Re: [PATCH documentation 1/2] nohz1: Add documentation.
From: Christoph Lameter
Date: Mon Apr 15 2013 - 13:08:27 EST
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 which are bound to be high latency
reaching may be more than microscond in total. A timer interrupt may last
2-4 microsecond at best without PCI transactions.
> not cheap. But also not INSANE expensive... and it breaks-even already if you
> save one or two cache misses elsewhere.
Ok then maybe go dynticks if we can save at least one timer tick?
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