HyperThreading on Intel Xeon Haswell, a benefit?

grarpamp grarpamp at gmail.com
Mon Dec 15 05:40:45 UTC 2014

On Wed, Dec 10, 2014 at 8:19 AM, Jia-Shiun Li <jiashiun at gmail.com> wrote:
> Yes, Haswell has an additional store addr but still only one store data unit.
> http://www.tomshardware.com/reviews/core-i7-4770k-haswell-review,3521.html
> But I guess they'd argue that they meant to saturate memory
> channels with all available cores as possible first, and additional
> threads are only for last resort. And that's probably what the most
> schedulers do.
> I benchmarked it on a 4th gen i3. Buildkernel got 5~10% benefit IIRC.
> The best way to tell is still to conduct tests with your own workload.
> If the claimed 5% transistor cost brings 10% benefits, that's already
> a win. OTTH how much you paid for it is another story.

Where is the claim of "5% transistor cost" from?
I don't see it linked in this thread.
Is it in terms of $ as a sales feature to get HT/SMT, or transistor
count to get it? I think SMT transistor count could change over
CPU generations optimized.

Any bump in price to get HT, is amortized over time.
Any bump in performance due to HT, is integrated over time.
A watt costs about $1/yr.
If SMT is 5% faster, over 4hr saves 12 minutes of your time, which
saves $n/day, which more than pays for purchase and watts.
If it is slower, it hurts similarly $hard unless you turn it off
and eat its purchase difference.

Thus to see what people were seeing perf wise.

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