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SubjectRe: [PATCH 2/11] x86: convert to generic helpers for IPI function calls


On Tue, 22 Apr 2008, Ingo Molnar wrote:
>
> > Or is it just a performance optimization? [...]
>
> yes and i gave in - Nick and Jens wants to do some crazy stuff and they
> had the numbers. Here's the previous discussion:
>
> http://lkml.org/lkml/2008/2/27/125

No, the previous discussion was about single *queues* vs single *vectors*.

I agree unconditionally with the decision to use a separate per-cpu queue
from the shared queue (in fact, I would argue that the "mask" code might
want to notice when the mask is just a single CPU, and turn a mask request
into a targeted request).

But I wonder why we want to then have two IPI target vectors, when it
would appear to be perfectly fine and cheap to have just a single vector
that can handle both the per-cpu case and the shared queue case (since the
thing would tend to be one or the other, not both).

A single vector is still pefectly fine, if 99% of all usage cases are the
targeted-to-a-single-cpu thing, because the shared queue will basically be
empty (and you can test that without even taking any locks).

Linus


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