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Date:      Wed, 2 Jan 2013 21:52:37 -0800
From:      Kevin Oberman <kob6558@gmail.com>
To:        Alexander Motin <mav@freebsd.org>
Cc:        Davide Italiano <davide@freebsd.org>, Ian Lepore <freebsd@damnhippie.dyndns.org>, Adrian Chadd <adrian@freebsd.org>, Marius Strobl <marius@alchemy.franken.de>, FreeBSD Current <freebsd-current@freebsd.org>, freebsd-arch@freebsd.org, Luigi Rizzo <rizzo@iet.unipi.it>
Subject:   Re: [RFC/RFT] calloutng
Message-ID:  <CAN6yY1vRJN8EpKpYARfkShRzmPfC4VEw33O1mfppZ%2BD%2B8iebgQ@mail.gmail.com>
In-Reply-To: <50E4AF4C.2070902@FreeBSD.org>
References:  <50CCAB99.4040308@FreeBSD.org> <50CE5B54.3050905@FreeBSD.org> <50D03173.9080904@FreeBSD.org> <20121225232126.GA47692@alchemy.franken.de> <50DB4EFE.2020600@FreeBSD.org> <1356909223.54953.74.camel@revolution.hippie.lan> <20121231061735.GA5866@onelab2.iet.unipi.it> <50E16637.9070501@FreeBSD.org> <20130102105730.GA42542@onelab2.iet.unipi.it> <50E418EA.7030801@FreeBSD.org> <20130102122743.GA43241@onelab2.iet.unipi.it> <CAO4K=PUHAH=UNzMde0V2TwkN5vj3gw9hHj5yCQxDvdUn%2Buqv7w@mail.gmail.com> <1357135374.54953.150.camel@revolution.hippie.lan> <CAJ-Vmo=mmm5zhwHyzKeg1VEL8hSz6_LxJAaLh74ArHF3_9KWaQ@mail.gmail.com> <50E4AF4C.2070902@FreeBSD.org>

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On Wed, Jan 2, 2013 at 2:06 PM, Alexander Motin <mav@freebsd.org> wrote:
> On 02.01.2013 18:08, Adrian Chadd wrote:
>>
>> .. I'm pretty damned sure we're going to need to enforce a "never
>> earlier than X" latency.
>
>
> Do you mean here that we should never wake up before specified time (just as
> specified by the most of existing APIs), or that we should not allow sleep
> shorter then some value to avoid DoS? At least on x86 nanosleep(0) doesn't
> allow to block the system. Also there is already present mechanism for
> specifying minimum timer programming interval in eventtimers(9) KPI.

I can see serious performance issues with some hardware (wireless
comes to mind) if things happen too quickly. Intuition is that it
could also play hob with VMs.

I believe that the proper way is to wake between  T_X and T_X + D.
This assumes that D is max_wake_delay, not deviation, which leaves us
at the original of (T_X) =< event_time =< (T_X + D).
-- 
R. Kevin Oberman, Network Engineer
E-mail: kob6558@gmail.com



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