Re: CURRENT as gateway on not-so-fast hardware: where is a bottlneck?

On Wed, 2012-08-15 at 14:40 +0400, Lev Serebryakov wrote:
> Hello, Alexander.
> You wrote 15 августа 2012 г., 14:18:05:
> 
> 
> AM> It is quite pointless to speculate without real info like mentioned
> AM> above KTR_SCHED traces. Main thing I've learned about schedulers, things
> AM> there never work as you expect. There are two many factors are relations
> AM> to predict behavior in every case.
>   I'll take these with as much variants (ULE and 4BSD, polling with
> HZ=1000 and interrupts with default HZ) as I can, in day or two.
>   Now I have kernels with KTR compiled in (GEN, NET and SCHED).
> 
> AM> About Soekris and idle CPU measurement, let's start from what kind of 
> AM> eventtimer is used there. As soon as it is UP machine, I guess it uses
> AM> i8254 timer in periodic mode. It means that it by definition can't
>  It doesn't have any other timers. You could think about this machine
> as about good old "true" i386, with PCI (and some additional fancy
> commands in CPU core, something like classic Pentium) but
> nothing more.
> 
> kern.eventtimer.choice: i8254(100) RTC(0)
> kern.eventtimer.et.RTC.flags: 17
> kern.eventtimer.et.RTC.frequency: 32768
> kern.eventtimer.et.RTC.quality: 0
> kern.eventtimer.et.i8254.flags: 1
> kern.eventtimer.et.i8254.frequency: 1193182
> kern.eventtimer.et.i8254.quality: 100
> kern.eventtimer.periodic: 1
> kern.eventtimer.timer: i8254
> kern.eventtimer.activetick: 1
> kern.eventtimer.idletick: 0
> kern.eventtimer.singlemul: 2
> 
> AM> properly measure load from treads running from hardclock, such as 
> AM> dummynet, polling netisr threads, etc.
>   You see, here are two different problems:
> 
> (a) with polling, system is responsive under any load, but wire2wifi
> performance  is hugely affected by wire2wire traffic (and mpd5
> inbetween). And, yes, "top" seems to lie about idle time.
> 
> (b) with interrupts, system works much better when it works (wire2wifi
> speed is affected by wire2wire traffic, but to much less extent), but
> it freezes every third minute for minute, when traffic is passed, but
> no user-level applications including BIND and DHCP server) works at
> all FOR MINUTE OR MORE. It not looks like 100ms lag, which could affect
> video playback. It looks like 60-120 seconds lag! At least, in case of
> ULE, I didn't try 4BSD yet.
> 

I had trouble earlier this year with an industrial single-board computer
that uses the same chipset as your Soekris (Geode 500 + CS5536) where
the interrupt handler for the RTC chip would occasionally get stuck in a
loop for a minute or more at a time, making userland processes
completely unresponsive during that time.

It's a long shot, but if the trouble you're seeing has the same cause,
it should be fixed by this patch:

http://lists.freebsd.org/pipermail/freebsd-hackers/2012-January/037233.html

-- Ian