Re: [PATCH documentation 1/2] nohz1: Add documentation.

[Arjan van de Ven <arjan@linux.intel.com>]

On 4/15/2013 9:53 AM, Christoph Lameter wrote:
> On Mon, 15 Apr 2013, Arjan van de Ven wrote:
>
>> to put the "cost" into perspective; programming a timer in one-shot mode
>> is some math on the cpu (to go from kernel time to hardware time),
>> which is a multiply and a shift (or a divide), and then actually
>> programming the hardware, which is at the cost of (approximately) a cachemiss
>> or two
>> (so give or take in the "hundreds" of cycles)
>> at least on moderately modern hardware (e.g. last few years)
>
> Well these are PCI transactions

eh no not on anything modern

they're touching the local apic which is core-local

> Ok then maybe go dynticks if we can save at least one timer tick?

switching between periodic versus not is actually non-trivial and much more expensive
(and complex) so not something you want to do all the time.
once during early boot is hard enough already