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From: Thomas Gleixner <tglx@linutronix.de>
To: Arjan van de Ven <arjanvandeven@gmail.com>
Cc: Eric Dumazet <edumazet@google.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
	Frederic Weisbecker <fweisbec@gmail.com>,
	Chris Mason <clm@fb.com>, Arjan van de Ven <arjan@infradead.org>,
	Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [patch 13/20] timer: Switch to a non cascading wheel
Date: Wed, 15 Jun 2016 18:43:46 +0200 (CEST)	[thread overview]
Message-ID: <alpine.DEB.2.11.1606151746270.5839@nanos> (raw)
In-Reply-To: <CADyApD3QNoS+dkv_XhzKbk+KQv_JC5p=jXT-=bcWh1xmfyNdQw@mail.gmail.com>

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




   

  reply	other threads:[~2016-06-15 16:45 UTC|newest]

Thread overview: 62+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-06-13  8:40 [patch 00/20] timer: Refactor the timer wheel Thomas Gleixner
2016-06-13  8:40 ` [patch 01/20] timer: Make pinned a timer property Thomas Gleixner
2016-06-13  8:40 ` [patch 02/20] x86/apic/uv: Initialize timer as pinned Thomas Gleixner
2016-06-13  8:40 ` [patch 03/20] x86/mce: " Thomas Gleixner
2016-06-13  8:40 ` [patch 04/20] cpufreq/powernv: " Thomas Gleixner
2016-06-13 13:18   ` Arjan van de Ven
2016-06-13  8:40 ` [patch 05/20] driver/net/ethernet/tile: " Thomas Gleixner
2016-06-13  8:40 ` [patch 06/20] drivers/tty/metag_da: " Thomas Gleixner
2016-06-13 13:13   ` Arjan van de Ven
2016-06-13  8:40 ` [patch 07/20] drivers/tty/mips_ejtag: " Thomas Gleixner
2016-06-13  8:40 ` [patch 08/20] net/ipv4/inet: Initialize timers " Thomas Gleixner
2016-06-13  8:40 ` [patch 09/20] timer: Remove mod_timer_pinned Thomas Gleixner
2016-06-13  8:40 ` [patch 10/20] timer: Add a cascading tracepoint Thomas Gleixner
2016-06-13  8:40 ` [patch 11/20] hlist: Add hlist_is_last_node() helper Thomas Gleixner
2016-06-13 10:27   ` Paolo Bonzini
2016-06-13  8:40 ` [patch 12/20] timer: Give a few structs and members proper names Thomas Gleixner
2016-06-13  8:41 ` [patch 13/20] timer: Switch to a non cascading wheel Thomas Gleixner
2016-06-13 11:40   ` Peter Zijlstra
2016-06-13 12:30     ` Thomas Gleixner
2016-06-13 12:46       ` Eric Dumazet
2016-06-13 14:30         ` Thomas Gleixner
2016-06-14 10:11       ` Ingo Molnar
2016-06-14 16:28         ` Thomas Gleixner
2016-06-14 17:14           ` Arjan van de Ven
2016-06-14 18:05             ` Thomas Gleixner
2016-06-14 20:34               ` Peter Zijlstra
2016-06-14 20:42               ` Peter Zijlstra
2016-06-14 21:17                 ` Eric Dumazet
2016-06-15 14:53                   ` Thomas Gleixner
2016-06-15 14:55                     ` Arjan van de Ven
2016-06-15 16:43                       ` Thomas Gleixner [this message]
2016-06-16 15:43                         ` Thomas Gleixner
2016-06-16 16:02                           ` Paul E. McKenney
2016-06-16 18:14                             ` Peter Zijlstra
2016-06-17  0:40                               ` Paul E. McKenney
2016-06-17  4:04                                 ` Paul E. McKenney
2016-06-16 16:04                           ` Arjan van de Ven
2016-06-16 16:09                             ` Thomas Gleixner
2016-06-15 15:05                     ` Eric Dumazet
2016-06-13 14:36   ` Richard Cochran
2016-06-13 14:39     ` Thomas Gleixner
2016-06-13  8:41 ` [patch 15/20] timer: Move __run_timers() function Thomas Gleixner
2016-06-13  8:41 ` [patch 14/20] timer: Remove slack leftovers Thomas Gleixner
2016-06-13  8:41 ` [patch 16/20] timer: Optimize collect timers for NOHZ Thomas Gleixner
2016-06-13  8:41 ` [patch 17/20] tick/sched: Remove pointless empty function Thomas Gleixner
2016-06-13  8:41 ` [patch 18/20] timer: Forward wheel clock whenever possible Thomas Gleixner
2016-06-13 15:14   ` Richard Cochran
2016-06-13 15:18     ` Thomas Gleixner
2016-06-13  8:41 ` [patch 19/20] timer: Split out index calculation Thomas Gleixner
2016-06-13  8:41 ` [patch 20/20] timer: Optimization for same expiry time in mod_timer() Thomas Gleixner
2016-06-13 14:10 ` [patch 00/20] timer: Refactor the timer wheel Eric Dumazet
2016-06-13 16:15 ` Paul E. McKenney
2016-06-15 15:15   ` Paul E. McKenney
2016-06-15 17:02     ` Thomas Gleixner
2016-06-15 20:26       ` Paul E. McKenney
2016-06-14  8:16 [patch 13/20] timer: Switch to a non cascading wheel George Spelvin
2016-06-14  8:50 ` Thomas Gleixner
2016-06-14 10:15   ` George Spelvin
2016-06-14 10:20 ` Peter Zijlstra
2016-06-14 12:58   ` George Spelvin
2016-06-14 16:48     ` Thomas Gleixner
2016-06-14 19:56       ` George Spelvin

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