linux-kernel - Re: [patch 13/20] timer: Switch to a non cascading wheel

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Date:	Wed, 15 Jun 2016 18:43:46 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Arjan van de Ven <arjanvandeven@...il.com>
cc:	Eric Dumazet <edumazet@...gle.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Chris Mason <clm@...com>,
	Arjan van de Ven <arjan@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [patch 13/20] timer: Switch to a non cascading wheel

On Wed, 15 Jun 2016, Arjan van de Ven wrote:
> what would 1 more timer wheel do?

Waste storage space and make the collection of expired timers more expensive.

The selection of the timer wheel properties is combination of:

    1) Granularity 

    2) Storage space

    3) Number of levels to collect

#1 Influences the accuracy of the wheel levels. The worst case error is the
   ratio of the shortest timeout in a level and the granularity of that level.

   Currently I've chosen 12.5%. If we go to 25% worst case, then we increase
   the per level capacity by factor (1 << lvl).

#2 It strikes me a bit silly to waste storage space for esoteric
   outliers. According to the data I collected we have no timers at all
   between 2hrs and 5 days.

#3 The more levels we have the more steps we have when collecting expired
   timers or searching the next expiring timer. It's all bound, but it still
   adds up.

Thanks,

	tglx