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    Jesse Pollard <> on 07/11/2001 01:08:02 AM

    To:, Timur Tabi <>
    cc: (bcc: Amol Lad/HSS)

    Subject: Re: What is the truth about Linux 2.4's RAM limitations?

    "Richard B. Johnson" <>
    > In Unix and Unix variants, it is by design, provided that the
    > kernel exist within every process address space. Early Nixes
    > like Ultrix, simply called the kernel entry point. Since it
    > was protected, this trapped to the real kernel and the page-fault
    > handler actually performed the work on behalf of the caller.
    > Unlike some OS (like VMS), a context-switch does not occur
    > when the kernel provides services for the calling task.
    > Therefore, it was most reasonable to have the kernel exist within
    > each tasks address space. With modern processors, it doesn't make
    > very much difference, you could have user space start at virtual
    > address 0 and extend to virtual address 0xffffffff. However, this would
    > not be Unix. It would also force the kernel to use additional
    > CPU cycles when addressing a tasks virtual address space,
    > i.e., when data are copied to/from user to kernel space.

    I believe the VAX/VMS implementation shared OS and user space:

    p0 - user application 0
    p1 - system shared libraries 0x3fffffff
    p2 - kernel 0x7fffffff
    rest was I/O, cache memory 0xffffffff

    It was a hardware design, not a function of the software.

    UNIX origins were on a PDP-11. there were two sets of addressing registers
    1 kernel, 1 user (except on 11/45 - 1 kernel, 1 user, 1 "executive"
    (never used except in some really strange form of extented shared library)

    A full context switch was required. Kernel had to map a single 4KW window
    to the user space for access to the parameters. Another 4KW window was used
    to map the IO space. The remaining 6 mapping registers were used for supporting
    the kernel virtual address. BTW, 1 KW = 2K Bytes, a mapping register could
    map anything from 16 bytes to 8K bytes, if I remember correctly. The PDP 11
    with memory management only had 16 mapping registers (8 user, 8 kernel) with
    a maximum address of 64K bytes (16 bit addresses... my how far we've come).
    The base hardware could only handle a maximum of 256 K bytes. More recent
    cpu's expanded the size of the mapping registers (more bits/register) but did
    not increase the number of registers. The last system (PDP-11/70 level) could
    handle 4 MB of physical memory, but with all of the restrictions of the small
    systems, just more processes were handled.

    It was not possible to share memory between kernel/user other than that one
    4KW window. The Linux 3G/1G split is a design choice for speed. It would still
    be Linux even if it did 4G/0, just a different MM architecture with a lot
    more overhead on intel x86 hardware.
    >>>>> Can you please write what exactly the 'overhead' is and how the same
    overhead is not there in 3G/1G split

    Jesse I Pollard, II

    Any opinions expressed are solely my own.
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