Dynamic Ticks/HZ

Joe Holden lists at rewt.org.uk
Mon Nov 5 09:40:36 UTC 2012

Davide Italiano wrote:
> On Nov 4, 2012 10:40 PM, "Joe Holden" <lists at rewt.org.uk> wrote:
>> Davide Italiano wrote:
>>> On Mon, Nov 5, 2012 at 7:12 AM, Joe Holden <lists at rewt.org.uk> wrote:
>>>> Hi guys,
>>>> Has some default changed between 9.1-RC2 and HEAD?
>>>> On identical machines, one with 9.1-RC2 and one with HEAD from yesterday
>>>> (GENERIC) I see the following in systat -v:
>>>> 9.1:
>>>> 65 cpu0:timer
>>>> 10 cpu1:timer
>>>> HEAD:
>>>> 1127 cpu0:timer
>>>> 22 cpu1:timer
>>>> These are Supermicro i3 boxes and as far as I can see they have matching
>>>> BIOS config.
>>>> Thanks,
>>>> J
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>>> Which is your refresh rate for systat?
>>> I generally measure sampling every one second (i.e. systat -vm 1).
>>> Also, are you making your measurements when the system is idle?
>>> In order to trace the source(s) of these interrupts you might consider
>>> to collect data via KTR.
>> I'm also using a one second refresh rate, the system is entirely idle and
> the interupt rate is almost entirely static at 1127, occasionally it will
> drop to 1119.
>> From what I understand the timer is hz/ticks which became dynamic in 9.0,
> although that behaviour doesn't appear to be in HEAD anymore, at least on
> this hardware.
>> Thanks,
>> J
> It should be available, AFAIK. As I can see from your previous post you get
> about 20 interrupts on cpu1. This number is about 1/100 of the value you
> get on a !tickless kernel.
> If you provide the required ktr infos, probably someone will take a look.
doh, running kernel wasn't as GENERIC as I thought it was, looks like 
device polling not only breaks dynamic ticks but also reduces rx ability 
significantly, exactly 150,000 pps per 1000hz on igb versus 650,000 without

Is this a known issue? (and if device polling isn't as useful as it once 
was, should it be removed?)

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