How does multiprocessing work

These methods are usually unnecessary for most code:. Indicate that no more data will be put on this queue by the current process. The background thread will quit once it has flushed all buffered data to the pipe. This is called automatically when the queue is garbage collected. Join the background thread. This can only be used after close has been called. It blocks until the background thread exits, ensuring that all data in the buffer has been flushed to the pipe.

It is likely to cause enqueued data to be lost, and you almost certainly will not need to use it. Without one, the functionality in this class will be disabled, and attempts to instantiate a Queue will result in an ImportError. See bpo for additional information. The same holds true for any of the specialized queue types listed below. It is a simplified Queue type, very close to a locked Pipe. A queue must not be used anymore after it is closed.

For example, get , put and empty methods must no longer be called. Indicate that a formerly enqueued task is complete. Used by queue consumers. Raises a ValueError if called more times than there were items placed in the queue. The count of unfinished tasks goes up whenever an item is added to the queue. When the count of unfinished tasks drops to zero, join unblocks. This number is not equivalent to the number of CPUs the current process can use.

The number of usable CPUs can be obtained with len os. Return the Process object corresponding to the current process. An analogue of threading. Add support for when a program which uses multiprocessing has been frozen to produce a Windows executable.

Returns a list of the supported start methods, the first of which is the default. The possible start methods are 'fork' , 'spawn' and 'forkserver'. On Windows only 'spawn' is available. On Unix 'fork' and 'spawn' are always supported, with 'fork' being the default.

Return a context object which has the same attributes as the multiprocessing module. If method is None then the default context is returned. Otherwise method should be 'fork' , 'spawn' , 'forkserver'. ValueError is raised if the specified start method is not available. The return value can be 'fork' , 'spawn' , 'forkserver' or None. Sets the path of the Python interpreter to use when starting a child process. By default sys. Embedders will probably need to do some thing like.

Set the method which should be used to start child processes. Timer , or threading. Connection objects allow the sending and receiving of picklable objects or strings. They can be thought of as message oriented connected sockets.

Connection objects are usually created using Pipe — see also Listeners and Clients. Send an object to the other end of the connection which should be read using recv. The object must be picklable. Return an object sent from the other end of the connection using send. Blocks until there is something to receive. Raises EOFError if there is nothing left to receive and the other end was closed.

If timeout is not specified then it will return immediately. If timeout is a number then this specifies the maximum time in seconds to block.

If timeout is None then an infinite timeout is used. Note that multiple connection objects may be polled at once by using multiprocessing. Send byte data from a bytes-like object as a complete message. If offset is given then data is read from that position in buffer. If size is given then that many bytes will be read from buffer. Return a complete message of byte data sent from the other end of the connection as a string. Raises EOFError if there is nothing left to receive and the other end has closed.

If maxlength is specified and the message is longer than maxlength then OSError is raised and the connection will no longer be readable.

Read into buffer a complete message of byte data sent from the other end of the connection and return the number of bytes in the message. If offset is given then the message will be written into the buffer from that position. Offset must be a non-negative integer less than the length of buffer in bytes.

If the buffer is too short then a BufferTooShort exception is raised and the complete message is available as e. New in version 3. The Connection. Therefore, unless the connection object was produced using Pipe you should only use the recv and send methods after performing some sort of authentication.

If a process is killed while it is trying to read or write to a pipe then the data in the pipe is likely to become corrupted, because it may become impossible to be sure where the message boundaries lie. Generally synchronization primitives are not as necessary in a multiprocess program as they are in a multithreaded program. See the documentation for threading module. Note that one can also create synchronization primitives by using a manager object — see Managers.

A barrier object: a clone of threading. A bounded semaphore object: a close analog of threading. A condition variable: an alias for threading. If lock is specified then it should be a Lock or RLock object from multiprocessing. A clone of threading. A non-recursive lock object: a close analog of threading.

Once a process or thread has acquired a lock, subsequent attempts to acquire it from any process or thread will block until it is released; any process or thread may release it. The concepts and behaviors of threading. Lock as it applies to threads are replicated here in multiprocessing. Lock as it applies to either processes or threads, except as noted. Note that Lock is actually a factory function which returns an instance of multiprocessing.

Lock initialized with a default context. Lock supports the context manager protocol and thus may be used in with statements. With the block argument set to True the default , the method call will block until the lock is in an unlocked state, then set it to locked and return True.

Note that the name of this first argument differs from that in threading. With the block argument set to False , the method call does not block. If the lock is currently in a locked state, return False ; otherwise set the lock to a locked state and return True.

When invoked with a positive, floating-point value for timeout , block for at most the number of seconds specified by timeout as long as the lock can not be acquired.

Invocations with a negative value for timeout are equivalent to a timeout of zero. Invocations with a timeout value of None the default set the timeout period to infinite. Note that the treatment of negative or None values for timeout differs from the implemented behavior in threading. The timeout argument has no practical implications if the block argument is set to False and is thus ignored.

Returns True if the lock has been acquired or False if the timeout period has elapsed. Release a lock. This can be called from any process or thread, not only the process or thread which originally acquired the lock. Behavior is the same as in threading. A recursive lock object: a close analog of threading.

A recursive lock must be released by the process or thread that acquired it. Once a process or thread has acquired a recursive lock, the same process or thread may acquire it again without blocking; that process or thread must release it once for each time it has been acquired.

Note that RLock is actually a factory function which returns an instance of multiprocessing. RLock initialized with a default context. RLock supports the context manager protocol and thus may be used in with statements. When invoked with the block argument set to True , block until the lock is in an unlocked state not owned by any process or thread unless the lock is already owned by the current process or thread.

The current process or thread then takes ownership of the lock if it does not already have ownership and the recursion level inside the lock increments by one, resulting in a return value of True. When invoked with the block argument set to False , do not block. If the lock has already been acquired and thus is owned by another process or thread, the current process or thread does not take ownership and the recursion level within the lock is not changed, resulting in a return value of False.

If the lock is in an unlocked state, the current process or thread takes ownership and the recursion level is incremented, resulting in a return value of True. Use and behaviors of the timeout argument are the same as in Lock. Note that some of these behaviors of timeout differ from the implemented behaviors in threading.

Release a lock, decrementing the recursion level. If after the decrement the recursion level is zero, reset the lock to unlocked not owned by any process or thread and if any other processes or threads are blocked waiting for the lock to become unlocked, allow exactly one of them to proceed.

If after the decrement the recursion level is still nonzero, the lock remains locked and owned by the calling process or thread. Only call this method when the calling process or thread owns the lock.

An AssertionError is raised if this method is called by a process or thread other than the owner or if the lock is in an unlocked unowned state. Note that the type of exception raised in this situation differs from the implemented behavior in threading. A semaphore object: a close analog of threading. This differs from the behaviour of threading where SIGINT will be ignored while the equivalent blocking calls are in progress.

Without one, the multiprocessing. It is possible to create shared objects using shared memory which can be inherited by child processes. Return a ctypes object allocated from shared memory. By default the return value is actually a synchronized wrapper for the object.

The object itself can be accessed via the value attribute of a Value. If lock is True the default then a new recursive lock object is created to synchronize access to the value. If lock is a Lock or RLock object then that will be used to synchronize access to the value. So if, for instance, you want to atomically increment a shared value it is insufficient to just do. Return a ctypes array allocated from shared memory. By default the return value is actually a synchronized wrapper for the array.

If lock is True the default then a new lock object is created to synchronize access to the value. Note that an array of ctypes. The multiprocessing. Although it is possible to store a pointer in shared memory remember that this will refer to a location in the address space of a specific process. However, the pointer is quite likely to be invalid in the context of a second process and trying to dereference the pointer from the second process may cause a crash.

Note that setting and getting an element is potentially non-atomic — use Array instead to make sure that access is automatically synchronized using a lock. Note that setting and getting the value is potentially non-atomic — use Value instead to make sure that access is automatically synchronized using a lock. The same as RawArray except that depending on the value of lock a process-safe synchronization wrapper may be returned instead of a raw ctypes array. The same as RawValue except that depending on the value of lock a process-safe synchronization wrapper may be returned instead of a raw ctypes object.

Return a ctypes object allocated from shared memory which is a copy of the ctypes object obj. Return a process-safe wrapper object for a ctypes object which uses lock to synchronize access. If lock is None the default then a multiprocessing. A multiprocessing operating system OS is one in which two or more central processing units CPUs control the functions of the computer. Each CPU contains a copy of the OS, and these copies communicate with one another to coordinate operations.

The use of multiple processors allows the computer to perform calculations faster, since tasks can be divided up between processors. Multiprocessing operating systems OSs perform the same functions as single-processor OSs. They schedule and monitor operations and calculations in order to complete user-initiated tasks. The difference is that multiprocessing OSs divide the work up into various subtasks and then assign these subtasks to different central processing units CPUs.

Multiprocessing uses a distinct communication architecture to accomplish this. A multiprocessing OS needs a mechanism for the processors to interact with one another as they schedule tasks and coordinate their completion. Because multiprocessing OSs rely on parallel processing, each processor involved in a task must be able to inform the others about how its task is progressing. This allows the work of the processors to be integrated when the calculations are done such that delays and other inefficiencies are minimized.

For example, if a single-processor OS were running an application requiring three tasks to be performed, one taking five milliseconds, another taking eight milliseconds, and the last taking seven milliseconds, the processor would perform each task in order.

The entire application would thus require twenty milliseconds. Does this mean multiprocessing is useless in my situation? PrivateUser PrivateUser 4, 12 12 gold badges 57 57 silver badges 92 92 bronze badges. If this is a hyperthreaded CPU, then you'll still be able to run two processes simultaneously.

But in general, you will be able to run simultaneously, as many processes as you have logical core CPUs on your machine. I would assume that you have a hyperthreaded CPU in this case, so multiprocessing shouldn't be useless - just not as useful — inspectorG4dget. Pavel updated my question — PrivateUser. Pavel Thats my ubuntu desktop cpu details. I'm not on a virtual machine — PrivateUser. Show 2 more comments. Active Oldest Votes.

Nicolas Defranoux Nicolas Defranoux 2, 1 1 gold badge 9 9 silver badges 12 12 bronze badges. So If I buy 4 core processor, does that mean I can run 16 threads?

When using the threading module, this can also happen when you're printing because the text can get jumbled up and cause data corruption. You can use a print lock to ensure that only one thread can print at a time. I find that many guides tend to skip the negatives of using the tool they've just been trying to teach you.

It's important to understand that there are both pros and cons associated with using all these tools. For example:. Python wasn't designed considering that personal computers might have more than one core shows you how old the language is , so the GIL is necessary because Python is not thread-safe and there is a globally enforced lock when accessing a Python object.

Though not perfect, it's a pretty effective mechanism for memory management. What can we do? Multiprocessing allows you to create programs that can run concurrently bypassing the GIL and use the entirety of your CPU core.