A tool to manage virtual memory allocation and deallocation on Linux operating systems
Utilising system calls mmap or munmap to respond the memory request from kernel
vm stores the virtual memory and 'datablock - metablock' pairs requested by applications
vm for families tracks all the applications
vm_for_ vm_for_
emp_t stu_t
--------- ---------
| | | |
|_______| | |
| 28 F | | |
vm_for_families --------- | |
| 36 | |_______| A: Allocated
--------- |_______| | 28 F | F: Freed
| | | 28 A | ---------
--------- --------- | |
| stu_t | s=56 | 36 | | 56 | <- datablock
--------- |_______| |_______|
| emp_t | s=36 | 28 F | | 28 A | <- metablock
--------- --------- ---------
Page Size = 4096 Bytes
vm_page_family : emp_t, struct size = 36
next = (nil), prev = (nil)
page family = emp_t
0x7fe6762b0018 Block 0 F R E E D block_size = 36 offset = 24 prev = (nil) next = 0x7fe6762b006c
0x7fe6762b006c Block 1 ALLOCATED block_size = 36 offset = 108 prev = 0x7fe6762b0018 next = 0x7fe6762b00c0
0x7fe6762b00c0 Block 2 F R E E D block_size = 3856 offset = 192 prev = 0x7fe6762b006c next = (nil)
vm_page_family : student_t, struct size = 56
next = (nil), prev = (nil)
page family = student_t
0x7fe6762a0018 Block 0 ALLOCATED block_size = 56 offset = 24 prev = (nil) next = 0x7fe6762a0080
0x7fe6762a0080 Block 1 F R E E D block_size = 3920 offset = 128 prev = 0x7fe6762a0018 next = (nil)
# Of VM Pages in Use : 2 (8192 Bytes)
Total Memory being used by Memory Manager = 8192 Bytes
emp_t TBC : 3 FBC : 2 OBC : 1 AppMemUsage : 84
student_t TBC : 2 FBC : 1 OBC : 1 AppMemUsage : 104=> store information of the following datablock
struct meta_block_t {
int size; /* == 14 */
struct meta_block_t *next; /* == NULL*/
bool is_free; /* == FALSE*/
} | |
-------- <- break point
| |
| 20 | <- datablock
| |
-------- <- void *ptr3 = malloc(20);
| 20 F | <- metablock
-------- <- 12 + 20 = 32 = 8 * 4 bytes
| |
| 20 | <- datablock
| |
-------- <- void *ptr2 = malloc(20);
| 20 F| <- metablock
--------
-------- <- 12 + 14 + (2) = 28 = 7 * 4 bytes
| |
| 14 | <- datablock
| |
-------- <- void *ptr1 = malloc(14);
| 14 F| <- metablock (4 + 4 + 4 for three items)
---------
| |
HEAP SEGMENT
ex. free(ptr2); => get the information from its metablock => free the data block => turn flag (meta_block.is_free=TRUE)
--------- <- 12 + 20 = 32 = 8 * 4 bytes
| |
| 20 | <- datablock
| |
-------- <- void *ptr2 = malloc(20);
| 20 T| <- metablock
---------
void *ptr4 = malloc(10);
-------- --------
| | | 8 |
| | |-------|
| 30 | | 8 T |
| | --------- <- void *ptr2_B
| | | 10 | <- datablock
-------- --------- <- void *ptr2 = malloc(20);
| 30 T| | 10 F | <- metablock
-------- ---------
=> if block is split to too small segments => waste many segments for metablocks => splitting is NOT an optimal way
--------- ---------
| 8 | | |
|-------| | |
| 8 T | | 26 | <- datablock
--------- | |
| 10 | | |
--------- --------- <- void *ptr2
| 10 T | | 26 F | <- metablock
--------- ---------
After block splitting, 8-byte block cannnot be used by any malloc(x) as x>8 It's called internal fragmentation
-------- ---------
| | | 8 |
| | |-------|
| 30 | | 8 T |
| | --------- <- void *ptr2_B
| | | 10 | <- datablock
-------- --------- <- void *ptr2 = malloc(20);
| 30 T| | 10 F | <- metablock
-------- ---------
If malloc(16), none of these two block can be taken, because they are not continuous. => external fragmentation
---------
| 8 | <= smaller than 16
|-------|
| 8 T |
---------
| 8 |
|-------|
| 8 F |
---------
| 10 | <= smaller than 16
---------
| 10 T |
---------