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

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Date:	Tue, 14 Jun 2016 10:14:29 -0700
From:	Arjan van de Ven <arjanvandeven@...il.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Chris Mason <clm@...com>,
	Arjan van de Ven <arjan@...radead.org>, rt@...utronix.de,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [patch 13/20] timer: Switch to a non cascading wheel

evaluating a 120 hours timer ever 37 hours to see if it should fire...
not too horrid.

On Tue, Jun 14, 2016 at 9:28 AM, Thomas Gleixner <tglx@...utronix.de> wrote:
> On Tue, 14 Jun 2016, Ingo Molnar wrote:
>> * Thomas Gleixner <tglx@...utronix.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