Shared Memory Modules

Discussion in 'Python' started by S Green, Nov 25, 2003.

  1. S Green

    S Green Guest

    Does any one now if a shared memory module exists, written in python
    for a windows platform. I now one exists for unix?


    help most appreciated,

    S Green
     
    S Green, Nov 25, 2003
    #1
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  2. S Green

    Peter Hansen Guest

    S Green wrote:
    >
    > Does any one now if a shared memory module exists, written in python
    > for a windows platform. I now one exists for unix?


    Your first message with this same question did in fact make it to
    Usenet and the mailing list.

    Generally speaking, you should allow several days for answers to questions,
    rather than getting anxious just because ten people don't jump up and answer
    it in the first few hours.

    (And my guess is that one doesn't exist, as Windows shared memory would
    probably be a completely unreliable bitch, but that's just my guess. :)

    -Peter
     
    Peter Hansen, Nov 25, 2003
    #2
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  3. (S Green) writes:

    > Does any one now if a shared memory module exists, written in python
    > for a windows platform. I now one exists for unix?
    >
    >
    > help most appreciated,
    >
    > S Green


    import mmap
     
    Thomas Heller, Nov 25, 2003
    #3
  4. S Green

    Peter Hansen Guest

    Thomas Heller wrote:
    >
    > (S Green) writes:
    >
    > > Does any one now if a shared memory module exists, written in python
    > > for a windows platform. I now one exists for unix?
    > >
    > >
    > > help most appreciated,
    > >
    > > S Green

    >
    > import mmap


    The docs suggest that mmap.mmap(x, x, mmap.MAP_SHARED) will work under
    Unix but not Windows.

    Does the Windows version really support shared memory, or is the
    multiple-maps-per-file feature only valid within a single process?

    -Peter
     
    Peter Hansen, Nov 25, 2003
    #4
  5. Peter Hansen <> writes:

    > Thomas Heller wrote:
    >>
    >> (S Green) writes:
    >>
    >> > Does any one now if a shared memory module exists, written in python
    >> > for a windows platform. I now one exists for unix?
    >> >
    >> >
    >> > help most appreciated,
    >> >
    >> > S Green

    >>
    >> import mmap

    >
    > The docs suggest that mmap.mmap(x, x, mmap.MAP_SHARED) will work under
    > Unix but not Windows.
    >
    > Does the Windows version really support shared memory, or is the
    > multiple-maps-per-file feature only valid within a single process?


    Yes, it does. You have to give this memory block a name, though:

    sharedmem = mmap.mmap(0, 16384, "GlobalSharedMemory")

    This allows to access 16384 bytes of memory, shared across processes,
    not backed up by a file in the filesystem.

    Thomas
     
    Thomas Heller, Nov 25, 2003
    #5
  6. S Green

    Peter Hansen Guest

    Thomas Heller wrote:
    >
    > Peter Hansen <> writes:
    >
    > > Thomas Heller wrote:
    > >>
    > >> (S Green) writes:
    > >>
    > >> > Does any one now if a shared memory module exists, written in python
    > >> > for a windows platform. I now one exists for unix?
    > >> >
    > >> >
    > >> > help most appreciated,
    > >> >
    > >> > S Green
    > >>
    > >> import mmap

    > >
    > > The docs suggest that mmap.mmap(x, x, mmap.MAP_SHARED) will work under
    > > Unix but not Windows.
    > >
    > > Does the Windows version really support shared memory, or is the
    > > multiple-maps-per-file feature only valid within a single process?

    >
    > Yes, it does. You have to give this memory block a name, though:
    >
    > sharedmem = mmap.mmap(0, 16384, "GlobalSharedMemory")
    >
    > This allows to access 16384 bytes of memory, shared across processes,
    > not backed up by a file in the filesystem.


    Thanks, Thomas.

    In my opinion the documentation on this is entirely unclear. I attach
    it for reference, but I can't offer any suggestions for improvement (as
    I don't know anything about shared memory) except that even after reading
    it a second time, Thomas' answer above is very much news to me:

    '''tagname, if specified and not None, is a string giving a tag name
    for the mapping. Windows allows you to have many different mappings
    against the same file. If you specify the name of an existing tag,
    that tag is opened, otherwise a new tag of this name is created. If
    this parameter is omitted or None, the mapping is created without a
    name. Avoiding the use of the tag parameter will assist in keeping your
    code portable between Unix and Windows. '''

    (from http://www.python.org/doc/current/lib/module-mmap.html)

    -Peter
     
    Peter Hansen, Nov 25, 2003
    #6
  7. Thomas Heller wrote:

    >>Does the Windows version really support shared memory, or is the
    >>multiple-maps-per-file feature only valid within a single process?

    >
    >
    > Yes, it does. You have to give this memory block a name, though:
    >
    > sharedmem = mmap.mmap(0, 16384, "GlobalSharedMemory")
    >
    > This allows to access 16384 bytes of memory, shared across processes,
    > not backed up by a file in the filesystem.


    That is très cool, it doesn't tell you this in the docs, does it?
    The first argument is 'the file handle' of the file to be mapped,
    and it doesn't say that 0 is valid and means 'no file at all'...

    However, I've just tried it, and managed to crash Python in mmap.pyd
    with an application exception... twice. But trying to reproduce it
    now fails-- Python keeps running.
    I did this:
    >>> import mmap
    >>> mem=mmap.mmap(0,3000000,'irmen')
    >>> mem[0]='a'
    >>> mem[2000000]='a'

    and initially, it crashed......... Python 2.3.2 on win xp)

    --Irmen
     
    Irmen de Jong, Nov 25, 2003
    #7
  8. Irmen de Jong <> writes:

    > Thomas Heller wrote:
    >
    >>> Does the Windows version really support shared memory, or is the
    >>> multiple-maps-per-file feature only valid within a single process?

    >> Yes, it does. You have to give this memory block a name, though:
    >> sharedmem = mmap.mmap(0, 16384, "GlobalSharedMemory")
    >> This allows to access 16384 bytes of memory, shared across processes,
    >> not backed up by a file in the filesystem.

    >
    > That is très cool, it doesn't tell you this in the docs, does it?
    > The first argument is 'the file handle' of the file to be mapped,
    > and it doesn't say that 0 is valid and means 'no file at all'...
    >
    > However, I've just tried it, and managed to crash Python in mmap.pyd
    > with an application exception... twice. But trying to reproduce it
    > now fails-- Python keeps running.
    > I did this:
    > >>> import mmap
    > >>> mem=mmap.mmap(0,3000000,'irmen')
    > >>> mem[0]='a'
    > >>> mem[2000000]='a'

    > and initially, it crashed......... Python 2.3.2 on win xp)


    This works for me even from the beginning (with 2.3 CVS version, XP Pro).

    Understading which parameters to pass apparently requires

    - reading the mmapmodule.c sources
    - and reading about CreateFileMapping and MapViewOfFile in MSDN.

    It would be great if someine could submit a patch for the docs <wink>.

    I vaguely remember, but am not able to find it anymore: didn't AMK once
    had an article about this somewhere?

    Thomas
     
    Thomas Heller, Nov 25, 2003
    #8
  9. Irmen de Jong wrote:
    > However, I've just tried it, and managed to crash Python in mmap.pyd
    > with an application exception... twice. But trying to reproduce it
    > now fails-- Python keeps running.
    > I did this:
    > >>> import mmap
    > >>> mem=mmap.mmap(0,3000000,'irmen')
    > >>> mem[0]='a'
    > >>> mem[2000000]='a'

    > and initially, it crashed......... Python 2.3.2 on win xp)
    > --Irmen


    Probably not enough actual memory hanging around. Some systems
    (and from this I'd guess XP) allocate virtual memory by reserving
    address space, not actually allocating the RAM and/or backing
    store for that memory. Python has no control over this, and you
    have nothing good to do if the memory is over-allocated. When
    you create the memory, you could walk across it writing into it
    (forcing it to exist), but that would just force the failure to
    happen earlier.


    -Scott David Daniels
     
    Scott David Daniels, Dec 1, 2003
    #9
  10. Scott David Daniels wrote:
    > Irmen de Jong wrote:
    >
    >> However, I've just tried it, and managed to crash Python in mmap.pyd
    >> with an application exception... twice. But trying to reproduce it
    >> now fails-- Python keeps running.
    >> I did this:
    >> >>> import mmap
    >> >>> mem=mmap.mmap(0,3000000,'irmen')
    >> >>> mem[0]='a'
    >> >>> mem[2000000]='a'

    >> and initially, it crashed......... Python 2.3.2 on win xp)
    >> --Irmen

    >
    >
    > Probably not enough actual memory hanging around. Some systems
    > (and from this I'd guess XP) allocate virtual memory by reserving
    > address space, not actually allocating the RAM and/or backing
    > store for that memory.


    Over-commit is that called, right?
    I'm sorry but that certainly wasn't the case here.
    My machine has 512 Mb RAM and about 250 Mb of them
    allocated when I tried it. (physical RAM that is)

    Very weird, I cannot reproduce the initial crash I experienced.

    --Irmen
     
    Irmen de Jong, Dec 1, 2003
    #10
  11. On Tue, 02 Dec 2003 00:44:15 +0100, Irmen de Jong <> wrote:

    >Scott David Daniels wrote:
    >> Irmen de Jong wrote:
    >>
    >>> However, I've just tried it, and managed to crash Python in mmap.pyd
    >>> with an application exception... twice. But trying to reproduce it
    >>> now fails-- Python keeps running.
    >>> I did this:
    >>> >>> import mmap
    >>> >>> mem=mmap.mmap(0,3000000,'irmen')
    >>> >>> mem[0]='a'
    >>> >>> mem[2000000]='a'
    >>> and initially, it crashed......... Python 2.3.2 on win xp)
    >>> --Irmen

    >>
    >>
    >> Probably not enough actual memory hanging around. Some systems
    >> (and from this I'd guess XP) allocate virtual memory by reserving
    >> address space, not actually allocating the RAM and/or backing
    >> store for that memory.

    >
    >Over-commit is that called, right?
    >I'm sorry but that certainly wasn't the case here.
    >My machine has 512 Mb RAM and about 250 Mb of them
    >allocated when I tried it. (physical RAM that is)
    >
    >Very weird, I cannot reproduce the initial crash I experienced.
    >

    What's that first zero argument?

    Snip from mmap docs:
    """
    mmap( fileno, length[, tagname[, access]])

    (Windows version) Maps length bytes from the file specified by
    the file handle fileno, and returns a mmap object. If length is 0,
    the maximum length of the map will be the current size of the file
    when mmap() is called.

    tagname, if specified and not None, is a string giving a tag name
    for the mapping. Windows allows you to have many different mappings
    against the same file. If you specify the name of an existing tag,
    that tag is opened, otherwise a new tag of this name is created.
    If this parameter is omitted or None, the mapping is created without a name.
    Avoiding the use of the tag parameter will assist in keeping your code portable
    between Unix and Windows.
    """

    Does that mean you are trying to use stdin as the open file handle?

    Regards,
    Bengt Richter
     
    Bengt Richter, Dec 2, 2003
    #11
  12. Bengt Richter wrote:
    > What's that first zero argument?

    [...]
    > Does that mean you are trying to use stdin as the open file handle?


    No it doesn't, according to Thomas Heller's post in this thread.
    Thomas said 0 means "not associated with a file" on windows...

    --Irmen
     
    Irmen de Jong, Dec 2, 2003
    #12
  13. Irmen de Jong <> writes:

    > Bengt Richter wrote:
    >> What's that first zero argument?

    > [...]
    >> Does that mean you are trying to use stdin as the open file handle?

    >
    > No it doesn't, according to Thomas Heller's post in this thread.
    > Thomas said 0 means "not associated with a file" on windows...


    mmapmodule.c internally uses INVALID_HANDLE_VALUE of a file handle of 0
    is used.

    As I said, read the source (and MSDN). And then submit a documentation
    patch <wink>.

    Thomas
     
    Thomas Heller, Dec 2, 2003
    #13
  14. On Tue, 02 Dec 2003 10:54:19 +0100, Thomas Heller <> wrote:

    >Irmen de Jong <> writes:
    >
    >> Bengt Richter wrote:
    >>> What's that first zero argument?

    >> [...]
    >>> Does that mean you are trying to use stdin as the open file handle?

    >>
    >> No it doesn't, according to Thomas Heller's post in this thread.
    >> Thomas said 0 means "not associated with a file" on windows...

    >
    >mmapmodule.c internally uses INVALID_HANDLE_VALUE of a file handle of 0
    >is used.

    Wouldn't passing None be more pythonic for a python interface?

    >
    >As I said, read the source (and MSDN). And then submit a documentation
    >patch <wink>.
    >

    How does one do that? (being lazy, I guess I could find out via www.python.org)

    Regards,
    Bengt Richter
     
    Bengt Richter, Dec 2, 2003
    #14
  15. (Bengt Richter) writes:

    > On Tue, 02 Dec 2003 10:54:19 +0100, Thomas Heller <> wrote:
    >
    >>Irmen de Jong <> writes:
    >>
    >>> Bengt Richter wrote:
    >>>> What's that first zero argument?
    >>> [...]
    >>>> Does that mean you are trying to use stdin as the open file handle?
    >>>
    >>> No it doesn't, according to Thomas Heller's post in this thread.
    >>> Thomas said 0 means "not associated with a file" on windows...

    >>
    >>mmapmodule.c internally uses INVALID_HANDLE_VALUE of a file handle of 0
    >>is used.

    > Wouldn't passing None be more pythonic for a python interface?
    >
    >>
    >>As I said, read the source (and MSDN). And then submit a documentation
    >>patch <wink>.
    >>

    > How does one do that? (being lazy, I guess I could find out via www.python.org)


    Finding out how to do this is the easier part ;-)

    Ok, to get you started:

    <http://cvs.sourceforge.net/viewcvs.py/python/python/dist/src/Modules/mmapmodule.c>
    <http://msdn.microsoft.com/library/en-us/fileio/base/creating_named_shared_memory.asp>
    <http://msdn.microsoft.com/library/en-us/fileio/base/createfilemapping.asp>
    <http://msdn.microsoft.com/library/en-us/fileio/base/mapviewoffile.asp>

    Thomas
     
    Thomas Heller, Dec 2, 2003
    #15
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