On Thu, Dec 15, 2011 at 2:28 PM, Eric Paris <eparis@xxxxxxxxxx> wrote:
>
> How expensive is an atomic_inc()/atomic_dec() combo? If it's mostly
> free we can do it in the right place.
It's pretty expensive, but it depends a lot on cache details etc. It
involves a memory barrier on x86 too, and depending on architecture it
might be anything from 150 cycles (P4 - but by now probably nobody
cares) to "a few tens" of cycles (roughly 10-35 on modern x86).
The cache miss itself - if it happens - is not counted in the above cost.
A totally uncontended spinlock is actually cheaper than a pair of
atomic ops. So if it's a hot path it probably should be moved
elsewhere if possible.
Linus
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