![]() The processes using the memory normally don't need to know when this happens, but if necessary, they can call msync to make it happen sooner. If there is a file behind the shared memory, the kernel occasionally syncs changed pages from RAM to the file. Writes by one process become visible to others directly, without needing to pass through the disk file (if any) that the memory is attached to. When multiple processes are using the same shared memory region, their individual virtual addresses will be pointing to the same physical address. It depends on how the memory region was created. Shared memory can be backed by a regular file, a block device, or swap. Memory between the communicating processes (Figure 9.23). The memory-mapped file serves as the region of shared Under this scenario, processes can communicate using shared memoryīy having the communicating processes memory-map the same file into their Quite often, shared memory is in fact implemented by memory mapping Memory between the communicating processes" seems to mean that it resides in main memory. It also introduces shared memory in the following.ĭo multiple processes share a memory region by sharing a memory-mapped file?ĭoes a "memory-mapped file" reside on disk or main memory? I think it is on the disk, but "The memory-mapped file serves as the region of shared Physical memory-the page that holds a copy of the disk block. The virtual memory map of each sharing process points to the same page of How the sharing of memory-mapped sections of memory is implemented: Given our earlier discussions of virtual memory, it should be clear Virtual memory and can be seen by all others that map the same section of ![]() Writes by any of the processes modify the data in Multiple processes may be allowed to map the same file concurrently, Operating System Concepts introduces sharing a memory-mapped file in the following.ĭo the multiple processes share the same file by sharing the same physical memory region holding the content of the file? When the file descriptor is specified as -1 an anonymous mmap is created that is a memory region, which is not backed by a disk file.Are sharing a memory-mapped file and sharing a memory region implemented based on each other? The following two quotes seem to say so, and seem a chicken-egg problem to me.Access level - ACCESS_READ for read only, ACCESS_WRITE for read and write and ACCESS_COPY for copy-on-write.Offset of the bytes from the beginning of the file.Number of bytes of the file to be mapped.To create a memory mapped file the following information needs to be provided to the mmap constructor.Python mmap constructor has variants for both Windows and Unix systems.A memory mapped file can be accessed similar to the way a bytearray is accessed in Python-using subscript notation or using the slice notation.When files are dealt as memory mapped files, the contents of the files can be accessed in the same way a region of memory is accessed, from a Python Program.The Python mmap constructor creates a memory-mapped file.While the usage of typical I/O APIs and their overhead is eliminated in using Memory Mapped Files, there are other things like "write order" by multiple programs to be taken care by the programmer.With memory mapped files programs access the data directly from the memory rather than using any I/O routines.The mechanism involved in memory-mapped files is the same mechanism used by operating systems for implementing virtual memory.A memory-mapped file is a region of memory that is mapped to a disk file.
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