Re: Hyperthreading on Core i7s: To use or not to use?

[Daniel J Blueman <daniel.blueman@gmail.com>]

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
-- 
Daniel J Blueman