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

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Message-ID: <20141005111911.GA35329@sesse.net>
Date:	Sun, 5 Oct 2014 13:19:11 +0200
From:	"Steinar H. Gunderson" <sgunderson@...foot.com>
To:	Chuck Ebbert <cebbert.lkml@...il.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: Slowdown due to threads bouncing between HT cores

On Sat, Oct 04, 2014 at 09:50:04AM -0500, Chuck Ebbert wrote:
> 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.

I can't get any good effect out of this. I tried both 50 ms (your value) and
500 ms, and while it seems (by eyeballing the per-cpu display in top) to
_sometimes_ lock the processes to cores, more often than not, they still
bounce around between all 40. Worse still, when it _does_, it seems to often
lock them to hyperthread pairs (e.g., I've seen it put threads only on
virtual cores 0-9 and 20-29, which means it has all of them on one socket!)

Notwithstanding top, the benchmarks don't improve; setting cores manually
with taskset still is much better.

/* Steinar */
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Homepage: http://www.sesse.net/
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