linux-kernel - Re: Slowdown due to threads bouncing between HT cores

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Date:	Sat, 4 Oct 2014 09:50:04 -0500
From:	Chuck Ebbert <cebbert.lkml@...il.com>
To:	"Steinar H. Gunderson" <sgunderson@...foot.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: Slowdown due to threads bouncing between HT cores

On Fri, 3 Oct 2014 21:44:29 +0200
"Steinar H. Gunderson" <sgunderson@...foot.com> wrote:

> Hi,
> 
> I did a chess benchmark of my new machine (2x E5-2650v3, so 20x2.3GHz
> Haswell-EP), and it performed a bit worse than comparable Windows setups.
> It looks like the scheduler somehow doesn't perform as well with
> hyperthreading; HT is on in the BIOS, but I'm only using 20 threads
> (chess scales sublinearly, so using all 40 usually isn't a good idea),
> so really, the threads should just get one core each and that's it.
> It looks like they are bouncing between cores, reducing overall performance
> by ~20% for some reason. (The machine is otherwise generally idle.)
> 

Try playing with /proc/sys/kernel/sched_migration_cost_ns. This sets
the number of nanoseconds the kernel will wait before considering
moving a thread to another CPU. I have mine set to 50000000.
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