linux-kernel - Re: [PATCH 3/7] locking/pvqspinlock: Implement wait-early for overcommitted guest

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Date:	Mon, 13 Jul 2015 12:50:22 -0700
From:	Davidlohr Bueso <dave@...olabs.net>
To:	Waiman Long <Waiman.Long@...com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org, Scott J Norton <scott.norton@...com>,
	Douglas Hatch <doug.hatch@...com>
Subject: Re: [PATCH 3/7] locking/pvqspinlock: Implement wait-early for
 overcommitted guest

On Sat, 2015-07-11 at 16:36 -0400, Waiman Long wrote:
>  /*
> + * Queued Spinlock Spin Thresholds
> + * -------------------------------
> + * Because of the cacheline contention effect of the ticket spinlock, the
> + * same spin threshold for queued spinlock will run a bit faster. So we set
> + * a slight larger threshold for the queue head (1.25X) while the other queue
> + * nodes will keep the same threshold.

How did you come to 1.25x? These sort of magic numbers scare me.

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