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Date: Tue, 14 Jun 2016 18:28:43 +0200 (CEST)
From: Thomas Gleixner <tglx@linutronix.de>
To: Ingo Molnar <mingo@kernel.org>
cc: Peter Zijlstra <peterz@infradead.org>, LKML <linux-kernel@vger.kernel.org>,
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        Arjan van de Ven <arjan@infradead.org>, rt@linutronix.de,
        Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [patch 13/20] timer: Switch to a non cascading wheel
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On Tue, 14 Jun 2016, Ingo Molnar wrote:
> * Thomas Gleixner <tglx@linutronix.de> wrote:
> > On Mon, 13 Jun 2016, Peter Zijlstra wrote:
> > > On Mon, Jun 13, 2016 at 08:41:00AM -0000, Thomas Gleixner wrote:
> > > > +
> > > > +	/* Cascading, sigh... */
> > > 
> > > So given that userspace has no influence on timer period; can't we
> > > simply fail to support timers longer than 30 minutes?
> > > 
> > > In anything really arming timers _that_ long?
> > 
> > Unfortunately yes. Networking being one of those. Real cascading happens once
> > in a blue moon, but it happens.
> 
> So I'd really prefer it if we added a few more levels, a hard limit and got rid of 
> the cascading once and for all!
> 
> IMHO 'once in a blue moon' code is much worse than a bit more data overhead.

I agree. If we add two wheel levels then we end up with:

  HZ 1000:  134217727 ms ~=  37 hours
  HZ  250:  536870908 ms ~= 149 hours
  HZ  100: 1342177270 ms ~= 372 hours 

Looking through all my data I found exactly one timeout which is insanely
large: 120 hours!

That's net/netfilter/nf_conntrack_core.c:
      setup_timer(&ct->timeout, death_by_timeout, (unsigned long)ct);

Anything else is way below 37 hours.

Thanks,

	tglx