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

From: Arjan van de Ven
Date: Mon Apr 15 2013 - 13:22:09 EST

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

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