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94 lines
2.6 KiB
Markdown
94 lines
2.6 KiB
Markdown
---
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title: "18-processes-sharing-and-threads"
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aliases:
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tags:
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- cosc204
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- lecture
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---
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# Shared memory for processes
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Can't assume all variables are initally zero.
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OS decide which process maps who which physical address
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memory mapping `mmap` is a family of system calls:
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- allow two processes to share some region of their memory space.
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- achieved by creating a memory region that can be shared by other processes
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shared memory by two processes mapping to the same file or using fork() after mmap()
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``` c
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random_fd = open("/home/hzy/test/zero", O_RDWR);
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ptr = mmap(NULL, 10240, PROT_READ|PROT_WRITE,
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MAP_SHARED | MAP_FILE, random_fd, 0);
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```
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private mapping is often used to set up new memory sections.
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``` c
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fd = open("mmap_test_file", O_RDWR); p
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tr = mmap(NULL, 10240, PROT_READ|PROT_WRITE,
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MAP_PRIVATE|MAP_FILE, fd, 0);
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```
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# Threads
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## product consumer problem
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- a producer process writes data to a buffer of fied size
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- a consumer reads data from the buffer
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these two processes are being scheduled independently by the CPU
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producer must wait if buffer is full
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consumer must wait if buffer is empty
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## threads
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- allows more convenient data sharing
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- threads within a single process share everything except they have their own program counter, registers and stack space
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## why
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- responsiveness - parrallelism - dont have to wait
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- resource sharing - automaticall share everything in same process
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- economy - lightweight, creating and switching between threads is faster than creating heavyweight processes
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- scalability - each to work on multiprocessor architectures
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## pthread
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- pthread_create(): create a thread
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- pthread_exit(): terminate a thread
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- pthread_join(): wait for a thread to terminate
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- pthread_detach(): detach a thread so that it won’t be waited
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- pthread_self(): get the thread id of the current thread
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- pthread_equal(): compare if two threads are the same
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- pthread_cancel(): send a cancellation request to a thread
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``` c
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void *myThreadFun(void *vargp) {
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sleep(1);
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printf("Hello from Thread \n");
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return NULL;
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}
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int main() {
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pthread_t thread_id;
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printf("Before Thread\n");
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pthread_create(&thread_id, NULL, myThreadFun, NULL);
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pthread_join(thread_id, NULL);
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printf("After Thread\n");
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exit(0);
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}
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```
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## web server thread
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- a web server is a process that provides data over the web to requesting clients
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- a thread is created for each request
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Lightweight process - shares everything except the stack
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