Re: [PATCH] genirq: ARM dyntick cleanup

[Linus Torvalds <torvalds@osdl.org>]

On Mon, 3 Jul 2006, Andrew Morton wrote:
> 
> void handle_dynamic_tick(struct irqaction *action)
> {
> }
> 
> consumes one byte, doesn't it?  That's not very far overboard ;)

Nope, it consumes - very fundamentally - about 64 bytes.

Even in the absense of alignment (which means that it generally takes up a 
minimum of 16 bytes of real memory), it takes up what would tend to be a 
much more critical resource: a cache line.

So an empty macro is a _lot_ more efficient than an empty function call. 

For interrupt handlers, the L1 I$ miss rate is basically 100%. Even the L2 
I$ miss rate is quite noticeable, _especially_ for things that are not 
easy to prefetch (ie stuff that is out-of-line). Which means that it also 
likely takes an inordinate amount of cycles, for doing zero work.

		Linus
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