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    SubjectRe: Hyperthreading on Core i7s: To use or not to use?
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    Hello,

    I have been done some benchmark things on the processors with HT,
    if you run lots of processes/threads that more than the physical
    cores,may be you should enable the HT to get more benefits.
    #unless your processes cause some cache competitions.

    I will send you some test data later.

    -huang

    2010-01-26 (火) の 10:56 +0000 に Daniel J Blueman さんは書きました:
    > On Jan 26, 10:10 am, Justin Piszcz <jpiszcz@lucidpixels.com> wrote:
    > > Hello,
    > >
    > > Should the 'correct' kernel [CPU] configuration for a core i7 860/870..?
    > >
    > > - Multi-core support
    > > - Cores: 8
    > > - SMT: Enabled/ON
    > >
    > > From CONFIG_SCHED_SMT:
    > >
    > > . 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
    > C6)?
    >
    > Daniel
    >
    > tip: modprobe msr and use turbostat to monitor turbo-boost and C-state
    > residency: http://bugzilla.kernel.org/attachment.cgi?id=24673


    --
    peng huang <huangpeng.linux@gmail.com>

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