copy-on-write | 易学教程

Does swift copy on write for all structs?

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问题 I know that swift will optimize to copy on write for arrays but will it do this for all structs? For example: struct Point { var x:Float = 0 } var p1 = Point() var p2 = p1 //p1 and p2 share the same data under the hood p2.x += 1 //p2 now has its own copy of the data 回答1: Array is implemented with copy-on-write behaviour – you'll get it regardless of any compiler optimisations (although of course, optimisations can decrease the number of cases where a copy needs to happen). At a basic level,

Does swift copy on write for all structs?

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Kernel copying CoW pages after child process exit

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问题 In Linux, whenever a process is forked, the memory mappings of the parent process are cloned into the child process. In reality, for performance reasons, the pages are set to be copy-on-write -- initially they are shared and, in the event one of the two processes writing on one of them, they will then be cloned ( MAP_PRIVATE ). This is a very common mechanism of getting a snapshot of the state of a running program -- you do a fork, and this gives you a (consistent) view of the memory of the

PHP null and copy-on-write

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问题 Suppose I want to have two variables and have them both equal to null . (More realistically, I am thinking about an array that contains a large amount of null s, but the "two variables" scenario is sufficient for the question.) Obviously, I can do this in more than one way. I can do this (method 1): $a = null; $b = $a; By my understanding, the result of this is that there is one zval that is pointed to by two entries in the symbol table: 'a' and 'b' . But alternatively one might do this

Does Go language use Copy-on-write for strings

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问题 Does Go language use Copy-on-write for strings as in Java? I.e. if I pass a string by value to a method and never change it will this allocate memory and copy the string (which will be time inefficient) or it will just reference a single copy. 回答1: It's not Copy-on-Write, as strings are immutable. But sharing a string will not make a copy of the underlying memory region either. In Go, a string is represented as a (length, data) pair. If you pass a string around, Go will copy the length and

PHP null and copy-on-write

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Suppose I want to have two variables and have them both equal to null . (More realistically, I am thinking about an array that contains a large amount of null s, but the "two variables" scenario is sufficient for the question.) Obviously, I can do this in more than one way. I can do this (method 1): $a = null; $b = $a; By my understanding, the result of this is that there is one zval that is pointed to by two entries in the symbol table: 'a' and 'b' . But alternatively one might do this (method 2): $a = null; $b = null; Naively one would expect that this should result in two different zvals,

Multiprocessing: why is a numpy array shared with the child processes, while a list is copied?

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问题 I used this script (see code at the end) to assess whether a global object is shared or copied when the parent process is forked. Briefly, the script creates a global data object, and the child processes iterate over data . The script also monitors the memory usage to assess whether the object was copied in the child processes. Here are the results: data = np.ones((N,N)) . Operation in the child process: data.sum() . Result: data is shared (no copy) data = list(range(pow(10, 8))) . Operation

How to Disable Copy-on-write and zero filled on demand for mmap()

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问题 I am implementing cp(file copy) command using mmap(). For that I mapped the source file in MAP_PRIVATE (As I just want to read)mode and destination file in MAP_SHARED mode(As I have to writeback the changed content of destination file). While doing this I have observed performance penalty due to lots of minor page faults that occurs due to 2 reason. 1) Zero fill on demand while calling mmap(MAP_PRIVATE) for source file. 2) Copy on write while calling mmap(MAP_SHARED) for destination file. Is

Does Go language use Copy-on-write for strings

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Does Go language use Copy-on-write for strings as in Java? I.e. if I pass a string by value to a method and never change it will this allocate memory and copy the string (which will be time inefficient) or it will just reference a single copy. It's not Copy-on-Write, as strings are immutable. But sharing a string will not make a copy of the underlying memory region either. In Go, a string is represented as a (length, data) pair. If you pass a string around, Go will copy the length and the pointer but not the data pointed to. For further information, see this recent thread on golang-nuts . Go

How does copy-on-write in fork() handle multiple fork?

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问题 According to wikipedia (which could be wrong) When a fork() system call is issued, a copy of all the pages corresponding to the parent process is created, loaded into a separate memory location by the OS for the child process. But this is not needed in certain cases. Consider the case when a child executes an "exec" system call (which is used to execute any executable file from within a C program) or exits very soon after the fork(). When the child is needed just to execute a command for the