x0
[   13] iload_3    pc: 0x630520 -> 0x1d
#local: 0x0 0x627c20 0x0 0x0 0x0     #stack: 0x627c20 0x0 0x0
#local: 0x0 0x627c20 0x0 0x0 0x0     #stack: 0x627c20 0x0 0x0
#local: 0x0 0x627c20 0x0 0x0 0x0     #stack: 0x627c20 0x0 0x0
#local: 0x0 0x627c20 0x0 0x0 0x0     #stack: 0x627c20 0x0 0x0
[   14] iastore    pc: 0x630521 -> 0x4f
#local: 0x0 0x627c20 0x0 0x0 0x0     #stack: 0x627c20 0x0 0x0
#local: 0x0 0x627c20 0x0 0x0 0x0     #stack: 0x627c20 0x0 0x0
#local: 0x0 0x627c20 0x0 0x0 0x0     #stack: 0x627c20 0x0 0x0
#local: 0x0 0x627c20 0x0 0x0 0x0     #stack: 0x627c20 0x0 0x0
#local: 0x0 0x627c20 0x0 0x0 0x0     #stack: 0x627c20 0x0 0x0
#local: 0x0 0x627c20 0x0 0x0 0x0     #stack: 0x0 0x0 0x0
[   15] iinc    pc: 0x630522 -> 0x84
#local: 0x0 0x627c20 0x0 0x0 0x0     #stack: 0x0 0x0 0x0
#local: 0x0 0x627c20 0x0 0x0 0x0     #stack: 0x0 0x0 0x0
#local: 0x0 0x627c20 0x0 0x0 0x0     #stack: 0x0 0x0 0x0
#local: 0x0 0x627c20 0x0 0x1 0x0     #stack: 0x0 0x0 0x0
[   16] goto    pc: 0x630525 -> 0xa7
[   17] iload_3    pc: 0x630519 -> 0x1d
#local: 0x0 0x627c20 0x0 0x1 0x0     #stack: 0x0 0x0 0x0
#local: 0x0 0x627c20 0x0 0x1 0x0     #stack: 0x0 0x0 0x0
#local: 0x0 0x627c20 0x0 0x1 0x0     #stack: 0x0 0x0 0x0
#local: 0x0 0x627c20 0x0 0x1 0x0     #stack: 0x1 0x0 0x0
[   18] iconst_2    pc: 0x63051a -> 0x5
#local: 0x0 0x627c20 0x0 0x1 0x0     #stack: 0x1 0x0 0x0
#local: 0x0 0x627c20 0x0 0x1 0x0     #stack: 0x1 0x2 0x0
[   19] if_icmpge    pc: 0x63051b -> 0xa2
#local: 0x0 0x627c20 0x0 0x1 0x0     #stack: 0x1 0x2 0x0
#local: 0x0 0x627c20 0x0 0x1 0x0     #stack: 0x1 0x0 0x0
#local: 0x0 0x627c20 0x0 0x1 0x0     #stack: 0x1 0x0 0x0
#local: 0x0 0x627c20 0x0 0x1 0x0     #stack: 0x0 0x0 0x0
[   20] aload_1    pc: 0x63051e -> 0x2b
#local: 0x0 0x627c20 0x0 0x1 0x0     #stack: 0x0 0x0 0x0
#local: 0x0 0x627c20 0x0 0x1 0x0     #stack: 0x0 0x0 0x0
#local: 0x0 0x627c20 0x0 0x1 0x0     #stack: 0x0 0x0 0x0
#local: 0x0 0x627c20 0x0 0x1 0x0     #stack: 0x627c20 0x0 0x0
[   21] iload_3    pc: 0x63051f -> 0x1d
#local: 0x0 0x627c20 0x0 0x1 0x0     #stack: 0x627c20 0x0 0x0
#local: 0x0 0x627c20 0x0 0x1 0x0     #stack: 0x627c20 0x0 0x0
#local: 0x0 0x627c20 0x0 0x1 0x0     #stack: 0x627c20 0x0 0x0
#local: 0x0 0x627c20 0x0 0x1 0x0     #stack: 0x627c20 0x1 0x0
[   22] iload_3    pc: 0x630520 -> 0x1d
#local: 0x0 0x627c20 0x0 0x1 0x0     #stack: 0x627c20 0x1 0x0
#local: 0x0 0x627c20 0x0 0x1 0x0     #stack: 0x627c20 0x1 0x0
#local: 0x0 0x627c20 0x0 0x1 0x0     #stack: 0x627c20 0x1 0x0
#local: 0x0 0x627c20 0x0 0x1 0x0     #stack: 0x627c20 0x1 0x1
[   23] iastore    pc: 0x630521 -> 0x4f
#local: 0x0 0x627c20 0x0 0x1 0x0     #stack: 0x627c20 0x1 0x1
#local: 0x0 0x627c20 0x0 0x1 0x0     #stack: 0x627c20 0x1 0x0
#local: 0x0 0x627c20 0x0 0x1 0x0     #stack: 0x627c20 0x1 0x0
#local: 0x0 0x627c20 0x0 0x1 0x0     #stack: 0x627c20 0x0 0x0
#local: 0x0 0x627c20 0x0 0x1 0x0     #stack: 0x627c20 0x0 0x0
#local: 0x0 0x627c20 0x0 0x1 0x0     #stack: 0x0 0x0 0x0
[   24] iinc    pc: 0x630522 -> 0x84
#local: 0x0 0x627c20 0x0 0x1 0x0     #stack: 0x0 0x0 0x0
#local: 0x0 0x627c20 0x0 0x1 0x0     #stack: 0x0 0x0 0x0
#local: 0x0 0x627c20 0x0 0x1 0x0     #stack: 0x0 0x0 0x0
#local: 0x0 0x627c20 0x0 0x2 0x0     #stack: 0x0 0x0 0x0
[   25] goto    pc: 0x630525 -> 0xa7
[   26] iload_3    pc: 0x630519 -> 0x1d
#local: 0x0 0x627c20 0x0 0x2 0x0     #stack: 0x0 0x0 0x0
#local: 0x0 0x627c20 0x0 0x2 0x0     #stack: 0x0 0x0 0x0
#local: 0x0 0x627c20 0x0 0x2 0x0     #stack: 0x0 0x0 0x0
#local: 0x0 0x627c20 0x0 0x2 0x0     #stack: 0x2 0x0 0x0
[   27] iconst_2    pc: 0x63051a -> 0x5
#local: 0x0 0x627c20 0x0 0x2 0x0     #stack: 0x2 0x0 0x0
#local: 0x0 0x627c20 0x0 0x2 0x0     #stack: 0x2 0x2 0x0
[   28] if_icmpge    pc: 0x63051b -> 0xa2
#local: 0x0 0x627c20 0x0 0x2 0x0     #stack: 0x2 0x2 0x0
#local: 0x0 0x627c20 0x0 0x2 0x0     #stack: 0x2 0x0 0x0
#local: 0x0 0x627c20 0x0 0x2 0x0     #stack: 0x2 0x0 0x0
#local: 0x0 0x627c20 0x0 0x2 0x0     #stack: 0x0 0x0 0x0
[   29] iconst_3    pc: 0x630528 -> 0x6
#local: 0x0 0x627c20 0x0 0x2 0x0     #stack: 0x0 0x0 0x0
#local: 0x0 0x627c20 0x0 0x2 0x0     #stack: 0x3 0x0 0x0
[   30] newarray    pc: 0x630529 -> 0xbc
#local: 0x0 0x627c20 0x0 0x2 0x0     #stack: 0x3 0x0 0x0
#local: 0x0 0x627c20 0x0 0x2 0x0     #stack: 0x0 0x0 0x0
#local: 0x0 0x627c20 0x0 0x2 0x0     #stack: 0x0 0x0 0x0
#local: 0x0 0x627c20 0x0 0x2 0x0     #stack: 0x627c80 0x0 0x0
[   31] astore_2    pc: 0x63052b -> 0x4d
#local: 0x0 0x627c20 0x0 0x2 0x0     #stack: 0x627c80 0x0 0x0
#local: 0x0 0x627c20 0x0 0x2 0x0     #stack: 0x0 0x0 0x0
#local: 0x0 0x627c20 0x0 0x2 0x0     #stack: 0x0 0x0 0x0
#local: 0x0 0x627c20 0x627c80 0x2 0x0     #stack: 0x0 0x0 0x0
[   32] return    pc: 0x63052c -> 0xb1
#local: 0x0 0x627c20 0x627c80 0x2 0x0     #stack: 0x0 0x0 0x0
jvm stack depth is zero.
interpret bytecode done.

 

最後更新：2017-04-03 07:57:05

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