Author davin
Recipients brett.cannon, davin, eric.snow, giampaolo.rodola, lukasz.langa, nascheme, osvenskan, pitrou, pmpp, rhettinger, ronaldoussoren, skrah, terry.reedy, yselivanov
Date 2019-02-23.16:42:57
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> FWIW I bumped into this lib: http://semanchuk.com/philip/sysv_ipc/

The author of that lib, Philip Semanchuk, is one of the people participating in this effort -- he has posted above in msg334934 here on b.p.o. and has helped review the PR in GH-11816.

He is also the author of the posix_ipc package which was the original basis for our POSIX Shared Memory implementation here.

The decision to base our Unix platform support upon POSIX and not SystemV libraries came after considerable research and there are important differences between the two.  To oversimplify:  POSIX Shared Memory support has now been available for some time on Linux, *BSD, MacOS, and others and is something of a successor to the SystemV.


> That assumes a single app/process which spawns a child (the "worker").

Not true.  A manager started by one process can be connected to by another process that is not a child.  This is covered in the docs here:  https://docs.python.org/3/library/multiprocessing.html#using-a-remote-manager  That child can then request that shared memory blocks it creates be remotely tracked and managed by that remote process's manager.  While I would not expect this to be a common use case, this is a feature of BaseManager that we inherit into SharedMemoryManager.

The SyncManager.Lock can be used as part of this as well.  Thus, two unrelated apps/processes *can* coordinate their management of shared memory blocks through the SharedMemoryManager.


> That would translate into a new Semaphore(name=None, create=False)
> class which (possibly?) would also provide better performances
> compared to SyncManager.Semaphore

Right!  You might have noticed that Philip has such a semaphore construct in his posix_ipc lib.

I opted to not attempt to add this feature as part of this effort to both (1) keep focused on the core needs to work with shared memory, and (2) to take more time in the future to work out how to get cross-platform support for the semaphore right (as you point out, there are complications to work through).


> Extra 1: apparently there are also POSIX msgget(), msgrcv() and
> msgsnd() syscalls which could be used to implement a System-V message
> Queue similar to SyncManager.Queue later on.

Right!  This is also something Philip has in his posix_ipc lib.  This should be part of the roadmap for what we do next with SharedMemory.  This one may be complicated by the fact that not all platforms that implement POSIX Shared Memory chose to also implement these functions in the same way.  We will need time to work out what we can or can not reasonably do here.


> Extra 2: given the 2 distinct use-cases I wonder if the low-level
> component (shared_memory.py) really belongs to multiprocessing module

Given what I wrote above about how multiprocessing.managers does enable these use cases and the existing "distributed shared memory" support in multiprocessing, I think it logically belongs in multiprocessing.  I suggest that "shm_open" and "shm_unlink" are our low-level tools, which appropriately are in _posixshmem, but SharedMemory and the rest are high-level tools; SharedMemoryManager will not be able to cover all life-cycle management use cases thus SharedMemory will be needed by many and in contrast, "shm_open" and "shm_unlink" will be needed only by those wishing to do something wacky.  (Note: I am not trying to make "wacky" sound like a bad thing because wacky can be very cool sometimes.)


Philip's ears should now be burning, I mentioned him so many times in this post.  Ah!  He beat me to it while I was writing this.  Awesome!

We would not be where we are with SharedMemory without his efforts over many years with his posix_ipc lib.
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2019-02-23 16:42:57davinsetrecipients: + davin, brett.cannon, nascheme, rhettinger, terry.reedy, ronaldoussoren, pitrou, osvenskan, giampaolo.rodola, skrah, pmpp, lukasz.langa, eric.snow, yselivanov
2019-02-23 16:42:57davinsetmessageid: <1550940177.77.0.0490177666158.issue35813@roundup.psfhosted.org>
2019-02-23 16:42:57davinlinkissue35813 messages
2019-02-23 16:42:57davincreate