linux-kernel - Re: [patch V2 00/20] timer: Refactor the timer wheel

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Date:	Fri, 17 Jun 2016 15:57:41 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Eric Dumazet <edumazet@...gle.com>
cc:	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.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>, rt@...utronix.de,
	Rik van Riel <riel@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	George Spelvin <linux@...encehorizons.net>,
	Len Brown <lenb@...nel.org>
Subject: Re: [patch V2 00/20] timer: Refactor the timer wheel

On Fri, 17 Jun 2016, Eric Dumazet wrote:
> >
> >    To achieve this capacity with HZ=1000 without increasing the storage size
> >    by another level, we reduced the granularity of the first wheel level from
> >    1ms to 4ms. According to our data, there is no user which relies on that
> >    1ms granularity and 99% of those timers are canceled before expiry.
> >
> 
> Ah... This might be a problem for people using small TCP RTO timers in
> datacenters (order of 5 ms)
> (and small delay ack timers as well, in the order of 4 ms)
> 
> TCP/pacing uses high resolution timer in sch_fq.c so no problem there.
> 
> If we arm a timer for 5 ms, what are the exact consequences ?

The worst case expiry time is 8ms on HZ=1000 as it is on HZ=250

> I fear we might trigger lot more of spurious retransmits.
> 
> Or maybe I should read the patch series. I'll take some time today.

Maybe just throw it at such a workload and see what happens :)

Thanks,

	tglx