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    SubjectOvercommittable memory (Was: Some questions about linux kernel.)
    Den 20-Mar-00 13:12:16 skrev James Sutherland følgende om "Re: Some questions about linux kernel.":
    > On Sun, 19 Mar 2000 15:50:07 -0600, you wrote:

    >>NOPE - if the process requests it and is granted it, then it should have
    >>access to it. It is not up to the system to say "here it is, but don't use
    >>that part of it, because I really didn't give it".

    > malloc() just allocates address SPACE. TOUCHING the memory then
    > populates that space, which is how I can use malloc() for a nice,
    > simple sparse matrix implementation. (On Win32, you have to mess
    > around trapping exceptions in userland and allocating the memory
    > yourself...)

    Note that malloc() is documented as allocating memory, regardless of
    whether you actually touch it or not. Any programmer relies on that.

    >>We are talking about the sum of all concurrent requests, and the system
    >>aborting when part of the requests already granted turns out to not be

    > No. The malloc() call requested address space, and that succeeded -
    > the address space was available.

    > What is NOT available, however, is memory to populate that area fully.
    > So what?

    So you have a broken malloc().

    > I could well be using malloc() to allocate a SPARSE matrix,
    > in which case I certainly do NOT want it to populate the whole thing!

    Then using malloc() to create the matrix in one go is the wrong thing to

    > I would be seriously irritated if your disk quota system denied me
    > permission to create a sparse file on the basis that, if fully
    > populated, it would overrun my quota. Why apply the same system to
    > memory?

    An lseek()/write() combination is documented to permit the file system
    to create a sparse file. malloc() etc. has no permission to create sparse
    memory blocks. That's why.


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