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    SubjectRe: FOR REVIEW: New x86-64 vsyscall vgetcpu()
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    >>>>> "Andi" == Andi Kleen <ak@suse.de> writes:

    Andi> On Thursday 15 June 2006 20:44, Tony Luck wrote:
    >> Another alternative would be to provide a mechanism for a process
    >> to bind to the current cpu (whatever cpu that happens to be). Then
    >> the kernel gets to make the smart placement decisions, and
    >> processes that want to be bound somewhere (but don't really care
    >> exactly where) have a way to meet their need. Perhaps a cpumask of
    >> all zeroes to a sched_setaffinity call could be overloaded for
    >> this?

    Andi> I tried something like this a few years ago and it just didn't
    Andi> work (or rather ran usually slower) The scheduler would select a
    Andi> home node at startup and then try to move the process there.

    Andi> The problem is that not using a CPU costs you much more than
    Andi> whatever overhead you get from using non local memory.

    It all depends on your application and the type of system you are
    running on. What you say applies to smaller cpu counts. However once
    we see the upcoming larger count multi-core cpus become commonly
    available, this is likely to change and become more like what is seen
    today on larger NUMA systems.

    In the scientific application space, there are two very common
    groupings of jobs. One is simply a large threaded application with a
    lot of intercommunication, often via MPI. In many cases one ends up
    running a job on just a subset of the system, in which case you want
    to see threads placed on the same node(s) to minimize internode
    communication. It is desirable to either force the other tasks on the
    system (system daemons etc) onto other node(s) to reduce noise and
    there could also be space to run another parallel job on the remaining
    node(s).

    The other common case is to have jobs which spawn off a number of
    threads that work together in groups (via OpenMP). In this case you
    would like to have all your OpenMP threads placed on the same node for
    similar reasons.

    Not getting this right can result in significant loss of performance
    for jobs which are highly memory bound or rely heavily on
    intercommunication and synchronization.

    Andi> So by default filling the CPUs must be the highest priority and
    Andi> memory policy cannot interfere with that.

    I really don't think this approach is going to solve the problem. As
    Tony also points out, tasks will eventually migrate. The user needs to
    tell the kernel where it wants to run the tasks rather than the kernel
    telling the task where it is located. Only the application (or
    developer/user) knows how the threads are expected to behave, doing
    this automatically is almost never going to be optimal. Obviously the
    user needs visibility of the topology of the machine to do so but that
    should be available on any NUMA system through /proc or /sys.

    In the scientific space the jobs are often run repeatedly with new
    data sets every time, so it is worthwhile to spend the effort up front
    to get the placement right. One-off runs are obviously something else
    and there your method is going to be more beneficial.

    IMHO, what we really need is a more advanced way for user applications
    to hint at the kernel how to place it's threads.

    Cheers,
    Jes
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