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    On Sun, 2009-08-23 at 12:09 +0300, raz ben yehuda wrote:
    > On Sun, 2009-08-23 at 07:21 +0200, Mike Galbraith wrote:

    > > Seems to me this boils down to a different way to make a SW box in a HW
    > > box, which already exists. What does this provide that partitioning a
    > > box with csets and virtualization doesn't?
    > OFFSCHED does not compete with cpu sets nor is
    > different.
    > 1. Neither virtuallization nor cpu sets provide hard real time. OFFSCHED
    > does this with a little cost and no impact on the OS.OFFSCHED is not
    > just accurate , it is also extremely fast,after all, it is NMI'ed
    > processor.

    Why not? Why can't I run an RT kernel with an RTOS guest and let it do
    it's deadline management thing?

    > 2. OFFSCHED has a access to every piece of memory in the system. so it
    > can act as a centry for the system, or use linux facilities. Also, the
    > kernel can access OFFSCHED memory, it is the same address space.

    Hm. That appears to be a self negating argument.

    > 3. OFFSCHED can improve the linux OS ( NAPI,OFFSCHED firewall,RTOP ),
    > while a guest OS cannot.
    > 4. cpu sets cannot replace softirqs and hardirqs. OFFSCHED can. cpu sets
    > deals with kernel threads and user space threads. in OFFSCHED we use
    > offlets.

    Which still looks like OS-fu to me.

    > 5. cpu sets and virtualization are services provided by the kernel to
    > the "system".who serves the kernel ? who protects the kernel ?

    If either one can diddle the others ram, they are in no way isolated or
    protected, so can't even defend against their own bugs.

    What protects a hard RT deadline from VM pressure, memory bandwidth
    consumption etc etc? Looks to me like it's soft RT, because you can't
    control the external variables.

    > 6. offlets gives the programmer full control over an entire processor.
    > no preemption, no interrupts, no quiesce. you know what happens , and
    > when it happens.

    If I can route interrupts such that only say network interrupts are
    delivered to my cset/vm core, and the guest OS is a custom high speed
    low drag application, I just don't see much difference.

    > I have this hard real time system several years on my SMP/MC/SMT
    > machines. It serves me well. The core of OFFSCHED patch was 4 lines.
    > So,i simply compile a ***entirely regular*** linux bzImage and that's
    > it. It did not mess with drivers, spinlocks, softirqs ..., OFFSCHED just
    > directed the cpu_down to my own hard real time piece of code. The rest
    > of the kernel remained the same.

    Aaaaanyway, I'm not saying it's not a useful thing to do, just saying I
    don't see any reason you can't get essentially the same result with
    what's in the kernel now.


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