reading blocksįirst, in most IO operations the characteristics of the underlying storage hardware dominates the performance. (It is possible to advise kernel in advance with madvise() so that the kernel may load the pages in advance before access).įor more details, there is related question on Stack Overflow: mmap() vs. Reading a large block with read() can be faster than mmap() in such cases, if mmap() would generate significant number of faults to read the file. If a page of the mapped file is not in memory, access will generate a fault and require kernel to load the page to memory. There is also no system call overhead when accessing memory mapped file after the initial call if the file is loaded to memory at initial mmap(). Using mmap() maps the file to process' address space, so the process can address the file directly and no copies are required. Kernel has to copy the data to/from those locations. Calls to read() and write() include a pointer to buffer in process' address space where the data is stored. Memory mapping a file directly avoids copying buffers which happen with read() and write() calls. How does memory mapping a file have significant performance increases over the standard I/O system calls? Then when it accesses the mapped memory, page faults happen. It takes a system call to create a memory mapping. If I am correct, memory mapping file works as following. Using the read() and write() system calls simplifies and speeds up fileĬould you analyze the performance of memory mapped file? Manipulating files through memory rather than incurring the overhead of Reads and writes to the file are handled as routine memory accesses. Read in more than a page-sized chunk of memory at a time). Read from the file system into a physical page (some systems may opt to However, a page-sized portion of the file is Initial access to the file proceeds through ordinary demand paging, Memory mapping a file isĪccomplished by mapping a disk block to a page (or pages) in memory. Lead to significant performance increases. Memory mapping a file, allows a part of the virtual address space to be logically associated with the file. Each file access requiresĪlternatively, we can use the virtual memory techniques discussed soįar to treat file I/O as routine memory accesses. System calls open(), read(), and write(). Consider a sequential read of a file on disk using the standard
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