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    SubjectRe: [PATCH 00/07][RFC] i386: NUMA emulation
    On 10/1/05, Dave Hansen <> wrote:
    > On Fri, 2005-09-30 at 16:33 +0900, Magnus Damm wrote:
    > > These patches implement NUMA memory node emulation for regular i386 PC:s.
    > >
    > > NUMA emulation could be used to provide coarse-grained memory resource control
    > > using CPUSETS. Another use is as a test environment for NUMA memory code or
    > > CPUSETS using an i386 emulator such as QEMU.
    > This patch set basically allows the "NUMA depends on SMP" dependency to
    > be removed. I'm not sure this is the right approach. There will likely
    > never be a real-world NUMA system without SMP. So, this set would seem
    > to include some increased (#ifdef) complexity for supporting SMP && !
    > NUMA, which will likely never happen in the real world.

    Yes, this patch set removes "NUMA depends on SMP". It also adds some
    simple NUMA emulation code too, but I am sure you are aware of that!

    I agree that it is very unlikely to find a single-processor NUMA
    system in the real world. So yes, "[PATCH 02/07] i386: numa on
    non-smp" adds _some_ extra complexity. But because SMP is set when
    supporting more than one cpu, and NUMA is set when supporting more
    than one memory node, I see no reason why they should be dependent on
    each other. Except that they depend on each other today and breaking
    them loose will increase complexity a bit.

    > Also, I worry that simply #ifdef'ing things out like CPUsets' update
    > means that CPUsets lacks some kind of abstraction that it should have
    > been using in the first place. An #ifdef just papers over the real
    > problem.

    Maybe. CPUSETS has two bitmaps, one for cpus and one for mems. So
    depending on SMP or NUMA seems logical to me. Regarding the #ifdef, it
    was added because partition_sched_domain() is only implemented for
    SMP. That symbol has no prototype or implementation when CONFIG_SMP is
    not set. Maybe it is better to add an empty inline function in
    linux/sched.h for !SMP?

    > I think it would likely be cleaner if the approach was to emulate an SMP
    > NUMA system where each NUMA node simply doesn't have all of its CPUs
    > online.

    Absolutely. And that removes the need for some of my patches. QEMU
    runs SMP kernels. It is possible to run SMP kernels on UP hardware.
    But there is of course a certain performance loss introduced by all
    the SMP locks. I'd rather not force !SMP users to run SMP kernels if
    they want coarse-grained memory resource control.

    Thanks for your input!

    / magnus
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