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    SubjectRe: Hyperthreading on Core i7s: To use or not to use?
    On Jan 26, 10:10 am, Justin Piszcz <> wrote:
    > Hello,
    > Should the 'correct' kernel [CPU] configuration for a core i7 860/870..?
    > - Multi-core support
    > - Cores: 8
    > - SMT: Enabled/ON
    > . SMT scheduler support improves the CPU scheduler's decision making .
    > . when dealing with Intel Pentium 4 chips with HyperThreading at a .
    > . cost of slightly increased overhead in some places. If unsure say .
    > . N here. .
    > Does this also 'help' and/or 'apply' as much when dealing with Core i7s?
    > --
    > Quick little benchmark (pbzip2 -9 linux kernel source), the benchmark is
    > really within the noise (8 on/off)
    > - Multicore(8)/HT(Off) = 73.72user 0.33system 0:09.50elapsed 779%CPU (0avgtext+0avgdata 458528maxresident
    > - Multicore(8)/HT(On) = 74.28user 0.40system 0:09.67elapsed 772%CPU (0avgtext+0avgdata 428304maxresident
    > - Multicore(4)/HT(On) = 68.76user 0.30system 0:17.44elapsed 396%CPU (0avgtext+0avgdata 213616maxresident)k
    > --
    > Has anyone done any in-depth benchmarking for the core i7s that have multiple
    > cores and HT disabled/enabled?

    With my Dell Studio 15 (model 1557) laptop, there is no option to
    disable HT in the current BIOS, so booting with maxcpus=4 (since the
    kernel enumerates non-sibling cores first) gave me a 5-15% speedup on
    some large image processing (convolution, FFTs, conversion) on all
    available cores, presumably due to better cache efficiency.

    Booting with maxcpus=4 prevents any of the cores sitting in C6, needed
    for turbo-boost and a lower thermal profile, though I did find
    scheduling latency and responsiveness better under load booting with
    maxcpus=4, so favour this when plugged in.

    Clearly, having the BIOS option allows benefit to certain applications
    - Dell should give their users the choice!

    Perhaps the 'noht' boot option should be reintroduced to initialise
    all cores, but only expose non-sibling cores to the OS (thus allowing


    tip: modprobe msr and use turbostat to monitor turbo-boost and C-state
    Daniel J Blueman

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