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Embedded Software Lab.

Sungkyunkwan University

Embedded Software Lab.

Dongkun Shin

전공핵심실습1:운영체제론Chapter 11. Accessing Files

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Embedded Software Lab.

• Tizen Z3 기기 반납– 일시: 2016년 6월 14일 화요일 오후 6:15~ (15주차 수업 시간)

– 구성품

• Samsung Z3(SM-Z300HZDDINS) 1대

• Micro-USB 케이블 1개

• Micro-SD 카드 (Sandisk micro-SDHC Ultra 16GB Class10) 1개

• Micro-SD 카드 리더기 (Memorette M2 리더기) 1개

– 실습장비 사용 중에 관리 소홀로 인한 고장, 부속품 분실 등으로비용이 발생할 경우 대여자의 책임으로 처리함.

Announcement

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• Reading and Writing a File

• Memory Mapping

• Practice 1. Accessing File in Linux

• Memory Area Allocation in malloc

• Practice 2. malloc in Linux

Contents

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Embedded Software Lab.

• File Accessing Modes– Canonical mode

• Read ops block the calling process, Write ops do not block the calling process. (Read : Sync, Write : Async by using page cache)

– Synchronous mode

• Write ops also block the calling process.

– Memory Mapping mode

• Map the file into memory that can be accessed directly by the applications. (User buffer has a role as page cache)

– Direct I/O mode

• Bypass the page cache.

– Asynchronous mode

• Requests for data transfers never block the calling process.

Accessing Files in Linux

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Linux I/O Stack

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• Reading from a file (Canonical/Synchronous mode)– Page-based

• If the data is not in RAM

• Kernel allocates a new page frame

• Fills the page with the data

• Adds the page to the page cache (Kernel Address Space)

• Copies the page into the process address space

• Writing to a file– Depends on the file system type.

• Usually do_generic_file_write()

Reading and Writing a File

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Embedded Software Lab.

• do_generic_file_read()

• Descriptors

– iovec : address & length of user mode buffer

– kiocb : keep track of the completion status of I/O operation

Reading from a file

Corresponding file object pointerSemantic descriptor for asynchronous I/OMethod called in cancelling this operation

Current position of the file in this operationBytes to transfer

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Embedded Software Lab.

Read()

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• Many disk accesses are sequential

• Enhance disk performance & Improve system responsiveness

• No use when performing random accesses to files

• Current Window– Consists of pages requested by process or read in advance in

kernel.

• Ahead Window– Consist of pages currently being read by kernel

– Not yet requested by process

Read-ahead

PG_READAHEAD

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Embedded Software Lab.

• Read ahead Descriptor

Read-ahead (cont’)

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Embedded Software Lab.

Write

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Embedded Software Lab.

• Memory Mapping (File I/O by generic memory instruction)– Memory region can be associated with regular file or block

device file

– Accessing pages of the memory region = operation on the mapped file

– Shared memory paging (MAP_SHARED)

• Each write changes the file on disk

• Visible to all other processes that map the same file

– Private memory paging (MAP_PRIVATE)

• Just a read the file, not to write it

• Much more efficient than shared memory mapping

Memory Mapping

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Memory Mapping Data Structures

Metadata about memory area of a process

Metadata about page cache memory area of a file

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Embedded Software Lab.

• mmap() system call– Parameters

• File descriptor

• Offset inside the file

• Length of the file

• Flag (Shared or Private)

• Permission (Read, Write and Execute)

– Procedure1. Check availability of mmap.

2. Find available virtual memory area with get_unmapped_area()

3. Check open mode

4. Initialize memory mapping descriptors

• Destroying a Memory Mapping– munmap()

• Flush all dirty data and remove vma

Creating a Memory Mapping

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Embedded Software Lab.

• Write the code and compile by gcc. (in Linux PC)1. $ gedit fileaccess.c

2. $ gcc -o fileaccess fileaccess.c

Practice 1. Accessing File in Linux (1/2)

#include <stdio.h>

int main() {FILE *fp1, *fp2;int readdata;printf("1. fopen()\n");fp1 = fopen("./a.txt", "w");printf("2. fprintf()\n");fprintf(fp1, "12345");printf("3. fclose()\n");fclose(fp1);

printf("4. fopen()\n");fp2 = fopen("./a.txt", "r");printf("5. fscanf()\n");fscanf(fp2, "%d", &readdata);printf("6. fclose()\n");fclose(fp2);return 0;

}

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Embedded Software Lab.

• Print system calls called on accessing file. – $ strace ./fileaccess

Practice 1. Accessing File in Linux (2/2)

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• Chunk– Memory allocation unit of malloc

• Bin– a doubly linked list aggregating

free chunks by size

• Small bin(16B≤|r|≤512B)– Allocated by brk() (data area)

– b : 1 bin per 8B unit = 62 bins

• Large bin(512B<|r|)– b : 64B, 512B, 4KB, 32KB, 256KB

– The bins includes chunks of which size is less that b.

– 128KB<r• Allocated by mmap() (heap area)

• Cannot be separated

Memory Area Allocation in malloc

r : requested memory sizeb : bin’s chunk unit criteria

b

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• Write the code and compile by gcc. (in Linux PC)1. $ gedit malloc.c

2. $ gcc -o malloc malloc.c

Practice 2. malloc in Linux (1/2)

#include <stdio.h>#include <stdlib.h>

int main() {void *mem1, *mem2;printf("1. malloc(100)\n"); // 100Bmem1 = malloc(100);printf("2. first free()\n");free(mem1);printf("3. malloc(1000000)\n"); // 1MBmem2 = malloc(1000000);printf("4. second free()\n");free(mem2);

}

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• Print system calls called on accessing file. – $ strace ./malloc

Practice 2. malloc in Linux (2/2)