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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