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Date: Fri, 17 Jun 2016 15:57:41 +0200 (CEST)
From: Thomas Gleixner <tglx@linutronix.de>
To: Eric Dumazet <edumazet@google.com>
cc: LKML <linux-kernel@vger.kernel.org>, Ingo Molnar <mingo@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
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        Linus Torvalds <torvalds@linux-foundation.org>,
        George Spelvin <linux@sciencehorizons.net>,
        Len Brown <lenb@kernel.org>
Subject: Re: [patch V2 00/20] timer: Refactor the timer wheel
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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