Date: Mon, 17 Dec 2012 22:11:12 +0100
From: Luigi Rizzo <rizzo@iet.unipi.it>
To: Davide Italiano <davide@freebsd.org>
Cc: freebsd-current <freebsd-current@freebsd.org>, freebsd-arch@freebsd.org
Subject: Re: regarding r242905 ('us' argument to some callout functions) was Re: [RFC/RFT] calloutng
Message-ID: <20121217211112.GA84347@onelab2.iet.unipi.it>
In-Reply-To: <CACYV=-HvsHmTGxQdiFx0Gcj1CW-ZHzGZsj5WBucBp3BWCMwWCw@mail.gmail.com>
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On Mon, Dec 17, 2012 at 12:17:54PM -0800, Davide Italiano wrote: > On Mon, Dec 17, 2012 at 11:27 AM, Luigi Rizzo <rizzo@iet.unipi.it> wrote: > > [addressing the various items separately] > > > > On Fri, Dec 14, 2012 at 01:57:36PM +0100, Davide Italiano wrote: > >> On Fri, Dec 14, 2012 at 7:41 AM, Luigi Rizzo <rizzo@iet.unipi.it> wrote: > > ... > >> > - for several functions the only change is the name of an argument > >> > from "busy" to "us". Can you elaborate the reason for the change, > >> > and whether "us" means microseconds or the pronoun ?) > >> > > >> > >> Please see r242905 by mav@. > > > > i see the goal of this patch is to pass along the amount of > > time till the next timer. > > > > I wonder why the choice is to use (actually, call) the value > > "microseconds" rather use a bintime or something scaled and with a > > well defined resolution. > > > > In fact looking at the relevant diff > > > > http://svnweb.freebsd.org/base/projects/calloutng/sys/kern/kern_clocksource.c?r1=242905&r2=242904&pathrev=242905 > > > > cpu_idleclock() actually returns a value that is not even microseconds > > but 1/(2^20) seconds. The value seems to be ignored right now > > so it would be a good time to discuss the resolution. > > > > I am concerned that at some point (5 years from now perhaps ?) > > microseconds might start to become too coarse and we would like > > something with a more fine-grained resolution. On the > > other hand, for the purposes of this change, we can probably > > live with an upper limit of some seconds (waking up the machine > > once per second is not going to kill performance). > > > > I would talk more about power consumption problem rather than performances. > Yes, you're right because now, even with calloutng changes, the CPU is > woken up at least twice per second. > The wheel scan, in case it doesn't find a new callout to schedule in > the next half-second, schedules an interrupt half a second from "now" > (where now is the time obtained using getbinuptime()/binuptime()). > This is a threshold we set up empirically, and it resulted is "good" > for now. But in the future someone may raise the threshold to 1 > second, 10 seconds, or something like. So, I don't agree with your > statement. this is only an issue if we want to use 32 bits. If we go to 64 bits, there is enogh space for picoseconds on the fractional part, and a few years in the integer part. but keep in mind, even powerwise, i doubt the exit from deep sleep and a callout takes more than 500us so even doing that once per second gives less than 0.5 per mille increase in power compared to a machine that is always idle This is really noise that we should not worry about. cheers luigi
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