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

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Message-ID: <87a95cq7es.fsf@tassilo.jf.intel.com>
Date:	Sat, 04 Oct 2014 06:41:15 -0700
From:	Andi Kleen <andi@...stfloor.org>
To:	"Steinar H. Gunderson" <sgunderson@...foot.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: Slowdown due to threads bouncing between HT cores

"Steinar H. Gunderson" <sgunderson@...foot.com> writes:
>
> So, benchmark:
>
>  - Default: 13266 kN/sec
>  - Change from ondemand to performance on all cores: 14600 kN/sec
>  - taskset -c 0-19 (locking affinity to only one set of hyperthreads):
>    17512 kN/sec

iirc the main reasons for unbound threads migrating away are:

- something else gets scheduled on these logical CPUs, so
the scheduler tries to balance to run queue lengths

You could check that with perf timechart or perf sched record/map
or kernelshark.

- there is some IO or communication which causes wakeup affinity.

You could try disabling WAKEUP_PREEMPTION or NEXT_BUDDY in 
/sys/kernel/debug/sched_features

-Andi
-- 
ak@...ux.intel.com -- Speaking for myself only
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