copy-on-write

问题 I have a class that returns large NumPy arrays. These arrays are cached within the class. I would like the returned arrays to be copy-on-write arrays. If the caller ends up just reading from the array, no copy is ever made. This will case no extra memory will be used. However, the array is "modifiable", but does not modify the internal cached arrays. My solution at the moment is to make any cached arrays readonly (a.flags.writeable = False) . This means that if the caller of the function may

问题 I have a class that returns large NumPy arrays. These arrays are cached within the class. I would like the returned arrays to be copy-on-write arrays. If the caller ends up just reading from the array, no copy is ever made. This will case no extra memory will be used. However, the array is "modifiable", but does not modify the internal cached arrays. My solution at the moment is to make any cached arrays readonly (a.flags.writeable = False) . This means that if the caller of the function may

问题 My filesystem (FS) (ZFS specifically) supports copy-on-write (COW), i.e. a copy (if done right) is a very cheap constant operation, and does not actually copy the underlying content. The content is copied only once I write/modify the new file. Actually, I just found out, ZFS-on-Linux actually has not implemented that for userspace yet (right?). But e.g. BTRFS or XFS has. (See here, here, here, here.) For the (GNU) cp utility, you would pass --reflink=always option (see here.) cp calls ioctl

问题 Say I have a process in Linux from which I fork() another identical process. After fork ing, as the original process will start writing to memory, the Linux copy-on-write mechanism will give the process unique physical memory pages which are different from the one used by the forked process. How can I, at some point of execution, know which pages of the original process have been copied-on-write? I don't want to use SIGSEGV signal handler and give read only access to all the pages in the

问题 I have a very large structure, which I want to ensure is not copied needlessly. How can I make a copy-on-write container for it? 回答1: A copy-on-write is usually a struct wrapper over some backing object. public final class MutableHeapStore<T>: NonObjectiveCBase { public typealias Storage = T public private(set) var storage: Storage public init(storage: Storage) { self.storage = storage } } public struct COW<T> { public typealias Storage = MutableHeapStore<T> public typealias Value = T public

问题 How to get sparse block size and check if data is present at the given offset in sparse file in reiserfs/ext3 in Linux? I want to use it to implement simple copy-on-write block device using FUSE. Or I should better keep a bitmap in a separate file? 回答1: /usr/src/linux/Documentation/filesystems/fiemap.txt The fiemap ioctl is an efficient method for userspace to get file extent mappings. Instead of block-by-block mapping (such as bmap), fiemap returns a list of extents. There's a quick example

问题 I want to know how copy-on-write happens in fork(). Assuming we have a process A that has a dynamical int array: int *array = malloc(1000000*sizeof(int)); Elements in array are initialized to some meaningful values. Then, we use fork() to create a child process, namely B. B will iterate the array and do some calculations: for(a in array){ a = a+1; } I know B will not copy the entire array immediately, but when does the child B allocate memory for array? during fork()? Does it allocate the

问题 How do I prevent the GC from provoking copy-on-write, when I fork my process ? I have recently been analyzing the garbage collector's behavior in Ruby, due to some memory issues that I encountered in my program (I run out of memory on my 60core 0.5Tb machine even for fairly small tasks). For me this really limits the usefulness of ruby for running programs on multicore servers. I would like to present my experiments and results here. The issue arises when the garbage collector runs during

问题 Say we have a certain parent process with some arbitrary amount of data stored in memory and we use fork to spawn a child process. I understand that in order for the OS to perform copy on write, the certain page in memory that contains the data that we are modifying will have its Read-only bit set, and the OS will use the exception that will result when the child tries to modify the data to copy the entire page into another area in memory so that the child gets it's own copy. What I don't

问题 Is a shared memory/copy on write implementation for general containers (like that found in Qt's containers) superseded by C++11 move semantics and rvalue references? Where does one fail and the other succeed? Or are they complementary rather than alternatives? 回答1: Both copy on write and move semantics have been used to optimize value semantics of objects that hold their data on the heap. std::string , for example has been implemented both as a copy-on-write object, and as a move-enabled