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    SubjectRe: 2.2.18Pre Lan Performance Rocks!
    On Mon, Oct 30, 2000 at 01:04:34AM -0700, Jeff V. Merkey wrote:
    > > >
    > > > When you say it reloads it's VM, you mean it reloads the CR3 register?
    > >
    > > Yes.
    > >
    > > No. In 2.4 you could probably use the on demand lazy vm mechanism ingo described for
    > > the nfsd processes. In 2.2 it is a bit more tricky, if I remember right lazy mm needed
    > > quite a few changes.
    > >
    > > But before doing too many changes i would first verify if that is really the problem.
    > We will never beat NetWare on scaling if this is the case, even in 2.4.
    > Andre and my first job will be to create an arch port with MANOS that
    > disables this and restructures the VM.

    I just guess the end result will be as crash prone as Netware when you install any third
    party software ;)

    lazy mm is probably a better path, as long as you stay in kernel threads and a single user mm
    it'll never switch VMs.

    > > PTEs are read for aging on memory pressure in vmscan.
    > >
    > > segment register reload happen on interrupt entry, to load the kernel cs/ds (are they that
    > > costly?). If it was really costly you could probably check in interrupt entry if you're
    > > already running in kernel space and skip it.
    > They are. segment register reloads will trigger the following:
    > IDT table atomic fetch to verify (LOCK#) (if triggered by task gate from INTR)
    > GDT table atomic fetch to verify (LOCK#)
    > LDT table atomic fetch to verify (LOCK#) (if present)
    > PDE table atomic fetch to verify (LOCK#)
    > The process has to verify that the loaded segment descriptor is valid, and
    > it will fetch from all these tables to do it, with up to 4 (LOCK#)
    > assertions occurring invisibly in the hardware underneath (which will
    > generate 4 non-cacheable memory references, in addition to wrecking
    > havoc on the affected L1/L2 cache lines). Oink. It only does this
    > when you load one, not when you save one, like pushing it on the
    > stack. Since you look at MANOS code, you'll note that in
    > CONTEXT.386, I do and add esp, 3 * 4 instead of poppping the segment
    > registers off the stack if they are in the kernel address space.
    > Linux should do the same, if possible as an optimization.

    Interesting. You could do easily the same in Linux by changing (in 2.2) the SAVE_ALL macro
    in arch/i386/kernel/irq.h and doing the same in ret_from_intr's RESTORE_ALL macro (after
    changing it to a RESTORE_ALL_INT or you'll break system calls)

    Actually I don't even know why irq.h's SAVE_ALL even loads __KERNEL_CS, it should be already
    set by the interrupt gate. __KERNEL_DS probably needs to be still loaded, but only when
    you came from user space.

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