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    SubjectRe: 2.2.18Pre Lan Performance Rocks!

    One more optimization it has. NetWare never "calls" functions in the
    kernel. There's a template of register assignments in between kernel
    modules that's very strict (esi contains a WTD head, edi has the target
    thread, etc.) and all function calls are jumps in a linear space.
    layout of all functions are 16 bytes aligned for speed, and all
    arguments in kernel are passed via registers. It's a level of
    optimization no C compiler does -- all of it was done by hand, and most
    function s in fast paths are layed out in 512 byte chunks to increases
    speed. Stack memory activity in the NetWare kernel is almost
    non-existent in almost all of the "fast paths"


    "Jeff V. Merkey" wrote:
    > A "context" is usually assued to be a "stack". The simplest of all
    > context switches
    > is:
    > mov x, esp
    > mov esp, y
    > A context switch can be as short as two instructions, or as big as a TSS
    > with CR3 hardware switching,
    > i.e.
    > ltr ax
    > jmp task_gate
    > (500 clocks later)
    > ts->eip gets exec'd
    > you can also have a context switch that does an int X where X is a gate
    > or TSS.
    > you can also have a context switch (like linux) that does
    > mov x, esp
    > mov esp, y
    > mov z, CR3
    > mov CR3, w
    > etc.
    > In NetWare, a context switch is an in-line assembly language macro that
    > is 2 instructions long for a stack switch and 4 instructions for a CR3
    > reload -- this is a lot shorter than Linux.
    > Only EBX, EBP, ESI, and EDI are saved and this is never done in the
    > kernel, but is a natural
    > affect of the Watcom C compiler. There's also strict rules about
    > register assignments that re enforced between assembler modules in
    > NetWare to reduce the overhead of a context switch. The code path is
    > very complex in NetWare, and priorities and all this stuff exists, but
    > these code paths are segragated so these types of checks only happen
    > once in a while and check a pre-calc'd "scoreboard" that is read only
    > across processors and updated and recal'd by a timer every 18 ticks.
    > Jeff
    > Andi Kleen wrote:
    > >
    > > On Tue, Oct 31, 2000 at 02:52:11PM -0700, Jeff V. Merkey wrote:
    > > > The numbers don't lie. You know where the code is. You notice that
    > > > there is a version of
    > > > the kernel hand coded in assembly language. You'l also noticed that
    > > > it's SMP and takes ZERO LOCKS during context switching, in fact, most of
    > > > the design is completely lockless.
    > >
    > > I suspect most of the confusion in this thread comes because you seem to
    > > use a different definition of context switch than Ingo and others. Could
    > > you explain what you exactly mean with a context switch ?
    > >
    > > -Andi
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